Exoplanets: Worlds Without End 9781466858985, 1466858982

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Exoplanets

Worlds Without End From the Editors of Scientific American Cover Image: NASA, ESA and G. Bacon (STScI) Letters to the Editor Scientific American 75 Varick Street, 9th floor New York, NY 10013-1917 or [email protected] Copyright © 2015 Scientific American, a division of Nature America, Inc. ™Scientific American, Inc. All rights reserved. Published by Scientific American www.scientificamerican.com ISBN: 978-1-4668-5898-5 Scientific American and Scientific American MIND are trademarks of Scientific American, Inc., used with permission.

EXOPLANETS

Worlds Without End From the Editors of Scientific American

Table of Contents Introduction Exoplanets: Worlds Without End by Jesse Emspak

SECTION 1 Exo-Search

1.1 A Plentitude of Planets by John Matson 1.2 The Dawn of Distant Skies by Michael D. Lemonick 1.3 New Mission Will Examine Chemical Signatures by John Matson 1.4 Gravitational Mesolensing and the Hunt for Exoplanets by Caleb A. Scharf 1.5 Should We Expect Other Earth-like Planets at All? by Caleb A. Scharf

SECTION 2

Giants: Hot and Cold 2.1 The Varied Lives of “Hot Jupiters” by Ken Croswell 2.2 Lonely Planets by Caleb A. Scharf 2.3 Atmosphere Sheds Light on Planetary Formation by Kelly Oakes

SECTION 3

Potential Earths: Places We Could Call Home 3.1 Planets with Lots to Offer by Dimitar D. Sasselov and Diana Valencia 3.2 Noisy Stars May Create Phantom Planets by Ron Cowen 3.3 Water Worlds by Ron Cowen 3.4 Do Small Planets Need a Heavy Metal Star? by John Matson 3.5 Tiny Planets with Big Potential by John Matson 3.6

Do Three Super-Earths Orbit a Nearby Star? by Lee Billings 3.7 Better Than Earth by René Heller

SECTION 4

Oddballs: Stranger than Fiction 4.1 Bones of Giants by Charles Choi 4.2 Diamond Planets by John Matson 4.3 Improbable Planets by Michael W. Werner and Michael A. Jura 4.4 Intergalactic Expat by John Matson 4.5 A Large Lump of Coal by George Musser 4.6 A Tale of Two Exoplanets by JR. Minkel 4.7 Worlds with Two Suns by William F. Welsh and Laurance R. Doyle

SECTION 5

The Search for Life 5.1 Anybody Home? by John Matson 5.2 The Search for Life on Faraway Moons by Lee Billings 5.3 Extraterrestrial Intelligence by John Matson 5.4 Contact: The Day After by Tim Folger 5.5 The Color of Plants on Other Worlds by Nancy Y. Kiang

SECTION 6

Going the Distance: Interstellar Travel 6.1 Reaching for the Stars by Stephanie D. Leifer

6.2 Starship Humanity by Cameron M. Smith

Exoplanets: Worlds Without End Science fiction writers have been dreaming of other planets from the start. There's the desert planet Arrakis from Dune, the jungles world Endor in Return of the Jedi, and a global ocean in Solaris. None of these fictional planets has proven as strange as the real thing. The very first planets orbiting another star, called exoplanets, were discovered in 1992. They were big, rocky worlds circling the remains of a star that exploded as it died. It's likely they are the stripped cores of gas giants that survived the cataclysm. Three years later astronomers found a planet around a star like our sun, called 51 Pegasi. The planet, 51 Pegasi b, wasn't any less weird. It was a gas giant like Jupiter, but the resemblance ended there. This one sped around in a 4.2-day orbit, closer to its star than Mercury is to our sun. By itself it upended theories of planet formation. Since then, scientists have found more than 1,000 exoplanets, with thousands more potential candidates. They seem to be everywhere. And if anything, our solar system seems to be the strange one. Or at least it’s the strange one so far. Improving the way we search has resulted in more findings, but also more questions about those findings and what they mean. This eBook is a collection of stories about what we currently know, and it’s exciting to think that it could all change tomorrow. Section 1, “Exo-Search,” delves into those improved techniques of how astronomers are looking for new worlds. Finding these miniscule specks of light seems impossible – by rights they should be drowned out by light from their stars. But in "A Plentitude of Planets" and "The Dawn of Distant Skies," we see that a combination of space-based telescopes, careful ground-based observations and better instruments opened up the field, and the discoveries have come thick and fast. Sometimes a little too fast. Section 3 focuses on the race to find other Earth-like planets. With excitement at an all-time high, author Ron Cowen cautions against publishing too quickly out of optimism. In “Noisy Stars May Create Phantom Planets,” Cowen describes how stellar activity can mimic the signs of tiny exoplanets. In the case of Gliese 581, a star located in an area called “the Goldilocks Zone,” one research team announced the discovery of a planet circling the star, but another used different method to analyze the data and saw nothing. Still, several possibilities have been found, and the other articles in this section look at the potential Earth-like planets out there. "Earth-like" is a loose term, referring to planets made of rock, but astronomers actually found the giant, Jupiter-like exoplanets first. Section 2 discusses these gas giants, which come in many sizes, and as "The Varied Lives of 'Hot Jupiters'" shows, they can start their journeys in the outer reaches of a system and drift in, or spin placidly far from their primary. It's lucky for us that the gas giants of our system stayed put. In Section 4, we get to the real oddballs. They may be remnants of gas giants whose atmospheres were stripped away, as in the piece, "The Bones of Giants," or have alien chemistries. Some trace their course around white dwarfs, the results of a second generation of planets forming around old stars. But is there life out there? That's the focus of the fifth section. Prospects look good in one sense – there are billions of planets. But how often does life arise, and how tenacious is it? In "Anybody Home?" John Matson outlines the search for the chemical signatures that scientists think are telltale signs that we're not the only living things in this part of the universe. Of course, no collection would be complete without a look at getting to those worlds we found, and the challenge encompasses both technology and the way we build societies, and even our biology. It won't be like the visions we see in science fiction. It will be stranger – wonderfully so.

--Jesse Emspak Book Editor

SECTION 1 Exo-Search

A Plentitude of Planets by John Matson The next time you look up at the night sky and find yourself marveling at the number of stars overhead, know that you are only seeing part of the magnificent bounty that our galaxy holds. Most of those Milky Way stars are not isolated orbs. Rather an average star has at least one planetary companion, invisible to the naked eye and in most cases as yet unseen by telescopes, according to a new analysis. That extrasolar planets should be even more common than stars, which themselves seem innumerable, lends support to the hope that somewhere up in the night sky, circling one of those stars, is a world like Earth where life may have had a chance to take root, and maybe even have evolved into an intelligent form. The analysis of planetary frequency in the Milky Way appeared in Nature. (Scientific American is part of Nature Publishing Group.) The researchers, led by astronomer Arnaud Cassan of the Paris Institute of Astrophysics at University Pierre and Marie Curie, used a small sample of planetary discoveries to infer the size of the overall planetary population. Extrapolating from a few known planets and the relatively low probability that each of those planets should be detectable from Earth, the researchers found that each star is home to an average of 1.6 planets. The process is a bit like estimating the average number of children in a typical family by peering into a handful of random homes, counting the number of children in view, and estimating how many more are at school or otherwise out of sight. As such, the planetary demographics are still rudimentary; given the small number of statistics, the actual average could be closer to one planet per star, or it could be well over two planets per star. But the general ubiquity of extrasolar planets, which other astronomical campaigns have also suggested in recent years, seems unassailable. "This is not a surprise, but it's a really interesting thing to know," says astronomer Scott Gaudi of The Ohio State University, who did not contribute to the new research. Perhaps most encouraging is the finding by Cassan and his colleagues that the frequency of planets rises as the mass of those planets decreases. Large planets akin to Jupiter are relatively rare, midsize planets such as Neptune are present around roughly 50 percent of stars, and small planets just five to 10 times the mass of Earth are even more numerous than that. "Planets are common, and low-mass planets are as common as dirt in some sense," Gaudi says. Cassan based the galactic census on a planet-finding method called gravitational microlensing. Using the Warsaw University Telescope in Chile, astronomers monitor roughly 200 million stars to look for the sudden and anomalous amplification in the light from any one of them. That brightening can be caused by another star passing in front of the background star, with the gravitational field of the intervening star acting like a lens to focus the light of the background star toward Earth. Such alignments are rare, but by monitoring so many stars for years on end, the campaign, known as the Optical Gravitational Lensing Experiment (OGLE), has recorded thousands of microlensing events. The brightening and subsequent dimming of the background star due to microlensing does not always follow a smooth bell curve, however. In about a dozen cases identified by OGLE and by the similar Microlensing Observations in Astrophysics (MOA) experiment based at Mount John University Observatory in New Zealand, irregularities in the lensing signal point to a planet orbiting the foreground star and distorting the symmetry of the lens. The duration of a deviation from the bell curve indicates the suspected planet's mass. Microlensing has its downside—the planetary signals are ephemeral, lasting only as long as the background star and the planet-hosting star remain in alignment (typically about a month). But it has one critical advantage over other planet-hunting techniques: it is sensitive to bodies not especially close to their parent stars. More prolific planet-search methods, including the technique employed by NASA's Kepler spacecraft, which detects periodic variations in starlight caused by orbiting planets eclipsing their stars, have the most success detecting planets that orbit very close to their host stars and hence complete an orbit very quickly.

"Microlensing can probe planets of all masses for a very large range of orbital separations," from about 0.5 times to 10 times the Earth–sun distance, Cassan says. He notes that the abundance estimates can only increase with exploration of a larger range of orbital distances and planetary masses. "Our results are given for masses between five Earths and 10 Jupiter masses," Cassan says. "If there are other planets farther or closer in, the average number of planets per star would increase accordingly." The conclusion that smaller planets occur more often than bigger ones reinforces what Kepler has shown for planets that orbit close to their stars. The spacecraft is designed to locate worlds similar to our own—small, rocky planets at temperate, Earth-like distances from their host stars. That hunt is still underway, but early results from the mission have revealed that smallish planets—those just a bit bigger than Earth—are common in the hotter, close-in orbits to which Kepler is already sensitive. "Kepler has already been finding that small planets are actually quite ubiquitous around stars," Gaudi says. "That bodes well for our goal of eventually finding an Earth-size planet in the habitable zone. All signs are pointing to the low-mass planets being common, so I think there's a good chance that we'll find a system like that in the coming years." --Originally published: Scientific American Online, January 11, 2012.

The Dawn of Distant Skies by Michael D. Lemonick Nobody who was there at the time, from the most seasoned astrophysicist to the most inexperienced science reporter, is likely to forget a press conference at the American Astronomical Society’s winter meeting in San Antonio, Tex., in January 1996. It was there that Geoffrey W. Marcy, an observer then at San Francisco State University, announced that he and his observing partner, R. Paul Butler, then at the University of California, Berkeley, had discovered the second and third planets ever found orbiting a sunlike star. The first such planet, 51 Pegasi b, had been announced by Michel Mayor and Didier Queloz of the University of Geneva a few months earlier—but a single detection could have been a fluke or even a mistake. Now Marcy was able to say confidently that it had been neither. “Planets,” he told the crowd, “aren’t rare after all.” The announcement shook the world of astronomy. Almost nobody had been looking for planets because scientists were convinced they would be too hard to find. Now, after searching a mere handful of stars, astronomers had discovered three worlds, suggesting billions more waiting to be found. If Butler and Marcy had merely settled a question on planetary formation theory, their discovery would not have been such a big deal. But it showed unequivocally that so-called extrasolar planets did exist and, with them, the possibility of answering a question that had vexed philosophers, scientists and theologians since the time of the ancient Greeks: Are we alone in the universe? After the initial celebration, scientists settled down to figuring out exactly how they were going to investigate the prospect of even a rudimentary form of life on a planet orbiting an alien sun. Short of picking up an extraterrestrial broadcast, à la Jodie Foster in the movie Contact, the only way to find out would be to search extrasolar planets for atmospheric biosignatures— evidence of highly reactive molecules such as oxygen that would quickly disappear unless some kind of metabolizing organisms were replenishing the supply. Marcy, Mayor and their colleagues had seen only the gravitational effect the planets had on their parent star; to detect a biosignature, you would need to image an exoatmosphere directly. To do this, NASA planned to launch an increasingly powerful series of space telescopes, a program that would culminate in an orbiting telescope called the Terrestrial Planet Finder Interferometer that would cost billions of dollars and fly sometime in the 2020s. In short, astronomers knew that they wouldn’t be learning anything about exoplanet atmospheres anytime soon. They were wrong. The discovery of those first few exoplanets inspired an entire generation of young scientists to get into what was suddenly the hottest specialty in astrophysics. It convinced many of their older colleagues to switch into exoplanetology as well. This sudden influx of brainpower led to fresh ideas for investigating exoplanet atmospheres and sped things up dramatically. By 2001 observers had identified sodium in the atmosphere of one exoplanet. Since then, they have identified methane, carbon dioxide, carbon monoxide and water as well. They have even found indirect hints, by examining exoplanet atmospheres, that some planets may be partly made of pure diamond. “At this point,” says Heather Knutson, a California Institute of Technology astrophysicist who was involved in many of these pioneering observations, “we’ve learned something about the atmospheres on the order of 30 to 50 planets—if you count stuff that’s not yet published.” These discoveries are still a long way from providing evidence of life—no surprise, since most of the worlds Knutson is talking about are hot, Jupiter-like planets that hug their star more tightly than fiery Mercury orbits the sun. Increasingly, however, Knutson and other observers have begun to probe the atmospheres of smaller planets, so-called super-Earths, which are between two and 10 times as massive as our home planet—something that nobody could have imagined just a decade ago. The announcement in April 2013 that the Kepler space telescope had found two planets less than twice Earth’s size, both in orbits where temperatures might permit life to survive, was one more hint that life-friendly worlds are almost certainly plentiful. So while these planets, named Kepler 62e and 62f, are too distant to study in detail, astronomers are convinced it will not be many more years before observers can look for biosignatures in the atmospheres of planets that are essentially twins of Earth.

THE PARKING-LOT PLANET Astronomers assumed it would take decades to start looking at planetary atmospheres because the first handful of exoplanets were discovered in directly, through the influence each had on its parent star. The planets themselves were invisible, but be cause each star and planet orbit a mutual center of gravity, the gravitational tug of the planet makes the star appear to wobble in place. When a star moves toward us, its light subtly shifts to ward the blue end of the visible-light spectrum; when it moves away, the light shifts to the red. The degree of shifting tells observers the star’s radial velocity, or how fast it moves toward and away from Earth, which in turn tells us how massive the exoplanet is. Another option for finding planets was also available, however. If the invisible planet’s orbit were perfectly edge-on as seen from Earth, the planet would pass directly in front of its star, in what is known as a transit. Yet at the time of those first discoveries nearly two decades ago, few astrophysicists were thinking about transits at all, simply because the search for planets was itself so far out on the fringe. (A notable exception was William J. Borucki of the NASA Ames Research Center, whose Kepler spacecraft would eventually find transiting objects by the thousands.) A few years later, in 1999, Timothy W. Brown, then at the National Center for Atmospheric Research, and David Charbonneau, at the time a graduate student at Harvard University, set up a tiny, amateur-size telescope in a parking lot in Boulder, Colo., and saw an exoplanet transit for the first time. The planet was HD 209458b, which had been detected earlier by the radial-velocity technique. Weeks later Gregory W. Henry of Tennessee State University, working with Marcy, watched the same planet transiting its star. Both teams have been given equal credit for the discovery because the two detections were published simultaneously. The successful detection of transits not only gave astronomers a second way to find exoplanets, it also gave them a way to measure their density. The radial-velocity technique had revealed HD 209458b’s mass. Now astronomers knew how physically large it was because the amount of starlight a planet blocks is directly proportional to its size. (Dividing its mass by its size showed HD 209458b to be 38 percent larger than Jupiter even though the planet is only 71 percent as massive , an unexpected result that Princeton University astrophysicist Adam Burrows refers to as “an ongoing problem to explain.”) By this time a number of astrophysicists had realized that transits also made it possible to study an exoplanet’s atmosphere, in what Knutson calls a “wonderfully clever shortcut.” Even before the first transit was reported, in fact, Sara Seager, an astrophysicist at the Massachusetts Institute of Technology, who at the time was Charbonneau’s fellow grad student at Harvard, had co-authored a paper with her adviser, Dimitar D. Sasselov, in which they predicted what an observer should see as light from a star passed through a planet’s atmosphere when the planet moved across the star’s face [see “Planets with Lots to Offer,” by Dimitar D. Sasselov and Diana Valencia]. Physicists have long known that different atoms and molecules absorb light at different wavelengths. If you look at planets in a wavelength that corresponds to the molecule you are searching for, any atmospheres containing that molecule will absorb the light. The wispy planetary atmosphere will become opaque, making the planet appear larger. Seager and Sasselov suggested that sodium would be especially easy to detect. “Sodium is like skunk scent,” Charbonneau says. “A little bit goes a long way.” He knows this better than anyone: in 2001 Charbonneau, Brown and their colleagues went back to HD 209458b, their original transiting planet, not with a puny amateur telescope but with the Hubble Space Telescope. Sure enough, the sodium signal was there, just as predicted. TOTAL ECLIPSE Astronomers also realized that there was a second, complementary way to inspect the atmospheres of transiting planets. When a planet passes in front of its star, it presents its nightside to the observer. At other times, it shows at least part of its dayside, and just before the planet goes behind the star, the dayside is facing Earth. Although the star is far, far brighter, the planet itself also glows, mostly in the infrared. That glow vanishes abruptly, however, when the planet moves behind the star; its contribution to the combined light of planet and star vanishes. If astrophysicists can do a before-and-after comparison, they can deduce what the planet alone would look like. “It changes the nature of the problem,” Knutson says. “Instead of having to detect a very faint thing close to a very bright thing, all you have to do is measure signals that change with time.” As early as 2001, L. Drake Deming, then at the NASA Goddard Space Flight Center, aimed an infrared telescope on Hawaii’s Mauna Kea at HD 209458b in an attempt to see this so-called secondary eclipse, but, he says, he couldn’t make a detection.

Illustration by Mark A. Garlick; Source: NASA/JPL-Caltech/K. Stevenson, University of Central Florida

He knew, however, that the Spitzer Space Telescope, scheduled for launch in 2003, would almost certainly be able to make such an observation, as did Charbonneau. Both astrophysicists, unbeknownst to each other, applied for time on Spitzer to make the observations. Both got the time and took their data. Then, one day in early 2005, Deming recalls, he got a voice message: “Drake, this is Dave Charbonneau of Harvard,” the voice said. “I hear you made some interesting observations lately. Maybe we should talk.” It turned out that Deming (working with Seager) and Charbonneau had independently made the first secondary eclipse detections in history, at virtually the same time, using the same observatory. The two groups announced their results for two different stars—the much worked over HD 209458b in Deming’s case and a planet named TrES-1 in Charbonneau’s—simultaneously. A year later Deming’s team detected the secondary eclipse of a planet called HD 189733b. “This,” wrote Seager and Deming in a 2010 review article, “unleashed a flood of secondary eclipse observational detections using Spitzer. . . . It is accurate to say that no one anticipated the full magnitude and stunning impact of the Spitzer Space Telescope as a tool to develop the field of exoplanet atmospheric studies.” In fact, Seager says, “we’re using the Hubble and the Spitzer in ways they were never designed to be used, going to decimal places they were never designed to reach.” ATMOSPHERIC LAYERS Those studies have shown a couple of things, Seager says. “This sounds trite in a way, but we’ve learned that hot Jupiters are hot. We’ve measured their brightness and temperatures,” and what scientists have observed is consistent with how they expect stars to heat their planets. “Number two,” she continues, “we’ve detected molecules. Now, has [what we’ve found] been very different from what we expected? You know, not really.” Seager notes that physicists can straightforwardly model a ball of gas at some temperature made of some combination of elements and ask what kind of molecules form. “The laws of physics and chemistry are universal,” she says. Seager and other astrophysicists have also learned, however, that despite the overall similarity of exoplanet atmospheres, individual planets can differ in several ways. One has to do with how temperature changes with altitude. Some planets, such as Jupiter and Saturn in our own solar system, show temperature inversions, in which temperature rises with altitude rather than falling. Others do not. “The problem,” Knutson observes, “is that we don’t know what’s causing the inversion, and we can’t predict, therefore, which exoplanets will and won’t have this feature.” Some astrophysicists suggest that exoplanets with inversions might have some kind of heat-absorbing molecule, such as titanium oxide, but so far this is just a hypothesis. Another question is whether certain planetary atmospheres are made from a different mix of molecules than others. Nikku Madhusudhan, now at the University of Cambridge, analyzed the visible and infrared signature of a planet named WASP-12b and deduced that its atmosphere is unusually rich in carbon, with about as much of that element as oxygen.

Theory suggests that a carbon-to-oxygen ratio of more than 0.8, if mirrored in other, smaller planets in the same system (as it presumably would be, given that planets in a solar system are thought to condense from a single disk of gas and dust), would lead to “rocks” made of carbides—carbon-rich minerals—rather than the silicon-rich silicate rocks found in our solar system. If that were true, an Earth-size planet in the WASP-12 system could have continents made of diamond. Seager and others have written theoretical papers suggesting that there is nothing to rule out planets made largely of carbon or even of iron. In the case of WASP-12, however, it may not be correct. Knutson says that Ian Crossfield of the Max Planck Institute for Astronomy in Heidelberg, Germany, recently found that the light from WASP-12 is contaminated with light from a fainter double star in the background. “His data perhaps seem to cast some doubt on the interpretation for this particular planet,” Knutson says. WATER WORLD By far the most intense focus of observations has been concentrated on a planet known GJ 1214b, which orbits a small, reddish “M-dwarf ” star lying about 40 light-years from Earth. Its proximity makes GJ 1214b relatively easy to study, and its size, just 2.7 times the width of Earth, makes it far closer to being Earth-like than the hot Jupiters found in the first years of planet hunting. “It is everybody’s favorite superEarth,” says Laura Kreidberg, a grad student at the University of Chicago who is leading the data analysis on one such observing project. GJ 1214b was found in 2009 during the so-called MEarth Project organized by Charbonneau to look for planets around M dwarfs. The idea was that small transiting planets would be easier to find around these small, dim stars than around bigger ones, for several reasons. First, an Earth-size planet would block a relatively greater percentage of the small star’s light. Such a planet would also exert a relatively greater gravitational pull on the star, making it easier to gauge the planet’s mass and thus its density. The habitable zone for a small, cool star would also be much closer in than it is for a hot, sunlike star, which makes transits more likely to be spotted (because the orbit of a close-in planet does not have to be so precisely aligned for it to pass in front of the star) . Finally, there are vastly more M dwarfs in the Milky Way than there are sunlike stars—about 250 of the former lie within 30 or so light-years of Earth, compared with only 20 of the latter. GJ 1214b is not quite a second Earth: it is 2.7 times wider and six and a half times as massive as Earth, which gives it an overall density in between that of Earth and Neptune. Unfortunately, as Charbonneau and others realized immediately after the planet was discovered, this density can come about in several different ways. GJ 1214b could, for example, have a small, rocky core surrounded by a huge atmosphere of mostly hydrogen. It could also have a bigger core surrounded by a deep ocean of water, with a thin, water-rich atmosphere on top. It is impossible, given the density alone, to distinguish between those two possibilities—although the possibility of an ocean world is naturally more ex citing, given that liquid water is considered a prerequisite for, if not a guarantee of, life as we know it. Yet when University of Chicago astronomer Jacob Bean observed the planet in various wavelengths, hoping to see a change in its apparent size that would indicate just how thick the atmosphere is, he saw nothing. This could mean one of two things. The planet could have a puffy hydrogen atmosphere but one full of clouds and haze that would make it hard to detect. Or it could have a thin, watery atmosphere but one too thin to delineate with ground-based telescopes. The situation could be analogous to looking at a mountain range from a distance, says Kreidberg, who began working with Bean in 2013. “There may be peaks,” she explains, “but if you’re too far away, they might look like a flat line.” To try to resolve the issue, Bean and his colleagues were awarded 60 orbits of the Hubble. This was not the first time astronomers observed GJ 1214b with the Hubble, but it is by far the most intensive program to date. Alas, when the team analyzed their new data, they could neither confirm nor refute the possibility that GJ 1214b is a waterworld. Instead the Hubble observations only bolstered the case that, whatever GJ 1214b’s composition might be, its skies are filled with obscuring clouds. THE HUNT FOR OXYGEN Now that astronomers have been in the planet-hunting business for some time, they have begun to find many more planets with long orbital periods. These planets are farther away from their stars and thus cooler than the early population of hot Jupiters. “For a long time we were limited to things that were 1,500 kelvins, 2,000 kelvins, so really quite hot,” Knutson says. In these conditions, “most of the carbon in the atmosphere gets bound up with oxygen, forming carbon monoxide,” she says. “The really interesting thing that happens as you drop below around 1,000 kelvins is that it switches to being incorporated into methane instead.” Methane is especially intriguing because it could be a sign of biological activity—though an ambiguous one because methane can be produced through purely geophysical processes. Oxygen—and especially ozone, a highly reactive molecule made from three oxygen atoms—would be far more likely to signal the presence of life. It would also be extremely difficult to detect because its spectral signature is subtle,

especially so in the relatively small atmosphere of an Earth-size planet. Yet for all the activity around medium-hot super-Earths, astronomers are still focused on the grand prize. “All of this is really just an exercise,” Seager says. “I mean, it’s interesting in and of itself, but for people like me, it’s just a stepping-stone to when we finally get from super-Earths to studying the atmospheres of Earths.” That likely won’t happen before the James Webb Space Telescope is launched into orbit, probably in 2018, and a new generation of huge, ground-based instruments, including the Giant Magellan Telescope and the Thirty Meter Telescope, come online in the 2020s. Even with those powerful instruments, Seager says, “it’s going to take hundreds and hundreds of hours” of observing time. It is not clear even then that it will be possible to detect the signature of life unambiguously; for that, observers might still need the Terrestrial Planet Finder, whose funding has been reduced so drastically that any hope of an actual launch date is pure guesswork at this point. Yet it is remarkable that, so far ahead of any schedule anyone dreamed of in the 1990s, Seager can even talk about the realistic prospect of finding biosignatures. We are no longer merely hoping that an alien civilization will spot us and point a message our way. We are actively exploring the air above distant worlds, searching their skies for signs that something is home --Originally published: Scientific American, 23(3s); 4-11 (August 2014).

New Mission Will Examine Chemical Signatures by John Matson NASA’s Kepler mission has been a smash hit. It has discovered thousands of probable exoplanets— worlds orbiting stars other than the sun—several hundred of which have already been vetted and confirmed. Many of those planets are among the most nearly Earth-size planets known: of the 25 smallest-diameter exoplanets discovered to date, all but one were spotted by Kepler. There is just one asterisk tacked to Kepler’s immensely productive haul: the planets are hundreds or even thousands of light-years away, too distant to investigate in any detail. Enter TESS, the Transiting Exoplanet Survey Satellite, which NASA has green-lit for a 2017 launch at a cost of $200 million. TESS will survey a much larger swath of sky than its predecessor to uncover a new population of nearby exoplanets that scientists can scan with forthcoming telescopes. “Altogether we’ll examine about half a million stars,” says TESS principal investigator George R. Ricker, a Massachusetts Institute of Technology astrophysicist. Thousands of those stars are within 100 lightyears of the solar system. Like Kepler and the European CoRoT satellite before it, TESS will search for planetary transits: brief dimmings of starlight, occurring at regular intervals, that betray the shadowing presence of an unseen exoplanet. Ricker estimates that TESS may discover some 500 to 700 planets that are Earth-size or a few times larger, of which a handful will be potentially habitable. Around the time that TESS compiles a list of nearby exoplanets at the end of its two-year baseline mission, astronomers may have a powerful new eye in the sky to examine the newfound worlds in detail. NASA’s James Webb Space Telescope (JWST), currently slated to launch in 2018, should be able to tease out the signatures of certain molecules in the atmospheres of nearby planets. Ultimately, those kinds of chemical signatures could be used to infer the presence of extraterrestrial life on a planet. By simulating the observing power of the JWST trained on a nearby, possibly habitable planet, “we can almost see biogenic signatures, but not quite,” Ricker says. “That could well take a next-generation space instrument to do that.” Regardless, if TESS can indeed locate hundreds of nearby planets, astronomers will have their hands full for the foreseeable future—finding out what those planets are like and what kinds of habitats they might support and, just maybe, flinging some future probe toward one enticing-looking world. --Originally published: Scientific American 308(6); 29 (June 2013).

Gravitational Mesolensing and the Hunt for Exoplanets by Caleb A. Scharf When astronomers talk about methods for finding exoplanets the list is relatively short. There is the radial velocity, or ‘wobble’ technique, which senses the motion of a star around a common center-of-mass with its planets. There is the transit technique, employed with great success by NASA’s Kepler mission, and there are direct imaging and phase-photometry techniques – challenging observations that seek the light being actually emitted or reflected from a planet. And then there is gravitational microlensing, the chance magnification of the light from a distant star by the distortion in spacetime due to the mass of a foreground star and its planets – with distinctive ‘blips’ or cusps of brightness due to any worlds aligned close to the right place in the star’s lensing field. This form of gravitational distortion of the pathway of photons is called ‘micro’ because the typical arrangements and masses of stars results in tiny images; while the light of a background object may be greatly magnified we can’t see its distorted image directly, its light merges with that of the ‘lens’ stars, mere thousandths of a second of arc from it. The gravitational effect of a planet around the lens star is effectively amplified when it is close enough to the zone of maximum magnification, the Einstein ring, but its effect is also only seen as an additional and asymmetric boost in photons arriving at our telescopes. But the key phrase here is ‘typical arrangements’. Given the rarity of alignments between two stars separated by great distances, caused by the endless motions of all objects within our galaxy’s gravity well, the majority of such events that we see occur between stars that are both very distant from us – perhaps more than halfway between here and the center of the Milky Way. At these enormous distances (many thousands of light years) we cannot measure the motion of a lens star (or its potential lensing victims) relative to others, and so have no idea when or if any given star will magnify the light of something aligned directly behind it. The situation is rather different however for much closer stars. Not only can we obtain their ‘proper motions‘ with careful high-precision astrometry, the zone around them that is optimal for magnifying a background object is that much bigger in angular diameter, it is ‘meso’ not micro. Thus, gravitational mesolensing opens up a number of intriguing possibilities. First, as discussed in a triplet of wonderful recent papers by Lepine and Di Stefano, and Di Stefano, Matthews, and Lepine, it becomes possible to predict when a nearby lens star may move close enough to the position of a distant object to magnify it, and the larger lensing angle may cause a directly measurable shift in the apparent position of that background star as well as its brightness. If there are also planets around the lens star the predictability of the event may allow us to snag the early or late lensing signature of worlds on larger orbits that we might otherwise miss. The larger scale of the lensing angles also offers a unique probe of the effect of any close-in, very short orbital period planets. And the icing on the cake is that nearby stars are much more amenable to the detailed astronomical measurements necessary to estimate their true masses – the true strength of the lens. So can this be done? The authors point out a specific case; the object VB 10 is a low-mass star (perhaps a tenth the mass of the Sun) a mere 19 light years away. VB 10 ‘scoots’ across the sky at about 40 kilometers a second in transverse velocity, a minuscule but measurable angular shift per year (see the animation to the left here). Earlier Hubble Space Telescope images reveal something small and faint in its path – a distant background star headed for the lensing zone of VB 10 in late 2011/early 2012. Has VB 10 produced gravitational mesolensing? We’re awaiting the authors’ report on their efforts to observe any possible event. It’s not an optimal case, a nearby lens moving very fast doesn’t provide the best combination of factors, but it really is a pioneer in what might just become a new tool for hunting exoplanets – welcome to the fold gravitational mesolensing. --Originally published: Scientific American Online, March 7, 2012

Should We Expect Other Earth-like Planets at All? by Caleb A. Scharf This decade has been a spectacular one for exoplanets. New discoveries and new insights have truly pushed the gateway to other worlds even further open. We’ve gained increasingly good statistics on the incredible abundance of planets around other stars and their multiplicity. We also finally seem to have evidence that our neighboring star Alpha Centauri B does indeed harbor at least one world. It is by any set of standards, a great haul. But I continue to be a bugged by the claims of ‘habitable’ worlds and ‘Earth-like planets’ that seem to beset many scientific announcements (including I’m ashamed to say my own). In the spirit of closing out the passage of our 4,500,000,000th or so orbit around the Sun I thought I’d try to set the record straight, because I think we have so much more to look forward to than simply finding ‘another Earth’. First, when press releases state that a ‘habitable’ world may have been found, the truth is far more complex. Astronomers and astrobiologists tend to use the term habitable as a shorthand for the presence of liquid water on a planetary surface, implying a range of temperatures between the freezing and boiling point of water. But this also requires a surface atmospheric pressure high enough for water to exist as a liquid without boiling off to vapor, and an atmosphere will alter the transfer of radiation to and from the surface – often by way of a greenhouse effect. And this is just the beginning. By this simple criterion even the Earth is only partially habitable – about 85% of its area remains amenable to liquid water over a year (a fact that my colleagues Dave Spiegel, Kristen Menou and I reiterated a few years ago). So strictly speaking ‘habitable’ includes a range of environments that we would find appallingly hostile, including high-pressure, high-temperature climates and those in a sub-arctic category with thin atmospheres. The problem with the newly proclaimed habitable, or even potentially habitable exoplanets of 2012, is that not only do we at present have absolutely no information on the presence or absence of water or an atmosphere, we also have absolutely no idea (beyond informed guesses) about their geophysical history or present state. Geophysics is the dirty little secret here. On Earth the long-term (read millions of years) stability of the Earth’s surface environment close to the ‘habitable’ state is a direct consequence of geophysical re-cycling, the so-called Carbon-Silicate Cycle. Or to put this more crudely, no volcanism or tectonics, and you get no temperate climate. This does not mean that some of them don’t hit the sweet spot, but it’s horrendously premature to say so. Which gets us to the other point, the cavalier use of the phrase ‘Earth-like’. Utterance of this can evoke all sorts of images. It may make us think of oceans, beaches, mountains, deserts, forests, fluffy clouds, fluffy bunnies, warm summers, snowy winters, the local pub, or the fabulous hubbub of the local souk. But this is typically far from the meaning attached by scientists. It can simply indicate a planet with a rocky surface, rather than a world with a thick gaseous envelope. It can mean a world that is roughly the same mass and density as Earth. It can mean a planet orbiting a star like the Sun. Or it can just mean that we got bored of saying things like ‘a two-Earth mass object in a close to circular orbit around a roughly 4 billion year old main-sequence star that is similar in mass to the Sun.’ Although at some level this is purely to do with semantics I think it’s important to consider. What I believe we really mean when we say ‘Earth-like’ is that a planet is Earth equivalent. That is to say that while the planet might feel completely alien to human senses it nonetheless matches many of the same physical and chemical characteristics of Earth. It’s a bit like renting a car at an airport where you’ve reserved the open top red sports-car, only to be told that they’ve run out but you can have ‘an equivalent’ vehicle. It’ll have four wheels, an engine, and yes you can wind the windows all the way down if you’d like. And this is critical because one thing we have learned about exoplanets in 2012 is that they are remarkably diverse, and that the configuration and contents of our solar system are somewhat (only

somewhat) unusual. For example, none of our planetary sisters belong to what may be the most numerous category of worlds – objects with masses between that of the Earth and that of Neptune. There’s an absolute load of those out there, but none here. Our system is also relatively spread out, it is increasingly apparent that the universe likes to build compact orbital architectures of numerous planets with orbital periods of days to weeks. It is also apparent that a majority of planetary systems have likely gone through a far more dramatic period of dynamical rearrangement or dynamical ‘cooling’ than ours has – leaving a tell-tale signature in the highly elliptical orbits of major planets. This is just the tip of the iceberg. Although we don’t yet know much about the elemental and chemical composition of exoplanets, we do see a diversity in the contents and conditions of proto-planetary environments. We also suspect that the pathway to any individual world includes an enormous variety of essentially random events – the process of building a planet is highly stochastic. On the face of it this might seem a little depressing. After all, finding an ‘Earth-like’ planet is often described as the holy grail of exoplanetary science, the location of another place that could harbor life in the cosmos. But if all worlds are unique it might be that there are no other places quite like Earth, and if a delicate set of conditions are critical for life this would put a damper on these prospects. However, the chemistry of the universe does seem to be the very same chemistry from which we’ve sprung. Molecules made with carbon are ubiquitous, whether in the sparse interstellar medium or the thickening clouds of forming proto-stellar systems, organic chemistry dominates. It also permeates the ancient material of our solar system that we see preserved in carbonaceous chondrite meteorites and cometary contents. And life on Earth not only got going fast some 3.5 to 4 billion years ago, it seems to have rapidly evolved into an interlinked world-wide chemical network that robustly preserves the vital genetic blueprints for metabolism and survival – the core planetary gene set. While it is true that we do not yet know the actual answer, my personal take on all of this is the following. There may be no ‘Earth-like’ planet out there, but there are almost certainly Earth-equivalent worlds – alien but nonetheless amenable to life. Only the most contrived and strained interpretation of what we see by way of the elemental and chemical composition of the cosmos and of terrestrial biochemistry would suggest there’s something ‘special’ about what happened here on Earth. Carbon-chemistry rules, and Earth 4 billion years ago was about as alien from Earth today as one can imagine, yet life started up fast. It’s going to happen in other places too. So, we should not necessarily hold our breath for other Earth-like planets, but we should expect an astonishing diversity of equivalent places. I for one can’t wait to find out how the complex rough and tumble of molecular evolution played out on those other worlds. --Originally published: Scientific American Online, February 26, 2012.

SECTION 2 Giants: Hot and Cold

The Varied Lives of “Hot Jupiters” by Ken Croswell Ever since Swiss astronomers astonished the world by finding a gas-giant planet orbiting close to its star, researchers have wondered how these so-called hot Jupiters arose. None exists in our solar system, where the planetary giants—Jupiter, Saturn, Uranus and Neptune—reside in the deep freeze beyond the asteroid belt. Now a surprising discovery is providing fresh insight, suggesting that the more iron a star was born with, the more likely it is that the star's hot Jupiter had a violent past. "Hot Jupiters are sort of a red flag that our simple picture of how planetary systems form and evolve, which we used to have, is not sufficient," says Rebekah Dawson, a graduate student in astronomy at Harvard University. In that picture, planets arose from a disk of gas and dust around a star and stayed as far from their sun as where they originated. But not hot Jupiters. Just as the fossil of a tropical plant in Antarctica suggests continental drift, so hot Jupiters reveal that planets can move toward their star. Gas giants form far from their stars, where the protoplanetary disk is so cold that ice condenses and conglomerates into ice-rock-iron cores roughly 10 times more massive than Earth. In our solar system, the gravity of two such cores attracted so much hydrogen and helium that they swelled into the gas giants Jupiter and Saturn, which are 318 and 95 times more massive than Earth. Two other cores failed to accrete much gas and remained smaller; they became Uranus and Neptune, the sun's "ice giants," and weigh in at only 14.5 and 17.2 Earth masses. So a hot Jupiter starts life far from its star and must move inward until it gets hotter than any world in our solar system. But exactly how does the planet move inward? Astronomers once favored a gentle process, in which the protoplanetary disk slowly drags the planet sunward, leaving it on a circular orbit in the plane of the star's equator. In 2008, however, astronomers began finding hot Jupiters with tilted orbits, suggesting instead past violence: The gravity of other gas giants had kicked the Jupiters inward. Now Dawson has discovered a startling correlation between a star's iron abundance and the planets' orbital shapes that reveals their origins. Because hot Jupiters are close to their stars, stellar tides tug on the planets and make their orbits circular. So she looked instead at gas giants somewhat farther out, where stellar tides are too weak to significantly alter orbital shapes, finding that stars with more iron than the sun tend to have gas giants on much more elliptical paths. In the Astrophysical Journal Letters, Dawson and her advisor, Ruth Murray-Clay, explain the finding as follows. If a star and its protoplanetary disk are born with lots of iron and other heavy elements, the cores grow fast, because they consist mostly of these elements; therefore, the cores attract hydrogen and helium and give rise to more gas giants. A multitude of gas giants increases the chance that one planet's gravity will fling another sunward, where it becomes a hot Jupiter. In contrast, stars born with less iron may host at most one gas giant, which can move inward only by inching through the disk. "The result is pretty striking," says Daniel Fabrycky, an astronomer at the University of Chicago. He believes the message is clear: There is more than one way to make a hot Jupiter. After Dawson posted her work online, Stuart Taylor, an astronomer in Hong Kong, emailed her to say he had discovered the same correlation and presented it last summer at a conference in Beijing. But Taylor thought a star's high iron content might be an effect rather than a cause, because giant planets on elliptical orbits kick other planets into their star, boosting its iron level. "I think that our explanations are really complementary," Taylor says, because both processes may operate: An iron-rich star has more gas giants, causing more planets to crash into their sun. In any event, it's a good thing "our" Jupiter stayed put. "No matter how a gas giant moves in, it's probably bad news for any small, Earth-sized planets that are in its way," Dawson says. "They would most likely get scattered into the sun." Indeed, if the sun had been born with more iron, Uranus and Neptune might have grown into gas giants —and catapulted Jupiter or Saturn into our corner of the solar system. --Originally published: Scientific American Online, April 1, 2013

Lonely Planets by Caleb A. Scharf Hot Jupiters are special beasts in the exoplanetary menagerie. These giant worlds orbit their parent stars incredibly tightly, sometimes zipping around in barely a day or two, and so close that they can disturb the stellar atmosphere itself – as well as throwing themselves at the mercy of gravitational tides and scorching radiation. They were also the very first type of exoplanets to be detected around normal, hydrogen-burning, stars like our Sun in 1995. This was both a great triumph of the ingenuity and perseverance of a few astronomers, and a great surprise. Up to this point it was almost an unwritten expectation that other planetary systems would in some way be like ours. Smaller rocky worlds would orbit closer to stars and gas and ice giants would orbit at a distance, mimicking the solar system, but also matching our relatively simple picture of how planets should form. Hot Jupiters threw all of this for a loop. There was no way that they could have formed in-situ, suggesting immediately that some mechanism had moved them, or migrated them, into their toasty environments from an origin much further out. There are a number of possibilities for how this can happen. A massive proto-planet, more than 10 or 15 times the mass of the Earth, can set up density waves, or wakes, in the vast disks of gas and dust surrounding a proto-star. These spiral patterns of matter in turn exert a gravitational force on the planet, sometimes driving it inwards by bleeding off angular momentum. A young giant can quite rapidly burrow its way in close to a parent star this way. Another good possibility is that proto-planets form in configurations that are inherently unstable. Their gravitational pulls on each other eventually perturbing orbits to a point where planets collide, dive into the parent star, are ejected altogether to interstellar space, or are simply pushed into elliptical paths with small periastrons (closest approaches to the star) where gravitational tides collapse the orbits into the tight, nearly circular shapes that hot Jupiters inhabit. What has remained somewhat unclear however is what any of these scenarios mean for other, smaller planets in these systems. Some computer simulations of planet formation have indicated that terrestrialsized planets could perhaps survive the inwards migration of giant worlds in roughly 30% of cases, but other scenarios predict that to make a hot Jupiter a system must sacrifice many of its worlds. A study by Steffen et al. in the Proceedings of the National Academies uses data from the Kepler planetfinding mission to look for the signatures of smaller worlds around stars already proven to harbor giant hot Jupiters. In 63 systems the authors searched for signals of other transiting planets, or planets that might be perturbing the hot Jupiters – tweaking their orbital timing by their gravitational pulls. The conclusion? Hot Jupiters with orbital periods of less than 3 days show no significant evidence for other planets (as small as 2/3rds to 5 times the mass of Earth) near to them. By contrast, ‘warm’ Jupiters on somewhat larger, though still small, orbits, and ‘hot Neptunes’ (lower mass giants) do show statistical evidence for neighboring worlds. This apparent isolation of hot Jupiters (although we don’t yet know for sure about even smaller mass planetary neighbors) indicates a very distinct pathway to their formation – planet on planet interactions that drive these worlds into elliptical orbits that then erode down to small sizes by tides. In this scenario very few, if any, hot Jupiter systems will ever harbor Earth-sized planets anywhere in the habitable zone. The situation for hot Neptunes, or hot Earths may however be different. So hot Jupiters seem likely to spend their hellish lifetimes isolated from planetary sisters, the price paid for a very specific type of wild youth. --Originally published: Scientific American Online, May 7, 2013

Atmosphere Sheds Light on Planetary Formation by Kelly Oakes Astronomers have found water vapour and carbon monoxide, but no methane, in the atmosphere of an alien planet orbiting a star 129 light years away. The star, known as HR 8799, is at the centre of the first planetary system beyond our solar system to be imaged directly, in 2008. The star has at least four gas giants orbiting it. One of them, HR 8799c, is seven times the size of Jupiter that orbits at roughly the same distance Pluto does the sun in our own solar system. The light from the HR 8799c can be distinguished from its star, partly due to its distant orbit. Having the light from the planet itself means that astronomers can see the planet’s atmosphere in unprecedented detail. Quinn Konopacky, of the Dunlap Institute for Astronomy and Astrophysics, Toronto, Canada, and her colleagues used one of the Keck telescopes in Hawaii to get the most detailed look at its light yet. They then analysed that to get the chemical composition of the distant planet’s atmosphere. The data came from 5.5 hours of observations, made up of 33 ten minute exposures. Carbon monoxide and water both have a “very distinctive chemical fingerprint”, said Travis Barman, second author on the Science paper, from the Lowell Observatory in Flagstaff, Arizona, US. “We’ve seen it in many other objects and it has a very recognizable pattern,” Barman said in a press teleconference. “So it was very easy to see right away in our in our data.” But the researchers found no methane in the planet’s atmosphere. “The fact that we see carbon monoxide and no methane in this object that’s cool enough to allow methane to exist tells us that the mixing of the atmosphere is relatively efficient,” said Barman. The planetary system is young, with an estimated age of 30 million years. HR 8799c could be as hot as 1000C, and its gravity around 10 times that of Earth, Konopacky and her colleagues found. The results announced today give astronomers an insight into how the planetary system formed. There are currently two competing scenarios. Either planets form in a top down sort of way, as a disk of dust and star stuff collapses because of gravity, or they form from the bottom up, as gas slowly builds up on a planetary core. HR 8799c’s atmosphere suggests that it formed from the bottom up, from a debris disk containing plenty of ice that gave rise to the water in its atmosphere today. Ice and dust would have clumped together to make the planet’s core, then once it was big enough gas would have started to surround the core, making the deep atmosphere that envelopes Jupiter-like planets. “Once this core grew large enough to maybe about 10 times the size of the Earth, then it had sufficient gravity to start attracting the gas that was in the disk and it eventually acquired enough gas where it made this large gaseous atmosphere that we see and observe today,” said Konopacky. Almost a thousand exoplanets are now known, and NASA’s Kepler mission alone has found nearly three thousand exoplanet candidates. But HR 8799 and its planets are one of only a handful of systems beyond our own solar system that we’ve seen directly. Most exoplanets are found through indirect methods. Kepler, for example, detects planets by looking for dips in brightness of a planet’s host star. In time, probing the atmospheres of other exoplanets directly, like Konopacky and her colleagues did here, will give astronomers much more information about the conditions beyond our solar system. “Each time we’re able to divide an exoplanet’s light up into even smaller increments, we learn more and more about the planet’s atmosphere, composition and other basic properties,” said Barman. --Originally published: Scientific American Online, March 14, 2013.

SECTION 3 Potential Earths: Places We Could Call Home

Planets with Lots to Offer by Dimitar D. Sasselov and Diana Valencia Imagine yourself gazing at the sky on a summer night. You look in the direction of a particular star that, you have heard, has a special planet orbiting around it. Although you cannot actually see the planet—you can barely see the star itself—you know it is several times larger than Earth and, like Earth, is made mostly of rock. Quakes sometimes shake its surface, much of which is covered by oceans. Its atmosphere is not too different from the one we breathe, and its sky is swept by frequent storms and often darkened by the ash of volcanoes. But most of all, you know that scientists think it could harbor life—and that they plan to seek evidence for it. This scenario could become reality within the next decade. Initially, many of the extrasolar planets found were giants more similar to Jupiter. Now, using information from NASA’s Kepler probe, astronomers are discovering some that may not be too different from Earth. Of course, these worlds are light-years away, so even our most advanced instruments cannot actually see the details of their surfaces—the mountains, the clouds, the volcanoes—and perhaps never will. Usually all our telescopes can do is detect indirect signs of a planet’s presence and help us estimate its mass and how wide its orbit is. In some cases, they can also give information about a planet’s diameter and perhaps a few other details. In the case of the giant exoplanets, these details may include estimates about the atmospheric composition and wind dynamics. That is a far cry from being able to measure anything specific about geology, chemistry or other features. Yet from those few numbers, researchers can deduce surprisingly complex portraits of the faroff planets, using theoretical modeling, computer simulations and even laboratory experiments, combined with established knowledge of Earth and other planets of the solar system. In our research, for example, we have modeled planets with a composition similar to Earth’s. We found that such planets, even when they are substantially more massive than our world, should be geophysically active and have atmospheres and climates that might be friendly to life. In fact, we have learned that Earth’s mass may be at the lower extreme of the range needed for a planet to be habitable. In other words, had Earth been any smaller, it might have turned out to be as lifeless as Mars and Venus seem to be. The First Super-Earths The dream of finding planets that could potentially harbor life was what first entwined the careers of this article’s authors. The more senior of us (Sasselov) entered the field somewhat serendipitously a decade ago. The first extrasolar planets had been discovered in the mid-1990s, mostly using the “wobble” method, which detects the presence of a planet by its gravitational effects on its star; the body’s gravity tugs on the host star, accelerating it in alternating directions, something that can be detected as a shift in the spectrum of light received from the star. Initially some skeptical scientists wondered whether the wobbles could be caused by a star’s physics rather than by orbiting planets. That was how Sasselov—an astrophysicist and thus an expert on stars, not planets—got involved: his specialty was stars that display periodic changes in the way they shine. He helped to settle the wobble issue: the wobble was really caused by planets. Astrophysicists had a powerful tool for hunting exoplanets. Sasselov then joined a group of scientists who were proposing to build the Kepler space observatory to look for exoplanets. The probe eventually went into orbit in 2009. It is designed to detect planets by tracking small dips in a star’s brightness, usually lasting a few hours; if such dips happen at regular intervals, they signify that a planet is in orbit about the star, periodically passing in front of it. The telescope is trained at one particular patch of sky near the constellation Cygnus. Its wide-angle digital camera monitored about 150,000 stars for three years straight. Despite a technical problem in May of 2013 that effectively ended it's planet-hunting mission, Kepler has found hundreds of new planets, some as small as Earth.

How To Spot a Planet Compared with the stars they orbit, planets are very faint sources of light. Consequently, only a handful of extrasolar planets, all very large and bright, have been “seen” directly—that is, resolved as dots separate from their stars. In some cases, astronomers have detected a planet’s colors mixed in with the glare of the parent star. In most other cases, astronomers have found planets only indirectly, usually by applying the “wobble” or “transit” techniques. Wobble method During a planet’s orbit, its gravity pulls on the parent star. By analyzing the spectrum of light from the star, astronomers can measure changes in the star’s relative velocity with respect to Earth as small as one meter per second or less. Periodic variations reveal the presence of the planet.

Transit method If a planet’s orbit crosses the line of sight between its parent star and Earth, it will slightly dim the light received from the star, just as a partial lunar eclipse dims the sun. A Jupiter-size planet dims its star by about one percent; for an Earth-size one, the dimming is about 0.01 percent—a change that is within the sensitivity of the new Kepler space telescope

Illustration by Peter and Maria Hoey

Early in the planning of the mission, Sasselov realized that although Kepler would produce a wealth of information, scientists would not necessarily know what to make of it all. To his surprise, he learned, for instance, that no one had ever tried to model the geologic processes of a large Earth-like planet. So he began a collaboration with Richard O’Connell, a Harvard University expert on Earth’s interior dynamics. At that time, the other of us (Valencia) had started work on her Ph.D. in geophysics at Harvard, intending to focus on seismology, and was taking a geodynamics class being taught by O’Connell. Following a conversation he had with Sasselov, O’Connell asked his class to ponder how the size of Earth would change if it had more mass. How much would the additional gravity compact its innards? The question grabbed Valencia and changed the course of her research career. In our solar system, Earth is the largest of the rocky, or terrestrial, planets. So scientists were not accustomed to thinking of planets with a similar composition but many times the mass— super-Earths, for lack of a better word. The field was so new that when in 2004 our collaboration submitted its first paper on super-Earths for publication, it took the journal editors nearly a year to find scientists with the right expertise to referee it. In fact, early on many planetary scientists were puzzled by our choice of research topic. The only exoplanets discovered until then were Jupiter-class gas giants, not super-Earths. Why would anyone want to study planets that may not exist? Only months later, in 2005, our efforts were vindicated. Using the wobble method, Eugenio Rivera of the University of California, Santa Cruz, and his collaborators discovered a planet orbiting the star Gliese 876, in the constellation Aquarius. It was the first known super-Earth. We know that the planet, named GJ 876d, orbits its sun in just two days and that its mass is roughly 7.5 times that of Earth. But that is about all we can say about it. In particular, we have no way to find out GJ 876d’s mean density (which is mass divided by volume) and thus to guess its composition, because we cannot measure its size. An orbital transit, however, can reveal size: the extent to which a planet dims the

light of the parent star tells you the planet’s diameter. If you also measure the wobble, then you have both mass and diameter, and hence you can calculate mean density. If the density is high, like that of rock, your planet could be a rocky one. The transit method was how, in early 2009, astronomers discovered the first transiting super-Earth, CoRoT-7b, using France’s CoRoT space telescope, a smaller predecessor of Kepler. This planet is so dense it is definitely made of rock. It orbits so close to its star—its year lasts less than one Earth-day—that its dayside surface must be permanently molten. (Planets in tight orbits become tidally locked to their stars, so that they always show the same face to it, just like our moon does to Earth.) Hardly 10 months later a ground-based project led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics discovered a second transiting super-Earth. Dubbed GJ1214b, it is unusual in that it has a density closer to that of water than to that of rock, suggesting that it must have a thick envelope of gas. Thus, neither planet is anything like ours. We are looking for habitable, Earth-like worlds but seem to encounter monsters. Other oddities are likely to show up as well. For example, around very carbon-rich stars, solid planets would not consist primarily of silicon-oxygen compounds, as is the case of our solar system’s terrestrial planets, but of silicon bound to carbon. This would be quite a different kind of planet, with an interior made largely of diamond as a result of the compression of carbon. But because most solar systems, including ours, have similar compositions, researchers expect that the makeup of most super-Earths will be close to that of Earth—mostly silicon bound to oxygen and magnesium, plus iron and smaller amounts of other elements—often with the addition of vast amounts of water. Soon we will be discovering many such planets, so it is worthwhile to try to learn more about them, beginning with the physics of their interiors. Journey to the Center of a Super-Earth Two main categories of super-Earths should exist, depending on where in their solar systems the planets formed. Those that formed far enough from the star would have swept up large quantities of primordial ice particles that were orbiting the new star, and water would end up making up a much larger share of the planets’ mass than it does in the terrestrial planets of the solar system. On the other hand, planets that formed closer to their stars, where it was too hot for ice to exist, would have ended up relatively dry, like Earth and its fellow terrestrial planets in our solar system. A rocky planet would start out as a hot, molten mix of material and would immediately begin to cool down by radiating heat into space. Iron-and silicate-based crystals would form in the solidifying magma. Depending on the amount of oxygen, some of the iron would not be incorporated into minerals. This iron would remain in liquid form and, being denser, would sink to the center. Just as with Earth, then, the planet would assume an onionlike structure, with an iron core and a predominantly silicate mantle. A difference would arise in the cores of larger planets compared with those of Earth-size ones. Inside Earth, over billions of years the core has cooled enough so that the inner part of the core has solidified, whereas the outer core is still liquid, so that it churns in convective currents. The convection of the outer core is believed to be the engine that creates the geomagnetic field. But at the pressures that exist in a large planet’s core, iron can solidify even at temperatures as high as 10,000 kelvins, according to recent theoretical calculations. These high temperatures are probably exceeded only when the planets are very young. But a little cooling would be sufficient for the cores of super-Earths to solidify. Thus, a typical super-Earth may have a completely solid iron core and no global magnetic field. On Earth the field helps to protect us from the noxious effects of solar wind and cosmic rays, especially on land. But we do not know for sure whether it is essential for habitability. A water-rich planet would develop an even less familiar feature. A thick water layer—a single ocean— would envelop the planet. And something bizarre would happen in the ocean’s depths. Water turns into ice when cooled but also when compressed. Thus, on top of the silicate mantle another solid mantle would form, made of white-hot glowing ice. This would not be ordinary ice but rather the crystal structures named ice VII, ice X and ice XI, which so far have been observed only in laboratory experiments. Whether or not it is rich in water, a super-Earth, being more massive, compresses its interior to unimaginable pressures. A more massive planet will thus be denser than a less massive one of the same composition. In such extreme conditions, hard, rocky materials get even harder than those inside our planet, perhaps harder than diamond. How does Earth-like material behave under these very high pressures? On this front, too, researchers are using theoretical models and experiments to understand super-Earths better. For example, in recent years scientists have discovered a new structural arrangement, or phase, of material on Earth, called postperovskite. Although it constitutes only a small portion of Earth’s mantle, it would make up most of the mantle of super-Earths. Theory suggests that there could be an even denser phase, but experiments have yet to confirm its existence.

Once we have an idea of the structure of a planet and of what materials make up those layers, we are only half done. The next step is to understand the dynamics of that structure—or lack thereof. In other words, to figure out whether the planet is geologically restless, like Earth, or nearly still and frozen, like Mars. On Earth, mantle convection is the engine of most geologic processes. Below the plates that make up the surface of Earth, the mantle churns as it transports its internal heat toward the surface and then sinks back after it cools, similar to the convection in a boiling pot of water. The heat is in part left over after the planet’s formation and in part comes from the decay of radioactive elements in the mantle. We expect rocky super-Earths to have a similar concentration of radioactive heat sources or at least of uranium and thorium, because these elements are uniformly distributed throughout the galaxy and also get easily incorporated into planets during formation. Hence, being bigger than our home planet and having, in absolute terms, more radioactive material, massive Earth analogues produce more internal heat, which would translate into a more vigorous mantle convection. Prime Real Estate The strong stirring has several consequences, which ultimately affect the planet’s habitability. A perhaps unexpected consequence is that larger planets should have thinner plates. Mantle convection manifests itself on the surface as plate tectonics. Plates move as the mantle churns underneath them. When two plates collide, one of them may slide under the other and then sink back into the mantle, in a process known as subduction. Plates start out very thin at mid-ocean ridges, where they form in part from melted mantle material that rises to the surface, and grow thicker with time as they cool and move toward the subduction zones. According to our models, convection on bigger planets gives rise to larger forces and churns faster. Thus, plates also move faster, so that they have less time to cool and thicken. Being thinner, the plates would be easier to deform, except that the stronger gravity puts more pressure on the faults, which makes them more resistant to sliding. The net effect is that the resistance of the faults is not very different among planets of different size. That plate tectonics seems easier to sustain on a super-Earth than on a smaller rocky planet is a good thing, because plate tectonics may be good for habitability. On Earth, geologic activity, and volcanism in particular, continually spews carbon dioxide and other gases into the atmosphere. CO2 reacts with calcium silicate, producing calcium carbonate and silicon dioxide, both of which are solid and eventually end up as sediment on the ocean floors. As oceanic crust subducts back into the mantle, it carries carbonrich sediment with it. Subduction thus replenishes the mantle with carbon, so that some of it eventually makes its way back into the atmosphere. This so-called carbon-silicate cycle acts as a thermostat to regulate the global surface temperature. On Earth this cycle has helped keep temperatures close to those of liquid water over billions of years. Similarly, plate tectonics recycles other minerals and gases that are important for life, including energy-rich chemicals, such as hydrogen sulfide, that may have fueled life before photosynthesis evolved. With a super-Earth’s more vigorous convection, the timescales of plate production and subduction become shorter, which makes the carbon-silicate cycle faster and more robust. In some respects, then, super-Earths could be even more hospitable to life than Earth-size planets. Moreover, their larger masses would help these planets keep their atmospheres and water from escaping into space. This is an issue particularly for planets that are closer to their stars than, say, Mars is to the sun. Comparing Earth with the theoretical models of super-Earths of different sizes, we find a rich diversity of stable Earth-like planetary conditions, but this is a family of planets that barely includes Earth. Being smaller, Earth is more vulnerable in many ways. And in our solar system, the smaller planets are geologically rather static. Venus seems marginally capable of moving its plates, but Mars became stagnant early in its history and now does not produce enough emissions to replace its thinning atmosphere. It seems that our planet is barely big enough to have escaped this fate. Still, it is unclear if plate tectonics is really essential for life to exist. Postcard Pictures What would the landscapes on a solid super-Earth look like? At first glance they might not seem too different from those on our planet—aside from signs of life, which may or may not be there. Geologic processes would give rise to continents, mountains, oceans and an atmosphere, with clouds and all. Yet tectonic plates would move up to 10 times faster than on Earth. Mountains would grow and erode at a faster rate, and, because of the stronger gravity, they would not rise as high. (Those mountains would contrast sharply with those of our smaller neighbor Mars, where Olympus Mons is the tallest mountain in the solar system, at 21 kilometers high.) The composition of the atmosphere might also be different because of higher volcanic activity and different rates at which atmospheric gases escape to space. The era of super-Earth planet exploration has only just begun. The next step after Kepler will be to study the atmospheres of those planets and see if we can find any signs of life. To accomplish that we

need to determine at least two things—what the planet is made of and what gases are abundant in its atmosphere, which is connected to the dynamics of the interior. By splitting the light from a planet into its rainbow of colors, scientists will be able to see in it the optical fingerprints of such molecules as water, carbon dioxide and methane. In a few years the successor to the Hubble Space Telescope, called the James Webb Space Telescope, should open its infrared eye and allow glimpses into the atmospheres of super-Earths. The new telescope will need targets to study—some of them will be selected from the best and nearest of the planets discovered by Kepler. With luck, all-sky ground-based searches and space missions being conceived as follow-ups to Kepler will discover a few transiting super-Earths that are very close to us and thus relatively easy to study. --Originally published: Scientific American, 303(2); 38-45 (August 2010).

Noisy Stars May Create Phantom Planets by Ron Cowen Planet hunters have dramatically improved their techniques in the two decades since first discovering worlds beyond our solar system, most of which were gas-giant scorchers. Now they are searching for small, Earth-size exoplanets, such as the one said to circle Alpha Centauri B, which made headlines. Yet optimism about finding such planets may be premature. The problem is that stars swarm with surface activity that can mask or mimic the signs of tiny exoplanets. The putative planet orbiting Alpha Centauri B may, in fact, only be a mirage of stellar jitter. Astronomers found the planet with a standard technique. Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA) and his colleagues monitored the star's light for periodic shifts in frequency, a sign that a planet's gravitational tug is causing the star to wobble. When Artie Hatzes, an astronomer at the Thüringian State Observatory in Tautenburg, Germany, reanalyzed the data with two different methods, he found conflicting results: one showed a wobble; the other found none at all. He described his work in the Astrophysical Journal. “If one analysis produces a planet and another doesn't, that's not robust,” he says. (To be fair, Dumusque and his team flagged significant uncertainties in their announcement in October 2012.) Alpha Centauri B's disputed world is not the first to come under close scrutiny. In 2010 an international team announced the discovery of a small planet around the star Gliese 581, smack-dab in the middle of the star's Goldilocks zone—the region where temperatures are just right for plentiful liquid water. Yet other researchers looking through their own data found no sign of the planet. Many other candidate detections are just as marginal, says David Latham, a planet-hunting veteran at the CfA, but remain unpublished. The growing catalogue of hazy claims suggests that researchers must gather more data and resist the pressure to publish Earthanalogue discoveries too early, Hatzes says. He knows from experience: he now suspects that a planet his team announced in 2009, a gas-giant world thought to circle the star 42 Draconis, might also be a noise-induced illusion. Going forward, Latham notes, planet hunters should focus on quieter stars and develop new models for sources of stellar jitter. Better spectrographs, such as the new HARPS-North on the island of La Palma in the Canary Islands, will help by reducing instrumental noise. Even so, Hatzes says, “at some point you're going to hit that wall, which is the noise level of the star.” --Originally published: Scientific American Online, July 17, 2013.

Water Worlds by Ron Cowen NASA’s Kepler spacecraft has discovered two planets that are the most similar in size to Earth ever found in a star’s habitable zone — the temperate region where water could exist as a liquid. The finding, reported in Science, demonstrated that Kepler is closing in on its goal of finding a true twin of Earth beyond the Solar System, said theorist Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who is a member of the Kepler discovery team. Both planets orbit the star Kepler-62, which is about two-thirds the size of the Sun and lies about 1,200 light years (368 parsecs) from the Solar System. The outermost planet from the star, Kepler-62f, has a diameter that is 41% larger than Earth’s and takes 267 days to circle its star. The inner planet, Kepler62e, has a diameter 61% larger than Earth’s and a shorter orbit of 122 days. Kepler detected the planets by recording the tiny decrease in starlight that occurs when either of them passes in front of their parent star. Astronomers used those measurements to calculate the planets’ relative size compared to that star. Worlds Apart In the Science paper, the Kepler team — led by principal investigator William Borucki of NASA — suggests that the planets are solid, but may be rocky or icy. But Sasselov believes that the two orbs are likely to be covered entirely by oceans, based on his own analysis published in an arXiv preprint and coauthored with colleagues at Harvard-Smithsonian and the Max Planck Institute for Astronomy in Heidelberg, Germany. They theorize that the two water worlds are either liquid all the way down to their core or have a solid surface just beneath a shallower ocean. The latter model would be more conducive to life as we know it on Earth, where a recycling of material and energy from hydrothermal vents can sustain organisms, Sasselov said But some recycling could also occur in a much deeper ocean, owing to deposits of methane and other volatiles trapped in a layer of high-pressure, underground ice that could later be released by convection into the liquid, he notes. Sasselov and two other collaborators described that model in a paper in the Astrophysical Journal. The planets’ properties “are all consistent with being habitable”, said Lindy Elkins-Tanton, who directs the department of terrestrial magnetism at the Carnegie Institution for Science in Washington DC. However, it is not guaranteed that the two planets have enough carbon dioxide, a greenhouse gas, to keep the planet warm enough so surface water can exist as a liquid as the researchers have assumed, she cautions. Jonathan Lunine, a planetary scientist at Cornell University in Ithaca, New York, said that it is impossible to know the composition of the two exoplanets because scientists have not been able to calculate their masses. That is because Kepler-62e and Kepler-62f exert too feeble a gravitational tug on their parent star to be detected by any existing telescope. The planets also lie too far away from Kepler-62 for astronomers to use its light to hunt for chemical elements in the planets' atmospheres. Nonetheless, “these are the closest yet to the Earth twin everyone wants to see,” Lunine said. --Originally published: Nature News, April 18, 2013.

Do Small Planets Need a Heavy Metal Star? by John Matson In planet formation, slow, steady and small wins the race. Astronomers using NASA's Kepler spacecraft have found that small planets such as Earth can form around all manner of stars, whereas massive gas giant planets like Jupiter tend to take shape around stars with large concentrations of heavy elements such as iron and oxygen. The researchers published their findings in Nature and announced the results at the semiannual meeting of the American Astronomical Society in June 2012. (Scientific American is part of Nature Publishing Group.) In the early years of exoplanetary science, beginning with the first discovery of a planet orbiting a sunlike star in 1995, the majority of known worlds outside of the solar system were giants much like Jupiter—and sometimes much more massive. Those heavyweights have the largest effects on their planetary systems and as such are the easiest to detect. Researchers noticed that the kinds of stars hosting giant planets tended to contain relatively high levels of so-called metals (an astronomer's term for any element heavier than hydrogen or helium.) The chemical fingerprints of those stars point back to the makeup of the ancient disks of dust and gas from which the planets congealed, hinting that, at least for large worlds, having lots of metals around encourages planets to form. "If there's a lot of stuff in the disk, then we have a higher chance of finding these hot Jupiter planets," said lead study author Lars Buchhave of the Niels Bohr Institute at the University of Copenhagen. The question was, do small planets—the Earth and Neptune analogues of the galaxy —follow the same trend? With the advent of modern planet-hunting instruments such as Kepler, a space telescope built to seek out Earth-size bodies, astronomers have finally gotten a peek at the smaller denizens of the planetary zoo. Buchhave and his colleagues took spectral measurements of 152 stars that Kepler has inspected and where the spacecraft has projected the presence of 226 planets in total, most of them smaller in diameter than Neptune and some as small as Earth. (The mission has identified more than 2,000 probable planets altogether, but only dozens have been confirmed with follow-up observations.) They found that the host stars of those diminutive worlds are a diverse bunch, spanning a wide range of metallicities. On average, the small planets orbit stars roughly as metal-rich as the sun, a star of fairly ordinary composition, whereas giant exoplanets tend to inhabit planetary systems more enriched in metals. That should not come as a total surprise, notes astronomer Andrew Howard of the University of California, Berkeley. After all, according to prevailing theoretical models, a giant planet acquires a solid core and then gathers gases and ices around that core to swell up to a Jupiter-like diameter. So that core must take shape before the gaseous disk dissipates under the intense radiation of the newly formed star. "To form a Jupiter, it's a race against the clock," Howard says. A metal-rich environment speeds the growth of the core, helping a gas giant take shape before it is too late. A smaller, rockier planet, on the other hand, is not as dependent on that ephemeral reservoir of gas; it can grow more gradually, even after the gas in the protoplanetary disk has evaporated. "In my opinion, it points unambiguously to the fact that the formation of gas giant planets is quite a constrained process," says astronomer Debra Fischer of Yale University. An interesting question to pursue now, she says, is how low stellar metallicity can go before planet formation shuts off entirely. The finding could bode well for Kepler's attempts and those of other exoplanet-finding campaigns, as small planets such as ours seem not to be picky about where they pop up. "Small planets could be widespread in our galaxy, simply because they don't need a special environment in which to form," Buchhave said. --Originally published: Scientific American Online, June 13, 2012.

Tiny Planets with Big Potential by John Matson A world called Kepler 37 b could easily blend in to the long and growing list of known extrasolar planets, given its nondescript name. But the new addition to the catalogue of more than 1,000 exoplanets stands out in at least one major respect—as of this publication, it is far smaller than any planet yet discovered outside of our solar system. In fact, it is just a shade larger than Earth’s moon. “What makes this very interesting is this is a planet smaller than anything we see in our own inner solar system,” says Thomas Barclay, a research scientist at the NASA Ames Research Center in Moffett Field, Calif. Barclay is lead author of a study published online February 20, 2013 in Nature announcing the discovery of Kepler 37 b and two slightly larger worlds in the same planetary system. (Scientific American is part of Nature Publishing Group.) The researchers used NASA’s Kepler space telescope to identify the three planets orbiting Kepler 37, a star some 200 light-years away that is somewhat smaller than the sun. The spacecraft monitors more than 150,000 stars in the Milky Way for occasional winks, or dips in brightness, that might be caused by a planet passing in front of its star, from the probe’s perspective. The Kepler mission has already discovered more than 100 new planets since its launch in 2009 and has identified thousands of additional candidates that await confirmation. Planets smaller than Earth block relatively small amounts of starlight, which limits astronomers’ ability to detect them with Kepler. But the star Kepler 37 is bright and relatively free of disturbances, such as starspots, that can obscure a faint planetary signal. By observing the planet Kepler 37 b as it transited, or passed in front of its star, more than 50 times, Barclay and his colleague drew out a subtle but recurrent pattern. Every 13 days or so the star dimmed by a tiny fraction—just 0.002 percent—as the tiny planet passed across the star’s face. The exoplanet that previously held the record on the tiny end of the size spectrum—a Mars-size object known as Kepler 42 d—is nearly twice the diameter of Kepler 37 b. The newfound body is just 80 percent Mercury’s diameter and 30 percent that of Earth. (Kepler measures the diameters and orbital properties of exoplanets but is usually unable to pinpoint their masses.) All three of the exoplanets found by Barclay and his colleagues, in fact, will rank among the smallest known: Kepler 37 c is 74 percent the diameter of Earth, and Kepler 37 d is roughly twice our planet’s diameter. Orbiting its star at one tenth the distance between Earth and the sun, tiny Kepler 37 b must be extremely hot. “Any water on the surface would disappear very quickly,” Barclay says. “There is almost no chance of an atmosphere or liquid on the surface.” The researchers predict that Kepler 37 b would be a barren, rocky world similar to Mercury. The larger worlds in the planetary system orbit somewhat farther out but would still suffer scorching heat from the star. All three planets keep closer to the star Kepler 37 than any planet orbits the sun. “It just shows that Kepler has just an extraordinary ability to see a wide diversity of planetary architectures,” says Greg Laughlin, a professor of astronomy and astrophysics at the University of California, Santa Cruz, who did not contribute to the new study. Kepler was built to search for exo-Earths—rocky planets in cooler orbits than the uninhabitable worlds of the Kepler 37 system. But in the meantime it has found numerous planetary systems that little resemble ours. “What Kepler is also showing, and this is a side dividend to the main mission, is that the galactic planetary census is a lot different than we had believed from looking at our own planetary system,” Laughlin says. “Our solar system just contains nothing whatsoever inside Mercury’s orbit. But it turns out that the average planetary system has a lot going on in the inner region.” There is one catch in Kepler’s search for worlds comparable with or smaller than Earth: Whereas giant Jupiter-size planets often exert a gravitational tug on their host stars that is detectable with Earth-based telescope spectrographs, smaller exoplanet discoveries have proved difficult to confirm with observations other than Kepler’s. So researchers have turned to statistical arguments instead to quantify the probability of a false positive—for instance, a pair of undetected binary stars whose regular eclipses

mimic a planetary signal. Barclay and his colleagues used computer modeling to identify potential false positives and then rule them out with additional observations from the ground. In the end, based on population estimates of exoplanets, binary stars and other astronomical objects, the researchers calculated the probability that the signal collected from Kepler represents a true planet. “In this case, with the innermost planet we are confident that it is a true planet orbiting the target star with a confidence of 99.95 percent,” Barclay says. “So we’re very confident that this is what we think it is.” --Originally published: Scientific American Online, February 20, 2013.

Do Three Super-Earths Orbit a Nearby Star? by Lee Billings From its position 22 light-years away in the constellation of Scorpius, the red M dwarf star Gliese 667 C doesn’t look like much. Its dim light is lost to the naked eye, washed out by two brighter companion stars. Yet this tiny, exceedingly average star could play a crucial role in establishing that small, potentially Earth-like planets are common throughout our galaxy. Researchers have announced that seven planets orbit that star—and, if their mathematical analyses are correct, three of them could be habitable. Previous surveys of Gliese 667 C had turned up two planets, including a potentially rocky "super-Earth" orbiting in the star's habitable zone, the region in which a planet might possess liquid water on its surface. Dubbed Gliese 667 C c, this world could be a "Goldilocks" planet like Earth, with a "just right" temperature neither too hot nor too cold for life as we know it. Now, after years of hints that more planets lurk in the data, an international team of astronomers led by Guillem Anglada-Escudé of the University of Göttingen in Germany and Mikko Tuomi of the University of Hertfordshire in England have announced their discovery of between three and five additional worlds around the star. Two of these additional bodies could be super-Earths orbiting in the habitable zone, raising the possibility that the star harbors three Goldilocks worlds. The journal Astronomy & Astrophysics published their study in 2013. Unlike our own solar system, with its spacious arrangement of small inner planets and large outer worlds orbiting a G-type yellow dwarf star, all the purported planets around Gliese 667 C are of intermediate mass, somewhere between that of Earth and Uranus. Stranger still, all but one are huddled interior to the orbit of Mercury, the closest planet to our sun. Such a system is said to be "dynamically packed," for its planets are jammed cheek by jowl in every available island of stability around the star. In recent years, as torrents of data streamed in from major planet surveys such as NASA's Kepler mission, astronomers were shocked to discover that such compact systems seem to be the default planetary arrangement in our galaxy. "We knew from Kepler that dynamically packed systems were prevalent around Sun-type stars, and now we have another around an M dwarf," Anglada says. The result suggests that many more compact systems—and potentially habitable planets—reside around nearby M dwarfs than previously thought. Finding those planets has not been easy, because small, potentially habitable worlds are usually barely discernible against a noisy background of stellar jitter. Unlike most of the more than 3,000 likely planets found by NASA's Kepler mission, which were discovered by their transits—the shadows they cast toward Earth when they happen to cross the faces of their stars—Gliese 667 C's planets were detected via a more indirect technique, by the back and forth wobble their bulk induces on the star as they whip to and fro in their orbits. For the Gliese 667 C system, each planet's orbital tug only shifts the entire star's position by about one meter per second —walking speed—yet the star's seething surface swarms with stellar activity that at any moment can swamp this faint signal. Discerning meter-per-second planetary wobbles across the light-years is a bit like listening for faint music emerging from washes of static on a poorly tuned radio. A lone planet's signal is like the sound of a single, steadily strummed guitar string, pure and repeated, almost immediately recognizable. Multiple planets, however, are much tougher to decipher: their overlapping wobbles are more akin to an out-oftune orchestra playing all at once; only by listening for long periods can you hope to decipher any signals from the noise. Early Exoplanet Hints The first clear hints of a large multiplanet system around Gliese 667 C emerged last year, through the work of Philip Gregory, an astronomer at the University of British Columbia in Vancouver. Gregory was analyzing public data from the European Southern Observatory's HARPS spectrograph, a world-class planet-hunting instrument in La Silla, Chile. He noticed several previously unreported, potentially planetary wobbles, including one that looked like a 2.5-Earth-mass planet in a 39-day orbit—that is, another rocky planet within the star's habitable zone in addition to the already discovered—Gliese 667 C c. Gregory wrote up his findings and submitted them to a journal, but he stopped just short of claiming that he had found new planets.

As Gregory wrote his paper, Anglada and his colleagues were also glimpsing the wobbly evidence of Gliese 667 C's wealth of worlds by combining the HARPS measurements with data from two other telescopes. They analyzed the combined data using two independent and distinct statistical methods. Both methods strongly supported the presence of the two previously announced planets as well as three "new" planets with orbits and masses essentially identical to what Gregory reported in 2012. One of the two methods also found tentative evidence for two additional small planets, one in a hot 17-day orbit and another in a frigid 256-day orbit. Several rounds of further simulations only increased their confidence the planets were real. Gregory praises the group's work as "a very significant step forward," and notes that although his paper "served to draw attention to the possibility of multiple planets in the habitable zone," the Anglada study contains "more definitive results." Wobbly Statistics Even so, doubts remain. According to Xavier Bonfils, the leader of the HARPS team's M-dwarf survey, Anglada's team took various statistical "shortcuts" that made their analyses easier to perform but less robust. A key point, Bonfils contends, is that the team assumed Gliese 667C's planets reside in nearcircular orbits, a notion supported more by dynamical simulations than actual data from the star. More elongated "eccentric" orbits would make such a close-packed system unstable. So, if the new planets are real, most must trace low-eccentricity orbits. Or perhaps there are simply fewer planets than claimed. "The analysis they propose seems mathematically correct, but it is a less conservative approach than what is usually done," Bonfils says, hastening to add that he hopes the planets prove to be genuine. "The signals are there, but that doesn't mean they are all planets.” Hundreds or thousands of additional costly, time-consuming measurements could be required to confirm the planetary provenance of Gliese 667 C's meter-scale wobbles, Bonfils says. This is not the first time Anglada, Tuomi and their collaborators have made similar claims, notes Sara Seager, a prominent exoplanet researcher at the Massachusetts Institute of Technology who was not involved with the group's study. In recent years the group has also announced small planets—including potentially habitable ones—around a few other stars, although many of those claims remain unconfirmed. The issue, Seager explains, is not necessarily that these planets aren't real, but rather that the statistical techniques used to reveal their presence are so abstruse that there are few clear precedents and outside experts to properly judge the claims. "They use highly sophisticated, specialized methods to pull very weak signals out of noisy data," Seager says. "Only a handful of other teams in the world can reproduce this kind of data analysis." If the Anglada results hold up, though, they could help reshape the future of planet-hunting. Multiplestar systems and red dwarfs like Gliese 667 are the most common types in the Milky Way, and if most of them harbor packed planetary systems, the closest habitable worlds outside the solar system could be quite nearby indeed. “The clichéd response to this is that 'extraordinary claims require extraordinary proof,'" says Greg Laughlin, another exoplanet expert at the University of California, Santa Cruz, who was unaffiliated with the study. "But you can't really consider this to be an extraordinary claim, because even though it's not at all like our own solar system, what's being proposed is an extraordinarily ordinary planetary arrangement.” He adds that the Kepler mission “has clearly indicated that systems like Gliese 667 C, rather than systems like ours, are the default mode of planet formation in the galaxy." --Originally published: Scientific American Online, June 26, 2013

Better Than Earth by René Heller DO WE INHABIT THE BEST OF ALL POSSIBLE WORLDS? German mathematician Gottfried Leibniz thought so, writing in 1710 that our planet, warts and all, must be the most optimal one imaginable. Leibniz’s idea was roundly scorned as unscientific wishful thinking, most notably by French author Voltaire in his magnum opus, Candide. Yet Leibniz might find sympathy from at least one group of scientists—the astronomers who have for decades treated Earth as a golden standard as they search for worlds beyond our own solar system. Because earthlings still know of just one living world—our own—it makes some sense to use Earth as a template in the search for life elsewhere, such as in the most Earth-like regions of Mars or Jupiter’s watery moon Europa. Now, however, discoveries of potentially habitable planets orbiting stars other than our sun— exoplanets, that is—are challenging that geocentric approach. Over the past two decades astronomers have found more than 1,800 exoplanets, and statistics suggest that our galaxy harbors at least 100 billion more. Of the worlds found to date, few closely resemble Earth. Instead they exhibit a truly enormous diversity, varying immensely in their orbits, sizes and compositions and circling a wide variety of stars, including ones significantly smaller and fainter than our sun. Diverse features of these exoplanets suggest to me and to others that Earth may not be anywhere close to the pinnacle of habitability. In fact, some exoplanets, quite different from our own, could have much higher chances of forming and maintaining stable biospheres. These “superhabitable worlds” may be the optimal targets in the search for extraterrestrial, extrasolar life. AN IMPERFECT PLANET Of course, our planet does possess a number of properties that, at first glance, seem ideal for life. Earth revolves around a sedate, middle-aged star that has shone steadily for billions of years, giving life plenty of time to arise and evolve. It has oceans of life-giving water, largely because it orbits within the sun’s “habitable zone,” a slender region where our star’s light is neither too intense nor too weak. Inward of the zone, a planet’s water would boil into steam; outward of the area, it would freeze into ice. Earth also has a life-friendly size: big enough to hold on to a substantial atmosphere with its gravitational field but small enough to ensure gravity does not pull a smothering, opaque shroud of gas over the planet. Earth’s size and its rocky composition also give rise to other boosters of habitability, such as climate-regulating plate tectonics and a magnetic field that protects the biosphere from harmful cosmic radiation. Yet the more closely we scientists study our own planet’s habitability, the less ideal our world appears to be. These days habitability varies widely across Earth, so that large portions of its surface are relatively devoid of life—think of arid deserts, the nutrient-poor open ocean and frigid polar regions. Earth’s habitability also varies over time. Consider, for instance, that during much of the Carboniferous period, from roughly 350 million to 300 million years ago, the planet’s atmosphere was warmer, wetter and far more oxygen-rich than it is now. Crustaceans, fish and reef-building corals flourished in the seas, great forests blanketed the continents, and insects and other terrestrial creatures grew to gigantic sizes. The Carboniferous Earth may have supported significantly more biomass than our present-day planet, meaning that Earth today could be considered less habitable than it was at times in its ancient past. Further, we know that Earth will become far less life-friendly in the future. About five billion years from now, our sun will have largely exhausted its hydrogen fuel and begun fusing more energetic helium in its core, causing it to swell to become a “red giant” star that will scorch Earth to a cinder. Long before that, however, life on Earth should already have come to an end. As the sun burns through its hydrogen, the temperature at its core will gradually rise, causing our star’s total luminosity to slowly increase, brightening by about 10 percent every billion years. Such change means that the sun’s habitable zone is not static but dynamic, so that over time, as it sweeps farther out from our brightening star, it will eventually leave Earth behind. To make matters worse, recent calculations suggest that Earth is not in the middle of the habitable zone but rather on the zone’s inner cusp, already teetering close to the edge of overheating. As Stars Age, They Turn Up the Heat on Habitable Planets

On human timescales, a star’s habitable zone appears to be static. But because stars brighten as they age, over eons the zone sweeps outward, eventually leaving living worlds behind. Earth is poised near the inner edge of the sun’s habitable zone and will become too hot to harbor liquid water in some 1.75 billion years. Smaller stars shine dimmer and longer than the sun, scarcely budging their habitable zones over tens of billions of years, potentially extending their planets’ lives.

Illustration by Jen Christiansen

Consequently, within about half a billion years our sun will be bright enough to give Earth a feverish climate that will threaten the survival of complex multicellular life. By some 1.75 billion years from now, the steadily brightening star will make our world hot enough for the oceans to evaporate, exterminating any simple life lingering on the surface. In fact, Earth is well past its habitable prime, and the biosphere is fast approaching its denouement. All things considered, it seems reasonable to say our planet is at present only marginally habitable. SEEKING A SUPERHABITABLE WORLD In 2012 I first began thinking about what worlds more suitable to life might look like while I was researching the possible habitability of massive moons orbiting gas-giant planets. In our solar system, the biggest moon is Jupiter’s Ganymede, which has a mass only 2.5 percent that of Earth—too small to easily hang on to an Earth-like atmosphere. But I realized that there are plausible ways for moons approaching the mass of Earth to form in other planetary systems, potentially around giant planets within their stars’ habitable zones, where such moons could have atmospheres similar to our own planet. Such massive “exomoons” could be superhabitable because they offer a rich diversity of energy sources to a potential biosphere. Unlike life on Earth, which is powered primarily by the sun’s light, the biosphere of a superhabitable exomoon might also draw energy from the reflected light and emitted heat of its nearby giant planet or even from the giant planet’s gravitational field. As a moon orbits around a giant planet, tidal forces can cause its crust to flex back and forth, creating friction that heats the moon from within. This phenomenon of tidal heating is probably what creates the subsurface oceans thought to exist on Jupiter’s Europa and Saturn’s moon Enceladus. That said, this energetic diversity would be a doubleedged sword for a massive exomoon because slight imbalances among the overlapping energy sources could easily tip a world into an uninhabitable state. No exomoons, habitable or otherwise, have yet been detected with certainty, although some may sooner or later be revealed by archival data from observatories such as nasa’s Kepler space telescope. For the time being, the existence and possible habitability of these objects remain quite speculative. Superhabitable planets, on the other hand, may already exist within our catalogue of confirmed and candidate exoplanets. The first exoplanets found in the mid-1990s were all gas giants similar in mass to Jupiter and orbiting far too close to their stars to harbor any life. Yet as planet-hunting techniques have improved over time, astronomers have begun finding progressively smaller planets in wider, more clement orbits. Most of the planets discovered over the past few years are so-called superEarths, planets larger than Earth by up to 10 Earth masses, with radii between that of Earth and Neptune. These planets

have proved to be extremely common around other stars, yet we have nothing like them orbiting the sun, making our own solar system appear to be a somewhat atypical outlier. Many of the bigger, more massive super-Earths have radii suggestive of thick, puffy atmospheres, making them more likely to be “mini Neptunes” than super-sized versions of Earth. But some of the smaller ones, worlds perhaps up to twice the size of Earth, probably do have Earth-like compositions of iron and rock and could have abundant liquid water on their surfaces if they orbit within their stars’ habitable zones. A number of the potentially rocky super-Earths, we now know, orbit stars called M dwarfs and K dwarfs, which are smaller, dimmer and much longer-lived than our sun. In part because of the extended lives of their diminutive stars, these super-sized Earths are currently the most compelling candidates for superhabitable worlds, as I have shown in recent modeling work with my collaborator John Armstrong, a physicist at Weber State University. THE BENEFITS OF LONGEVITY We began our work with the understanding that a truly long-lived host star is the most fundamental ingredient for superhabitability; after all, a planetary biosphere is unlikely to survive its sun’s demise. Our sun is 4.6 billion years old, approximately halfway through its estimated 10-billion-year lifetime. If it were slightly smaller, however, it would be a much longer-lived K dwarf star. K dwarfs have less total nuclear fuel to burn than more massive stars, but they use their fuel more efficiently, increasing their longevity. The middle-aged K dwarfs we observe today are billions of years older than the sun and will still be shining billions of years after our star has expired. Any potential biospheres on their planets would have much more time in which to evolve and diversify. A K dwarf ’s light would appear somewhat ruddier than the sun’s, as it would be shifted more toward the infrared, but its spectral range could nonetheless support photosynthesis on a planet’s surface. M dwarf stars are smaller and more parsimonious still and can steadily shine for hundreds of billions of years, but they shine so dimly that their habitable zones are very close in, potentially subjecting planets there to powerful stellar flares and other dangerous effects. Being longer-lived than our sun yet not treacherously dim, K dwarfs appear to reside in the sweet spot of stellar superhabitability. Today some of these long-living stars may harbor potentially rocky super-Earths that are already several billion years older than our own solar system. Life could have had its genesis in these planetary systems long before our sun was born, flourishing and evolving for billions of years before even the first biomolecule emerged from the primordial soup on the young Earth. I am particularly fascinated by the possibility that a biosphere on these ancient worlds might be able to modify its global environment to further enhance habitability, as life on Earth has done. One prominent example is the Great Oxygenation Event of about 2.4 billion years ago, when substantial amounts of oxygen first began to accumulate in Earth’s atmosphere. The oxygen probably came from oceanic algae and eventually led to the evolution of more energy-intensive metabolisms, allowing creatures to have bigger, more durable and active bodies. This advancement was a crucial step toward life’s gradual emergence from Earth’s oceans to colonize the continents. If alien biospheres exhibit similar trends toward environmental enhancement, we might expect planets around long-lived stars to become somewhat more habitable as they age. To be superhabitable, exoplanets around small, long-lived stars would need to be more massive than Earth. That extra bulk would forestall two disasters most likely to befall rocky planets as they age. If our own Earth were located in the habitable zone of a small K dwarf, the planet’s interior would have grown cold long before the star expired, inhibiting habitability. For example, a planet’s internal heat drives volcanic eruptions and plate tectonics, processes that replenish and recycle atmospheric levels of the greenhouse gas carbon dioxide. Without those processes, a planet’s atmospheric CO2 would steadily decrease as rainfall washed the gas out of the air and into rocks. Ultimately the CO2-dependent global greenhouse effect would grind to a halt, increasing the likelihood that an Earth-like planet would enter an uninhabitable “snowball” state in which all of its surface water freezes. Beyond the potential breakdown of a planet-warming greenhouse effect, the cooling interior of an aging rocky world could also cause the collapse of any protective planetary magnetic field. Earth is shielded by a magnetic field generated by a spinning, convecting core of molten iron, which acts like a dynamo. The core remains liquefied because of leftover heat from the planet’s formation, as well as from the decay of radioactive isotopes. Once a rocky planet’s internal heat reservoir became exhausted, its core would solidify, the dynamo would cease, and the magnetic shield would fall, allowing cosmic radiation and stellar flares to erode the upper atmosphere and impinge on the surface. Consequently, old Earth-like planets would be expected to lose substantial portions of their atmospheres to space, and higher levels of damaging radiation could harm surface life. Rocky super-Earths as much as twice our planet’s size should age more gracefully than Earth, retaining their inner heat for much longer because of their significantly greater bulks. But planets larger than about three to five Earth masses may actually be too bulky for plate tectonics because the pressures and viscosities in their mantles become so high that they inhibit the required outward flow of heat. A rocky planet only two times the mass of Earth should still possess plate tectonics and could sustain its geologic

cycles and magnetic field for several billion years longer than Earth could. Such a planet would also be about 25 percent larger in diameter than Earth, giving any organisms about 56 percent more surface area than our world on which to live. LIFE ON A SUPERHABITABLE SUPER-EARTH What would a superhabitable planet look like? Higher surface gravity would tend to give a middling super-Earth planet a slightly more substantial atmosphere than Earth’s, and its mountains would erode at a faster rate. In other words, such a planet would have relatively thicker air and a flatter surface. If oceans were present, the flattened planetary landscape could cause the water to pool in large numbers of shallow seas dotted with island chains rather than in great abyssal basins broken up by a few very large continents. Just as biodiversity in Earth’s oceans is richest in shallow waters near coastlines, such an “archipelago world” might be enormously advantageous to life. Evolution might also proceed more quickly in isolated island ecosystems, potentially boosting biodiversity. Of course, lacking large continents, an archipelago world would potentially offer less total area than a continental world for land-based life, which might reduce overall habitability. But not necessarily, especially given that a continent’s central regions could easily become a barren desert as a result of being far from temperate, humid ocean air. Furthermore, a planet’s habitable surface area can be dramatically influenced by the orientation of its spin axis with respect to its orbital plane around its star. Earth, as an example, has a spin-orbit axial tilt of about 23.4 degrees, giving rise to the seasons and smoothing out what would otherwise be extreme temperature differences between the warmer equatorial and colder polar regions. Compared with Earth, an archipelago world with a favorable spin-orbit alignment could have a warm equator as well as warm, ice-free poles and, by virtue of its larger size and larger surface area on its globe, would potentially boast even more life-suitable land than if it had large continents. Taken together, all these thoughts about the features important to habitability suggest that superhabitable worlds are slightly larger than Earth and have host stars somewhat smaller and dimmer than the sun. If correct, this conclusion is tremendously exciting for astronomers because across interstellar distances super-Earths orbiting small stars are much easier to detect and study than twins of our own Earth-sun system. So far statistics from exoplanet surveys suggest that super-Earths around small stars are substantially more abundant throughout our galaxy than Earth-sun analogues. Astronomers seem to have many more tantalizing places to hunt for life than previously believed. One of Kepler’s prize finds, the planet Kepler-186f, comes to mind. Announced in April 2014, this world is 11 percent larger in diameter than Earth and probably rocky, orbiting in the habitable zone of its M dwarf star. It is probably several billion years old, perhaps even older than Earth. It is about 500 lightyears away, placing it beyond the reach of current and near-future observations that could better constrain predictions of its habitability, but for all we know, it could be a superhabitable archipelago world. Closer superhabitable candidates orbiting nearby small stars could soon be discovered by various projects, most notably the European Space Agency’s PLATO mission, slated to launch by 2024. Such nearby systems could become prime targets for the James Webb Space Telescope, an observatory scheduled to launch in 2018, which will seek signs of life within the atmospheres of a small number of potentially superhabitable worlds. With considerable luck, we may all soon be able to point to a place in the sky where a more perfect world exists. --Originally published: Scientific American 312(1); 32-39 (January 2015).

SECTION 3 Oddballs: Stranger than Fiction

Bones of Giants by Charles Choi The first rocky worlds astronomers detect circling other stars could resemble Inferno more than Earth. The existence of such lava-coated planets, which may prove commonplace, will force a reconsideration of theories about planetary formation. Since 1991 observers have discovered over 1000 exoplanets—worlds outside our solar system. Most are gas giants, Jupiter-mass or larger, and of those roughly a third are “hot Jupiters” surprisingly near their stars, all closer than Mercury is to our sun. Some hot Jupiters live just too close to their stars for comfort. In 2003, the Hubble Space Telescope provided the first evidence of an evaporating atmosphere, from an exoplanet, HD 209458b, that circles its star at a distance of less than 1⁄ 20 the distance between the sun and Earth. The star roasts the exoplanet and rips at it with its gravity. The result: the exoplanet blows away at least 10,000 tons of gas a second, which streaks off in a vast plume 200,000 kilometers long. Astronomer Alfred Vidal-Madjar of the Institute of Astrophysics in Paris and his team dubbed the world “Osiris,” after the Egyptian god torn to pieces by his evil brother Set. In contemplating the fate of Osiris, Vidal-Madjar and his team calculated how long it and other giants might live. At roughly 220 times Earth’s mass, Osiris boasts a gravitational pull strong enough to hold its atmosphere until its star dies. But the researchers speculate the hellish rate of evaporation might completely scour all gas off smaller hot Jupiters or those closer to their stars than Osiris. This could lead to a new class of planets—a dead giant’s hard, bare heart. The astronomers named such worlds “chthonians,” after primeval Greek deities of the underworld. In findings that appeared in Astronomy and Astrophysics, astronomer Alain Lecavelier des Etangs of the Institute of Astrophysics and his co-workers figured that many exoplanets may one day become chthonians. Though remnants of far larger worlds, chthonians would still weigh in at roughly 10 to 15 times Earth’s mass and six to eight times Earth’s diameter. With searing temperatures of roughly 1,000 degrees Celsius at their surfaces, they would look “like lava planets,” Lecavelier des Etangs imagines. If chthonian exoplanets exist, “it is probable that they will be the first rocky planets to be detected around other stars,” Vidal-Madjar remarks. (Three planets, two about three to four times Earth’s mass and the third twice the mass of the moon, were discovered in the 1990s and most likely are solid, but they all orbit a pulsar.) Spotting chthonians would help answer questions regarding planetary formation, explains astronomer Adam Burrows of the University of Arizona. Researchers think that worlds are born from disks of gas and dust encircling stars. The most popular idea proposes that solid cores amass from protoplanetary disks and behave like seeds, attracting gas to grow into giant planets. The alternative theory suggests that giant planets may not possess hard cores. Instead they may have fluid centers, after having condensed directly from protoplanetary disks without forming solid hearts. Scientists have not conclusively identified whether the centers of giants in our own solar system are solid. Detecting chthonians could prove one scenario of planetary formation right. The European Southern Observatory telescope in Chile has an outside chance of finding them: one of the instruments there can detect planets as low as about 15 times Earth’s mass by looking for the gravitational tugs each has on its star. The French satellite COROT, launched in 2006, may have spotted one such world, called COROT 7b, in 2009. NASA’s Kepler might also have spotted one, called Kepler 10b, in 2011. Burrows thinks that chthonian exoplanets may not turn out to be all rock. If a chthonian’s star does not strip off its atmosphere, ices found in a giant’s core might survive underneath. Lecavelier des Etangs says that chthonians might even support life, although it would almost certainly be “very different from what we know on Earth.” --Originally published: Scientific American 290(5); 22-24, May 2004.

Diamond Planets by John Matson The study of exoplanets—worlds orbiting distant stars—is still in its early days. Yet already researchers have found hundreds of worlds with no nearby analogue: giants that could steamroll Jupiter; tiny pebbles broiling under stellar furnaces; puffy oddballs with the density of peat moss. Still other exoplanets might look familiar in broad-brush, only to reveal a topsy-turvy realm where rare substances are ordinary, and vice versa. Take carbon, for instance: the key constituent of organic matter accounts for some of humankind’s most precious materials, from diamonds to oil. Despite its outsize importance, carbon is uncommon— it makes up less than 0.1 percent of Earth’s bulk. On other worlds, though, carbon might be as common as dirt. In fact, carbon and dirt might be one and the same. An exoplanet 40 light-years away was recently identified as a promising candidate for just such a place—where carbon dominates and where the pressures in the planet’s interior crushes vast amounts of the element into diamond. The planet, known as 55 Cancri e, might have a crust of graphite several hundred kilometers thick. “As you go be neath that, you see a thick layer of diamond,” says astrophysicist Nikku Madhusudhan, a postdoctoral fellow at Yale University. The crystalline diamond could account for a third of the planet’s thickness. Carbon-based worlds would owe their distinct makeup to a planet-formation process very different from our own. If the composition of the sun is any indication, the cloud of dust and gas that coalesced into the planets of our solar system ought to have contained about twice as much oxygen as carbon. Indeed, Earth’s rocks are mostly based on oxygen-rich minerals called silicates. Astronomers have determined that 55 Cancri e’s host star, however, contains slightly more carbon than oxygen, which may reflect a very different planet-forming environment. And Madhusudhan and his colleagues calculated that the planet’s bulk properties—denser than a water world but less dense than a world made of Earth-like minerals— match those predicted for a carbon planet. The researchers published their findings in the November 10, 2012, Astrophysical Journal Letters. Life-forms on a carbon planet—if they exist—would little resemble the oxygendependent organisms of Earth. Precious oxygen would prove valuable as a fuel in much the same way that humans covet hydrocarbon fuels on Earth, says Marc Kuchner of the NASA Goddard Space Flight Center. Even courtship customs would be worlds apart from ours. “You would not be impressed if someone gave you a diamond ring,” Kuchner muses. “If your suitor showed up with a glass of water, that would be really exciting.” Editor's note: Since this article was published, another study indicated that carbon may be less abundant relative to oxygen than previously thought. The group, lead by Joanna Teske, found that the planet’s host star contains almost 25 percent more oxygen than carbon, about midway between the Sun and what the previous study suggested. “In theory, 55 Cancri e could still have a high carbon-to-oxygen ratio and be a diamond planet, but the host star does not have such a high ratio,” Teske said in a statement. “So in terms of the two building blocks of information used for the initial ‘diamond-planet’ proposal — the measurements of the exoplanet and the measurements of the star — the measurements of the star no longer verify that.” So uncertainty still remains about both the formation and composition of 55 Cancri e, which Teske described as "more of a diamond in the rough." --Originally published: Scientific American 308(1); 12 (January 2013).

Improbable Planets by Michael W. Werner and Michael A. Jura Among the most poignant sights in the heavens are white dwarfs. Although they have a mass comparable to our sun’s, they are among the dimmest of all stars and becoming ever dimmer; they do not follow the usual pattern relating stellar mass to brightness. Astronomers think white dwarfs must not be stars so much as the corpses of stars. Each white dwarf was once much like our sun and shone with the same brilliance. But then it began to run out of fuel and entered its stormy death throes, swelling to 100 times its previous size and brightening 10,000-fold, before shedding its outer layers and shriveling to a glowing cinder the size of Earth. For the rest of eternity, it will sit inertly, slowly fading to blackness. As if this story were not gloomy enough, it gets worse. We and our colleagues have found more than a dozen white dwarfs in our galaxy that are orbited by asteroids, comets and perhaps even planets—entire graveyards of worlds. While the stars were still alive, they rose every day in the skies of these worlds. They gently warmed the soil and stirred the wind. Living organisms may have soaked up their rays. But when the stars died, they vaporized or engulfed and incinerated their inner planets, leaving only the bodies that resided in the chilly outposts. Over time the dwarfs shredded and consumed many of the survivors as well. These decimated systems offer a grim look at the fate of our own solar system when the sun dies five billion years from now. Astronomers have always suspected that planets might orbit stars other than our sun. We imagined, though, that we would find systems much like our own solar system, centered on a star much like the sun. Yet when a flood of discoveries began in the 1990s, it was apparent right away that extrasolar planetary systems can differ dramatically from our solar system. The first example was the sunlike star 51 Pegasi, found to have a planet more massive than Jupiter on an orbit smaller than that of Mercury. As instruments became more sensitive, they found ever stranger instances. The sunlike star HD 40307 hosts three planets with masses between four and 10 Earth masses, all on orbits less than half the size of Mercury’s. The sunlike star 55 Cancri A has no fewer than five planets, with masses ranging from 10 and 1,000 Earth masses and orbital radii ranging from one tenth that of Mercury to about that of Jupiter. Planetary systems imagined in science fiction scarcely compare. The white dwarf systems demonstrate that the stars do not even need to be sunlike. Planets and planetary building blocks can orbit bodies that are themselves no larger than planets. The variety of these systems equals that of systems around ordinary stars. Astronomers hardly expected the ubiquity of planetary systems, their hardiness and the apparent universality of the processes by which they form. Solar systems like our own might not be the most common sites for planets, or even life, in the universe. Phoenix from the Ashes It is sometimes forgotten today, but the first confirmed discovery of any extrasolar planets was around a very unsunlike star: the neutron star PSR 1257+12, an even more extreme type of stellar corpse than a white dwarf. It packs a mass greater than the sun’s into the size of a small asteroid, some 20 kilometers across. The event that created this beast, the supernova explosion of a star 20 times the mass of the sun, was more violent than the demise of a sunlike star, and it is hard to imagine planets surviving it. Moreover, the star that exploded probably had a radius larger than 1 AU (astronomical unit, the Earth-sun distance), which is larger than the orbits of the planets we see today. For both reasons, those planets must have risen up out of the ashes of the explosion. Although supernovae typically eject most of their debris into interstellar space, a small amount remains gravitationally bound and falls back to form a swirling disk around the stellar remnant. Disks are the birthing grounds of planets. Astronomers think our solar system took shape when an amorphous interstellar cloud of dust and gas collapsed under its own weight. The conservation of angular momentum, or spin, kept some of the material from simply falling all the way to the newborn sun; instead it settled into a pancake shape. Within this disk, dust and gas coagulated into planets. Much the same process could have occurred in the postsupernova fallback disk. Astronomers discovered the system around PSR 1257+12 by detecting periodic deviations in the timing of the radio pulses it gives off; such deviations arise because the orbiting planets pull slightly on the star,

periodically shifting its position and thus altering the distance the pulses must travel. Despite intensive searches of other stars’ pulses, observers know of no other comparable system. Another pulsar, PSR B1620–26, has at least one planet, but it orbits so far from the star that astronomers think it did not form in a fallback disk but rather was captured gravitationally from another star. In 2006, however, NASA’s Spitzer Space Telescope discovered unexpected infrared emission from the neutron star 4U 0142+61. The infrared light might arise from the star’s magnetosphere or from a circumstellar disk. This star formed in a supernova explosion about 100,000 years ago, and it typically takes about a million years or so for planets to agglomerate, so if the radiation does signal the presence of a disk, this system may one day resemble that revolving around PSR 1257+12. Many white dwarfs also have disks, albeit of a somewhat different type: disks that indicate the actual presence of orbiting bodies rather than merely the potential to form them. As with 4U 0142+61, the clue is the unexpected emission of infrared light. The first hint dates to 1987, when one of NASA’s groundbased observatories, the Infrared Telescope Facility on the summit of Mauna Kea in Hawaii, found excess infrared light from the white dwarf G29–38. The spectrum of this excess was that of a body with a temperature of 1,200 kelvins, much cooler than the surface of the star, which is 12,000 kelvins. Initially astronomers thought that the dwarf must be orbited by a second, cooler star. But in 1990 they showed that the infrared emission varied in unison with the star’s own brightness, indicating that it was reflected or reprocessed starlight. The most plausible explanation is a circumstellar disk heated by the star. This star has another peculiar property. Its outermost layers contain heavy elements such as calcium and iron, which is odd because the gravitational field near the surface of a white dwarf is so strong that those elements should sink into the interior. In 2003 one of us (Jura) proposed a simple explanation for both the infrared excess and the presence of heavy elements: the white dwarf recently shredded an asteroid that strayed into its intense gravitational field. A cascade of collisions reduced the debris to an orbiting dust disk, which dribbled onto the star. Asteroids for Dessert Observations have since confirmed this scenario. Astronomers using both ground-based telescopes and the Spitzer telescope have identified some 15 white dwarfs with similar infrared excesses and elemental anomalies. For G29–38 and seven other stars, Spitzer has gone further and identified infrared emission from silicates in the disks. These silicates resemble those in dust particles in our solar system and appear quite different from those in dust in interstellar space. Moreover, although the stars’ outer layers contain heavy elements, they do not contain those elements in equal amounts. They are deficient in volatile elements such as carbon and sodium compared with elements that tend to remain in solid form, such as silicon, iron and magnesium. This elemental pattern matches that of the asteroids and rocky planets of the solar system. Both these facts support the contention that the disks are ground-up asteroids. The disks around white dwarfs are much smaller than the disks that give rise to planets around newborn sunlike stars. Judging from their infrared emission, they extend to only about 0.01 AU and have a mass as low as that of an asteroid 30 kilometers in diameter—a fact consistent with their possible origin in the disintegration of such an object. They are not potential sites of the formation of new planets but rather indicators that some planetary material survived the demise of the star. Theoretical calculations suggest that asteroids and Earth-like planets can escape destruction if they orbit farther than 1 AU. When our sun dies, Mars should make it, but Earth may or may not. To study how parts of a planetary system might endure, in 2007 Spitzer observed the white dwarf WD 2226–210. This dwarf is so young that the outer layers of the original sunlike star remain visible as the Helix nebula, one of the best-known planetary nebulae. Consequently, WD 2226–210 provides the missing link between sunlike stars and older white dwarfs such as G29–38. Around it is a dusty disk at a distance of 100 AU, comparable to the scale of our solar system. That is much farther than disks around other white dwarfs extend— too far, in fact, to consist of asteroids torn up by the dwarf’s gravity. This disk must instead consist of dust released as asteroids and comets collide. Similar debris disks exist around the sun and sunlike stars. This discovery confirms that when a sun like star dies, distant asteroids and comets can survive. And if asteroids and comets can survive, planets (which are, if anything, more dur able) should be able to survive as well. As WD 2262–210 cools, it will give off less light to illuminate the dust, and the distant belt of asteroids and comets will fade into invisibility. But occasionally one of its members may wander close enough to the white dwarf to be shredded. Starlets A third type of nonsunlike star that might host planets is the brown dwarf. Brown dwarfs are very different from white dwarfs, despite the similar names. They are not stellar corpses but stellar runts.

They form in the same way stars do, but their growth is stunted, leaving them with less than about 8 percent the mass of the sun—the threshold required for a stellar core to become hot and dense enough to ignite sustained nuclear fusion. The most they manage is a feeble infrared glow as they radiate away the heat they accumulated during their formation (and perhaps a brief early period of fusion). Over the past 15 years astronomical surveys have found hundreds of brown dwarfs, and the least massive of them is scarcely heavier than a giant planet. Astronomers have found that these bodies, even the smallest among them, can have disks and therefore perhaps planets as well. The possibility of planets is supported by observations showing that brown dwarf disks undergo a series of systematic changes—including a drop in the prominence of the infrared emission from silicates—attributable to coagulation of the dust particles. The same changes also occur in disks around larger stars and signal the growth of planetary building blocks. The brown dwarf disks are too meager for planets as large as Jupiter to form but contain plenty of material for a Uranus or Neptune. Some astronomers have claimed the discovery of planets that formed around brown dwarfs, but none of these claims is definitive. In short, astronomers have found planets around at least one neutron star; asteroids and comets around more than a dozen white dwarfs; and evidence for the early stages of planet formation around brown dwarfs. Ultimately, the study of these and other extrasolar systems has two goals: First, astronomers hope to learn more about our own solar system, particularly about its evolution and large-scale structure, features that are hard to discern from our limited temporal and spatial perspective. We also hope to place our solar system in its context. Is it average or an outlier? Despite the diversity of planetary systems, do they follow some common pathways in their formation? The similarity between the composition of asteroids in our solar system and of the material that has fallen onto white dwarfs suggests that the answer is yes. The second goal is to determine how widespread life might be in the universe. In our galactic neighborhood, brown dwarfs are roughly as numerous as stars. Might the nearest “star” to our sun be a yet to be discovered brown dwarf? Might the nearest planets to our solar system orbit a brown dwarf? The Widefield Infrared Survey Explorer (WISE) satellite, which NASA plans to launch at the end of the year, may well discover several brown dwarfs closer than the nearest known star. The formation of terrestrial planets around brown dwarfs would not only extend the range of potential habitats but also lead to the intriguing possibility that the nearest extraterrestrial life may wake up in the morning to a brown dwarf. Similarly, the presence of asteroids and comets around white dwarfs raises the possibility not only that planets can survive the demise of a sunlike star but also that life, if it could adapt to the changing conditions, may hold out in the environs of these dead stars. Perhaps, then, white dwarfs are not such a gloomy sight after all. --Originally published: Scientific American 300(6), 38-45 (June 2009).

Intergalactic Expat by John Matson Nearly everything we can see in the night sky without the aid of a telescope is in Earth's cosmic neighborhood, the Milky Way Galaxy. And the hundreds of planets that have been discovered outside our solar system all orbit stars within the Milky Way; their relative proximity permits the kind of careful look needed to identify an orbital companion. Astronomers have located yet another extrasolar planet, or exoplanet, within our galaxy. But this one seems to have started out in another galaxy that was then consumed by, and incorporated into, the Milky Way several billion years ago. The planet, which is somewhat more massive than Jupiter, was announced November 2010 by a group of researchers from the Max Planck Institute for Astronomy (M.P.I.A.) in Heidelberg, Germany, and the European Space Agency. The team used a telescope on a Chilean mountaintop to observe HIP 13044, an aging star about 2,000 light-years from our solar system. The star showed a periodic wobble on a timescale of 16.2 days that the researchers concluded was most plausibly explained by the gravitational tug of a massive planet orbiting very close by. Astronomers have found the vast majority of known exoplanets by tracking these stellar wobbles through Doppler shifts in the star's light. But the discovery of the newfound HIP 13044 b, named by convention for its host star, is surprising for a few reasons. One is that HIP 13044 b is the first verifiable exoplanet of extragalactic origin. (A tentative detection of a possible planet in Andromeda was announced in 2009, but the kind of one-off observation used in that study is not confirmable.) HIP 13044 belongs to the Helmi stream, a population of stars that stretches through the Milky Way with similar, unusual orbits and compositions. The Helmi stream was determined in 1999 to have originated in a small galaxy, similar to the Sagittarius dwarf galaxy, that was cannibalized by the larger Milky Way. Later work found that the ingested galaxy must have been devoured six billion to nine billion years ago. Another peculiarity is that the new planet's host star is quite evolved, having already passed through the red giant phase that awaits our sun in about five billion years. When the sun becomes a red giant, it will swell to many times its current size, and most likely swallow the planets of the inner solar system. But HIP 13044 b sticks close to its host star, well within the region that would be expected to be engulfed by a red giant, meaning that it may have migrated inward from an earlier, wider orbit as the star contracted. The researchers speculate that the planet may await destruction when the star swells again in the next phase of its evolution, when it becomes a so-called asymptotic giant branch star. Additionally, most known exoplanets orbit stars that are at least as metallic as the sun—that is, they are enriched in elements heavier than hydrogen and helium. But HIP 13044 and its fellow Helmi stream stars are extremely metal-poor. HIP 13044, which is nearly as massive as the sun, has only about 1 percent the metallicity of our planet's host star, making it the most metal-deficient star known to harbor a planet. "In the current picture of planet formation, you have the core accretion scenario—you need heavy elements to form planetary embryos," says study co-author Rainer Klement, an astronomer at M.P.I.A. "Apparently this doesn't work for the planet we found now." An alternate theoretical mechanism, called disk instability, would allow planets to form without first assembling a rocky core; the discovery of HIP 13044 b could add observational weight to the model. "Now we found a planet around a very metal-poor star, and there has to be some other mechanism at work to form this planet," Klement says. --Originally published: Scientific American Online November 18, 2010.

A Large Lump of Coal by George Musser Astronomy is the science of the exotic, but the thing that astronomers most want to find is the familiar: another planet like Earth, a hospitable face in a hostile cosmos. The Kepler spacecraft, which was launched in March 2009, is their best instrument yet for discovering Earth-like planets around sunlike stars, as opposed to the giant planets that have been planet finders’ main harvest so far. But if the giant planets, which looked nothing like what astronomers had expected, are any indication, those Earths may not be so reassuringly familiar either. It has dawned on theorists in recent years that other Earth-mass planets may be enormous water droplets, balls of nitrogen or lumps of iron. Name your favorite element or compound, and someone has imagined a planet made of it. The spectrum of possibilities depends largely on the ratio of carbon to oxygen. After hydrogen and helium, these are the most common elements in the universe, and in an embryonic planetary system they pair off to create carbon monoxide. The element that is in slight excess ends up dominating the planet’s chemistry. In our solar system, oxygen dominates. Although we tend to think of our planet as defined by carbon, the basis of life, the element is actually a fairly minor constituent. The terrestrial planets are made of silicate minerals, which are oxygen-rich. The outer solar system abounds in another oxygen-rich compound, water. A study shows in detail how carbon lost out. Jade Bond of the University of Arizona and the Planetary Science Institute (PSI), Dante Lauretta of Arizona and David O’Brien of PSI have simulated how chemical elements got distributed around the solar system as it formed. They found that carbon remained in a gaseous state within the protoplanetary disk and was eventually blown out into deep space; the embryonic Earth wound up with none at all. The carbon in our bodies must have been delivered later, by asteroids and comets that formed under conditions that allowed them to incorporate the element. Had the carbon-oxygen balance tilted the other way, Earth would have turned out very differently, as Marc Kuchner, then at Princeton University, and Sara Seager, then at the Carnegie Institution of Washington, argued in 2005. It would consist not of silicates but of carbon-based compounds such as silicon carbide and, indeed, pure carbon itself. The crust would be mainly graphite, and a few kilometers underground the pressures would be high enough to form a rigid shell of diamond and other crystals. Instead of water ice, the planet would have carbon monoxide or methane ice; instead of liquid water, it might have oceans of tar. The galaxy could be filled with such worlds. According to an observational survey Bond cited, the average planet host star has a higher carbon-to-oxygen ratio than the sun does, and her team’s simulations predict that the most enriched systems give birth to carbon planets. “Some of these compositions differ greatly from solar and as a result produce terrestrial planets with vastly different compositions,” Bond said. To be sure, other surveys have found that the sun is indistinguishable from the average star in its class. The Kepler spacecraft may help settle the question, because even the limited amount of information it can glean about planets—their mass and radius— is enough to tell their general composition. Carbon Earths might be especially prevalent in more bizarre settings, such as the environs of white dwarfs and neutron stars. Regions of the galaxy that are rich in heavy elements generally, such as the galactic center, have higher carbon-to-oxygen ratios. As time passes and stars continue to manufacture heavy elements, the balance everywhere will tilt in favor of carbon. These and other astronomical discoveries turn the tables on our notions of the familiar and unfamiliar. Most of the galaxy is dark matter; most suns are dimmer and redder than our sun; and now, it seems, other Earths may not be especially Earth-like. If anything departs from the norm and deserves to be called exotic, it is us. --Originally published: Scientific American 302(1); 26 (January 2010).

A Tale of Two Exoplanets by JR. Minkel Temperature measurements revealed extreme behavior in two planets outside our solar system. One study indicated that HD 149026 b—a relatively small but extremely dense planet orbiting a distant star— has an atmospheric temperature of 2,300 kelvins (about 3,700 degrees Fahrenheit), or twice that of the hottest previously studied planet. Astronomers have also mapped the surface temperature of one of those next-to-hottest planets, the larger and less dense HD 189733 b. They conclude that winds are evening out its day and night temperatures by stirring together hot and cold gas. For HD 149026 b to reach such blistering heat, researchers say, it must suck up nearly all the energy it receives from its big bluish star. If so, the gaseous planet could be nearly pitch-black in color. Experts cannot fully explain the planet's intense heat, but they speculate that it may have something to do with its unusually high concentration of heavy elements. "This is a weird planet, and this is yet another weird thing about it," says planetary scientist Joseph Harrington of the University of Central Florida in Orlando. The two planets transit in front of their stars as viewed from Earth. Dubbed hot Jupiters for their typical size and closeness to their stars, they always present those stars with the same face. By comparing the intensity of infrared starlight as one of the gas giants goes behind and front of its star, researchers can deduce the temperatures of its day and night sides. Harrington and colleagues used the Spitzer Space Telescope (SST) to gauge the daytime temperature of HD 149026 b's upper atmosphere in this way. Infrared data had pegged three other exoplanets, including HD 189733 b, in the 1,000-to-1,200-kelvin range, which implied that the planets reflected about 30 percent of incoming starlight. To reach 2,300 kelvins, HD 149026 b must have zero reflectivity, or albedo, said Harrington and colleagues in a report published by Nature. Moreover, it must radiate energy back into space as quickly as it receives it. The atmosphere would heat up by absorbing blue-white starlight, radiating out lower energy infrared light, and pocketing the energy difference. Harrington says the team has debated what exactly the planet would look like. Zero albedo is "blacker than coal," he says, but the infrared light could spill into red where the heat is strongest. He says he pictures a deep-black planet glowing like an ember at the region closest to the star. HD 149026 b was already an oddball among hot Jupiters for its Saturn-like size and mass and its high density. More than two thirds of the planet must consist of elements heavier than helium, which are uncommon in gas giant planets and may introduce unexpected compounds into the atmosphere that contribute to the still mysterious total absorption, Harrington says. A separate team trained the SST on HD 189733 b for 33 hours of its 2.2-day orbital period, giving them a map of its surface heat. In a second Nature paper they reported that its day-and night-side temperatures were relatively similar—1,200 and 970 kelvins (about 1,700 and 1,285 degrees F), respectively—and that the day side's hottest spot did not face the star dead-on but was offset by 30 degrees longitude. Both features are signs of extreme wind speeds of perhaps 5,000 to 6,000 miles per hour, says astronomer Heather Knutson, a graduate student at the Harvard-Smithsonian Center for Astrophysics. "It tells us that winds are kind of shifting things around in the atmosphere," she says. The size and mass of HD 189733 b is average among hot Jupiters, Knutson says, but time will tell if such strong winds are also typical. The researchers say their next goal is to study the planets at other infrared frequencies in order to get more accurate readings and possibly learn about their atmospheres' constituents. As Harrington notes, "we're at the beginning of understanding how planets handle their heat."

--Originally published: Scientific American Online, May 9, 2007.

Worlds with Two Suns by William F. Welsh and Laurance R. Doyle For years the two of us wondered if paired, or “binary,” stars could support planets. Could worlds like the fictional Tatooine from Star Wars, where the sky is lit with the glow of two different suns, really exist? Astronomers had reason to think such systems might exist, yet some theorists disagreed. The environment around a pair of stars, they argued, would be too chaotic for planets to form. Unlike a body circling a single star, a planet orbiting a pair of stars would have to contend with two gravitational fields. And because the stars themselves orbit each other, the strength of the gravitational forces would constantly change. Even if a planet could form in such a dynamic environment, its long-term stability would not be assured—the planet could wind up being ejected into deep space or crashing into one of the stars. Observations of binary star systems had shown some indirect evidence for these “circumbinary” planets, but direct evidence remained elusive. Over two decades of effort by William Borucki and his collaborators to get an exoplanet-hunting spacecraft launched finally came to fruition in March 2009. NASA's Kepler Mission has since proved to be spectacularly successful, quickly revealing hundreds, then thousands, of planet candidates via the transit method, which searches for the mini eclipse that occurs when a planet orbits in front of the star, blocking some of its light. But after two years, no circumbinary planets had been detected. The frustrating lack of evidence began to take its toll. In a weekly Kepler telephone conference in the spring of 2011, one of us offered an attempt at black humor: “Maybe we should write a paper on why they don’t exist.” Silence followed. Our fears were misplaced. Within six months of that conversation we had a press conference to announce the discovery of the first transiting circumbinary planet. This planet was called Kepler-16b. Within months the Kepler Eclipsing Binary Working Group discovered two more circumbinary planets (Kepler-34b and Kepler-35b), showing that while exotic, such systems are not rare. A new class of planet system had been established. The current tally of Kepler circumbinary planets is seven, and that number could double in a short time. In fact, calculations suggest that tens of millions likely exist in the Milky Way. SEARCH STRATEGIES The quest for circumbinary planets began in the 1980s, even before astronomers found the first evidence of any “exoplanets” outside our solar system. Although transits can be much more complicated in a binary system, hopes for discovering such a system were fueled by a simple expectation: if a planet did orbit an eclipsing binary star system, we would expect it to travel in the same orbital plane as the stars themselves. In other words, if from our perspective on Earth the stars eclipsed each other, then the planet would be much more likely to eclipse one or both stars. This assumed that the planet and the stars had co-planar orbits, a reasonable hypothesis—and one that could be tested. Eclipsing binary stars are in many ways the foundation on which stellar astrophysics is built. Their special orientation along our line of sight means that the stars pass in front of each other once per orbit, blocking some of the light. By precisely modeling how the light dims during the eclipses, we can learn the sizes and shapes of the stars and the geometry of their orbits. Coupled with other measurements, we can measure the stars’ radii and masses. Eclipsing binary stars thus provide a fundamental calibration of stellar masses and radii, which in turn are used to estimate the stellar properties for noneclipsing and single stars. If the two stars in a binary system are very far apart, with an orbital period of, say, hundreds of years, the stars hardly affect each other, and they act almost as if in isolation. Planets may orbit either one of the stars and in general will not be much influenced by the presence of the other star. These are known as circumstellar, or S-type, planets, and dozens of such planets have been discovered in the past decade. Things get more interesting when stars are so close together that they take only weeks, or even days, to orbit each other. For a planet in such a binary to have a stable orbit, it would have to orbit both stars, not just one. Numerical calculations show that the planet’s orbital separation from the stars has to be larger

than a minimum critical distance; too close and the rotating binary system would destabilize the planet’s orbit, either swallowing it up or ejecting it out into the galaxy. The minimum stable separation is roughly two to three times the size of the stars’ separation. These kinds of planets are known as circumbinary, or P-type, planets. While planets around single stars and around individual stars in widely separated binaries are common, we wondered if nature could make planetary systems in the circumbinary configuration, where the planet orbits both stars. Two Types of Binary Planets Binary star systems come in many different forms. Some binary stars make huge, looping orbits around their common center of mass, taking hundreds of years to complete a single orbit. These stars tend to act almost as if isolated; an “S-type” planet can orbit one star in the pair without being bothered by the other. In contrast, stars close together can orbit each other in weeks or days. For years it was an open question whether “P-type” planets could survive the chaotic gravitational environment and orbit around the pair of stars.

Illustration by Ron Miller, Graphic by Jen Christiansen

In a simple one-star, one-planet system, transits will occur with a metronomic periodicity that greatly assists in their detection. Add another star, though, and the three-body system will start to display all manner of complicated effects. The complexity arises because the stars are quickly moving—in contrast to the single-star system where the star is effectively stationary. In fact, because the two stars are closer to each other than they are to the planet, they must orbit each other faster than the planet will orbit them— a manifestation of Johannes Kepler’s famous laws of planetary motion. Thus, the planet will be transiting a quickly moving target, and sometimes it will cross the star early and sometimes late. While precisely predictable (if the masses and orbits are known), the transits will not be periodic. In addition, the duration of the transit will change depending on the relative motion of the planet to the star being crossed—if they are moving in the same direction, the transit will be longer in duration, but when the star is on the other half of its orbit and moving the other direction, the transit will be shorter. These variations make detecting circumbinary planets difficult, but they also offer an important benefit: once the binary star’s orbit is deciphered, the pattern of changing transit times and durations can be used to unequivocally confirm the presence of an orbiting circumbinary body. No other astronomical phenomenon exhibits such a pattern. This is a unique characteristic of a circumbinary object—a smoking gun signature. The First Detection Until technical problems sidelined it in 2013, Kepler had kept its eye trained on a single patch of sky, looking for the characteristic dimming caused by planets crossing in front of host stars. In its quest for planets, Kepler also discovered more than 2,000 new eclipsing binary star systems. Several exotic systems were discovered, including the first known eclipsing triple-star systems.

In 2011 one of us (Doyle), along with associate Robert Slawson, working with him at the SETI Institute in Mountain View, Calif., noticed extra eclipse events in the binary stars known as KIC 12644769. The two stars eclipsed each other every 41 days, but there were three other unexplained eclipse events. The first two occurred 230 days apart. The next occurred 221 days later—nine days earlier than expected. This was just the kind of signature one would get from a circumbinary planet. These transits thus provided evidence of a third body orbiting the binary. But it could have been just a dim, small star grazing part of the large star—and as Kepler was showing us, such triple-star eclipsing systems are not exceptionally rare. The slight dimming indicated that the object could have a small radius, but starlike objects such as brown dwarf stars are also small, so we could not say for sure if the object was a planet. We had to measure its mass. In a three-body system, an unseen companion to a binary can make its presence known in two main ways. Imagine two stars eclipsing each other, with a relatively large planet circling the pair farther away. The binary stars orbit each other, but in addition, the center of mass of the pair is orbiting the center of mass of the three-body system. As a consequence, sometimes the binary stars will be displaced a little bit closer to Earth; at other times, they will be farther away. When they are farther away, light from the stars will take longer to reach us and the eclipses will occur slightly late. When the stars are closer to us, the eclipses will be early. The larger the mass of the third body, the larger the change. Thus, this cyclic lighttravel time effect allows one to infer the presence and estimate the mass of any unseen object. Also, the farther away the third body is from the binary, the greater the effect because the added distance will act as a lever, but the farther away, the longer the cycle time. In the case of our candidate circumbinary planet, there was no detectable cyclic change in the eclipse timing on the order of 230 days, implying that the hidden body had a low mass. But how low? The other way for a third body to affect the binary is through direct gravitational interaction, called the dynamical effect. This method dominates the light-travel time effect for closer objects. The unseen companion slightly alters the orbits of the binary stars, and these changes can be picked up through variations in the occurrence times of the eclipses. Because the smaller star comes closer to the third body, its orbit will be perturbed more. Unlike the light-travel time effect, the dynamical effect alters the times of the eclipses in complex ways. One of our colleagues on the Kepler Science Team, Daniel C. Fabrycky, now at the University of Chicago, noted that a stellar-mass object would strongly affect the eclipse times, whereas a planet would produce a much more subtle—but potentially measurable— signature. And for this system, the dynamical effects should be very much stronger than the light-travel time effect. We looked for and subsequently found the changes in eclipse timing, revealing that the tug on the stars was not anywhere near what a stellar-mass companion would produce. The grand finale of the investigation was provided when Joshua A. Carter of the Harvard-Smithsonian Center for Astrophysics was able to create a sophisticated computer model of the system. It matched the complete data set perfectly for a planet with a mass similar to Saturn’s. The excellent match between the observations and the modeling proved the existence of the planet and provided exquisitely precise values for the radii, masses and orbital characteristics of the system. This planet was designated Kepler-16b and was the first transiting circumbinary planet discovered. The combination of the transits and the clear dynamical effects made this detection unambiguous. Because the binary stars would appear as sun-size disks as seen from this planet, Kepler-16b soon acquired the nickname “Tatooine” from the fictional planet in Star Wars and its iconic image of a double sunset. Science fiction had become science fact. A New Class of Planet Kepler-16b appeared, at first, to be a very strange planet. Its orbit is uncomfortably close to its host stars, being only 9 percent farther out than the minimum critical distance needed for orbital stability. And because this was the only transiting circumbinary planet at the time, we asked ourselves: Is Kepler-16b just a fluke? Fortunately, the answer came quickly. Working with Jerome A. Orosz of San Diego State University, we had already been searching for circumbinary planets that do not transit their stars. These should be far more common than transiting cases, since the special alignment of the planet’s orbit to create a transit is not required. As mentioned, small variations in eclipse timings should reveal such planets. We had been pursuing this line of research for a few months and had identified a few candidate systems. Then, on a Tuesday afternoon in August 2012, one of us (Welsh) noticed transits in one of the binary star systems. Within hours Fabrycky had created a computer model that reproduced the variable transit times and durations, confirming the transiting object as a planet. We had discovered Kepler-34b. Working feverishly, the very next day Orosz found transits in another eclipsing binary star system, and it, too, harbored a planet—Kepler-35b.

Over the next few months Orosz would go on to discover Kepler-38b, showing that smaller, Neptunemass circumbinary planets also exist, and then the Kepler-47 planetary system with at least two planets, showing that binary stars can harbor multiple planets. The most recent circumbinary planet discovered, Kepler-64b (also known as PH1) was simultaneously and independently discovered by Johns Hopkins University graduate student Veselin Kostov and by amateur astronomers working as part of the Planet Hunters project. It is part of a quadruple-star system, further extending the diversity of places where planets can form. The seven circumbinary planets found so far tell us that these objects are not extremely rare but rather that we have uncovered a whole new class of planetary system. For every transiting planetary system detected, geometry tells us that there are roughly five to 10 planets that we do not see because they do not have the correct orientation to pass in front of the binary stars as seen from our vantage point. Given that seven planets were found out of roughly 1,000 eclipsing binaries searched, we can conservatively estimate that the galaxy is home to tens of millions of such circumbinary planetary systems. All the Kepler transiting circumbinary planets to date are gas-giant planets, worlds without the rocky crust that would allow an astronaut to stand on its surface and marvel at the double sunsets. The search continues for smaller rocky planets, although Earth-size circumbinary planets are going to be extremely difficult to detect. But even with such a small sample of planets, a number of interesting questions arise. For instance, half of all the Kepler eclipsing binaries have an orbital period of less than 2.7 days, so we expected that half of the binaries with planets would also have periods less than 2.7 days. But none of them do; the shortest orbital period is 7.4 days. Why? We speculate that it might be related to the process that brought the stars so close together in the first place. In addition, the planets tend to orbit their stars very closely. If they were in much closer, the planets’ orbits would be unstable. What, then, causes them to live so dangerously? Understanding why the circumbinary planets orbit so close to their critical instability radius will help us improve theories about how planets form and how their orbits evolve over time. Even though we do not know why these planets seem to prefer such precarious orbits, we can nonetheless infer something deep: the discovery that planets can live so near a chaotic environment is telling us that planet formation is vigorous and robust. A Dynamic Habitable Zone The tendency of the Kepler circumbinary planets to lie near the critical stability radius has an interesting consequence. For the Kepler sample of stars, the critical radius is generally close to the habitable zone—the region around a star (or in this case, around two stars) where the energy from that star makes the planet’s temperature just right for water to persist in the liquid state. Too close to the star, and the planet’s water boils; too far away, and the water freezes. And water is a prerequisite for life as we know it. For a single star, the habitable zone is a spherical-shell region around that star. In a binary system, each star has its own habitable zone, which merge into a distorted spheroid if the stars are close enough together, as is the case for the Kepler circumbinary planets. As the stars orbit each other, the combined habitable zone also revolves with the stars. Because the stars orbit faster than the planet does, the habitable zone swings around more quickly than the planet orbits. Unlike Earth, which is in a near-circular orbit around the sun, the distance a circumbinary planet has to each of its host stars can change radically over the course of the planet’s orbital year. Thus, planetary seasons could wax and wane in only a few weeks as the stars whirl about each other. These climate changes could be large and only quasi regular—“It would be a wild ride,” Orosz notes. Two of the seven known transiting circumbinary planets are in their system’s habitable zone, a remarkably high percentage. Although being in the habitable zone does not guarantee conditions suitable for life—Earth’s moon is in the sun’s habitable zone and yet is as desolate as can be imagined because its small mass is too feeble to retain an atmosphere, for example—the high fraction of circumbinary planets in their habitable zones does cause one to pause and wonder. With its severe and rapidly changing seasons, what would life, and indeed a civilization, be like on a circumbinary world? --Originally published: Scientific American 309(5); 40-47 (November 2013).

SECTION 1 The Search for Life

Anybody Home? by John Matson Even as astronomers work toward the hotly anticipated milestone discovery of an Earth-like twin orbiting another star, researchers are already asking what it will take to detect the existence of extraterrestrial life on such a planet. The good news is that observatories now being planned could have a shot. Yet it is hardly a lock. The next generation of giant, ground-based telescopes may be able to tease out biomarker signals from the starlight filtering through exoplanetary atmospheres, according to research published in the Astrophysical Journal and in Astronomy & Astrophysics. The two groups of scientists calculated what possible biomarkers might be detectable with the planned European Extremely Large Telescope (E-ELT), which would dwarf the twin Keck telescopes on Mauna Kea in Hawaii that are now on the cutting edge of astronomy. Living organisms on Earth leave numerous chemical imprints on the environment via, for instance, the production of oxygen by plants and bacteria, the release of methane during digestion, and the generation and consumption of carbon dioxide in the global carbon cycle. Measurements of those chemical species in an exoplanet’s atmosphere could provide strong indications of the presence of life there. Astronomers using the world’s best telescopes have already identified specific atoms and molecules in the atmospheres of massive, highly irradiated exoplanets. To do the same for smaller planets in cooler orbits—objects from which photons are relatively scarce—will require much bigger telescopes and many years of observations. With a high-resolution spectrograph to break down the collected light from an exoplanet into its component wavelengths, the E-ELT would, in principle, be able to spot oxygen gas in the atmosphere of a temperate, Earth-like world. The giant observatory may also be able to identify water, which is thought to be important but not sufficient for life as well as ozone (O3), a molecule closely related to oxygen gas. “When we are sure there is ozone, we could be pretty sure that there is oxygen in the atmosphere,” explains astrophysicist Pascal Hedelt of the German Aerospace Center and the Laboratory of Astrophysics of Bordeaux in France, who is the lead author of the Astronomy & Astrophysics study. Methane might also be detectable in some of the scenarios explored by Hedelt’s group. The connection between chemistry and life is not always straightforward, however, and the detection of oxygen, methane or some other biologically relevant molecule will require careful interpretation. Venus has an ozone layer, and Mars, according to research that is somewhat controversial in the planetary science community, releases occasional plumes of methane. Yet no solid evidence indicates that either planet hosts any microbes. “Only finding oxygen in principle is not enough,” says Ignas Snellen of Leiden University in the Netherlands, who led the Astrophysical Journal study. Regarding future exoplanet studies, Snellen cautions, “You really need to characterize the atmosphere as a whole.” --Originally published: Scientific American, 308(5); 26 (May 2013).

The Search for Life on Faraway Moons by Lee Billings We now know of more than 1,000 planets orbiting other stars. In all likelihood, hundreds of billions more call the Milky Way home. Many of the known “exoplanets” are large, gaseous worlds like Jupiter or Neptune—hostile places for life. But like those giants of our solar system, distant exoplanets may also have large moons. And if they do, moons—not planets—may be the most common home for life in the universe. The frontier of the search for moons of exoplanets—exomoons— lies deep in the basement of the Harvard-Smithsonian Center for Astrophysics, inside a gloomy room lined with computers in wire-mesh cages. Raising his voice over the mechanical whine of the cooling fans, British astronomer David Kipping remarks that nearly all of this computing power is currently devoted to analyzing a single planet, Kepler22b, which orbits a sunlike star some 600 light-years away from Earth. The distant world is named for NASA’s planet-hunting Kepler space telescope, which first spotted it. Kipping’s hope is that on closer inspection, the data that first revealed Kepler-22b’s presence may also divulge the subtler signals of lunar companions. He calls his project the Hunt for Exomoons with Kepler, or HEK. Kipping’s project is the most advanced exomoon hunt today. The intense computing power is necessary, Kipping says, because even the largest conceivable exomoon would leave a vanishingly faint signal in the data. Because of this, he intensively searches for evidence of exomoons around just a few carefully selected targets. He may not find as many exomoons as he would with a quick search of lots of targets, but “I’m not sure I’d believe those results,” he says. “Our goal is nice, clean, solid detections that everyone can agree on.” He has reason to be circumspect. Any claim of an exomoon discovery would be controversial, not only because the work is difficult but also because the find potentially has profound implications. For instance, Kipping explains, Kepler-22b resides in its star’s habitable zone, the region where liquid water could exist. The planet is so large it is likely to be an inhospitable, gas-shrouded orb rather than a rocky, terrestrial world like Earth. If, however, Kepler-22b has a massive lunar companion, that moon might be a pleasant place to live and a possible target for future astronomical searches for extraterrestrial life and intelligence. “Moons could be habitable,” he says. “And if that’s true, there’s a hell of a lot more opportunities for life out there than anyone has previously appreciated.” MAKING MOONS Many astronomers (as well as science-fiction authors) had long assumed that other planetary systems would mirror our own, with bountiful icy moons orbiting cold, giant worlds, similar to the arrangements we see around Jupiter and Saturn. With the first exoplanet discoveries of the 1990s, however, new possibilities arose; planet hunters began finding gas-giant planets that, after forming in the outer dark, somehow migrated in to closer, hotter orbits around stars. Some even occupied their stars’ habitable zones. Such positioning raised the question: Might some moons around those warm giants have rocky compositions, protective atmospheres and oceans like on Earth? Three researchers at Pennsylvania State University—Darren Williams, Jim Kasting and Richard Wade— were the first to study in detail how feasible it would be for an exomoon to possess an Earth-like environment. Their study, published in 1997 in Nature (Scientific American is part of Nature Publishing Group), asked how large a habitable-zone moon must be to maintain a substantial atmosphere and liquid water on its surface. “We found that moons smaller than Mars, about a tenth the mass of Earth, couldn’t hold on to an atmosphere for more than a few million years,” Williams says. Below that threshold, a moon would not generate enough gravitational force to retain a substantial atmosphere. The atmosphere of such a too tiny moon would boil off in radiation from the nearby star. The trouble is that moons as big as a terrestrial planet do not seem very easy to build. Astronomers believe that most moons form in much the same way that planets do—gradually coalescing out of a spinning disk of gas, ice and dust. Most computer simulations of this piece-by-piece lunar assembly

struggle to produce anything much bigger than Jupiter’s Ganymede, the largest moon our solar system managed to make. According to the 1997 study, such a moon would need to bulk up fourfold or fivefold to hang on to a permanent atmosphere. How to Make a Moon Scientists don’t expect gaseous Jupiter-like planets to harbor much life, but if such a planet is home to a sufficiently large moon, the moon just might. A fertile moon would have to be massive enough to gravitationally hold on to a thin atmosphere, however. Different methods of moon formation can lead to moons of vastly different sizes. Lumps from a Disk Example: Jupiter’s moons Planets are thought to form out of a disk of dust, gas and ice spinning around a star. Around these young planets additional disks might form, like eddies in a current (1). Over millions of years these secondary disks of matter clump into rings and moons (2 and 3). Yet these processes can build moons only as big as Jupiter’s Ganymede— not large enough to hold on to an atmosphere.

Massive Collision Example: Earth’s moon Soon after Earth formed, astronomers believe it was struck by a Mars-size object (1). The resulting cataclysm spit out a shower of rock and iron (2) that, over time, cooled and turned into the moon (3). In theory, such collisions could result in two objects that are nearly equal in size. In this double-planet scenario, the “moon” would be just as big as its “planet.”

Binary Capture Example: Neptune’s moon Triton Once a double-planet system forms—perhaps by collision—the pair could encounter another, larger planet (1). As the pair flies by (2), the larger planet could pull in one of the objects and fling the other off into space (3). Captured moons that come from binary planet systems could also be relatively large.

Illustrated by Ron Miller

Fortunately, nature has devised other ways to make massive moons. Earth’s moon, for example, is too large to have quiescently formed alongside our planet from a shared disk of gas and dust. Many astronomers think, instead, that our Earth-moon system was forged out of a cataclysmic collision early in our solar system’s history. Pluto and its largest moon, Charon, are thought to be another collision-forged duo, albeit on a much smaller scale. These pairs could account for other types of moons. In so-called binary-exchange reactions, a giant planet encountering a binary pair captures one member as a moon while the other member gets ejected into space. This kind of exchange has happened at least once before in our solar system: Neptune’s biggest moon, Triton, has a bizarre orbit that moves in the opposite direction of the giant planet’s rotation. Astronomers believe that Triton is the captured remnant of a binary pair that Neptune tore apart long ago. These large moons could potentially support liquid water—and thus life—even if they orbit a planet located outside of a star’s habitable zone. Extra warmth could come from the reflected light and emitted heat of a host planet, as well as the planet’s gravitational pull. Just as the moon raises tides in Earth’s oceans, the gravitational tug of a gas giant could send tidal energy rippling through a nearby moon, flexing the lunar interior and pumping it full of frictional heat. The effect is akin to heating up a metal paper clip by bending it back and forth in your hand. Indeed, if a moon orbits too close to its gas giant

planet, it could experience so much tidal heating that it boils off its atmosphere or melts into a glowing ball of slag, according to recent work by René Heller of McMaster University and Rory Barnes of the University of Washington. In wider lunar orbits, just the right amount of tidal heating could keep moons comfortably toasty, even if the planet is far from its star’s warming rays. Tidal forces could also change a moon’s orbit so that it would eternally present only one hemisphere toward its host planet, just as the moon does to Earth. Envisioning the night skies of such tidally locked worlds, Heller says, yields a deeply strange picture. “Imagine, for example, standing on the planet-facing hemisphere of a tidally locked moon,” he says. “The planet would be huge and would not move in the sky. At ‘noon’ on the moon, which corresponds to the point in its orbit where the star would be highest in the sky, the star would pass behind the planet, and there would be no reflected light from the planet. You would see stars all around but only a black disk directly overhead. At ‘midnight,’ when the moon’s orbit would be taking the star beneath your feet, the planet’s illuminated face would shift from a crescent to converge on a full circle, and you’d get all that reflected light. So at midnight, your sky would be much brighter than at noon.” Search Strategy Moons large enough to hold on to an atmosphere should, in theory, be visible in data from the Kepler satellite. Since its launch in 2009 until gyroscope problems cut short the mission last year, Kepler gazed unceasingly at a single patch of sky, continuously monitoring the brightness of more than 150,000 target stars. It searched for planets by detecting transits: shadows cast toward our solar system as planets crossed the faces of their suns. Each transit manifests as a distinct, recurring dip in a star’s “light curve,” its brightness plotted over time. The smallest planet Kepler has found so far, Kepler-37b, is exceedingly small—only slightly larger than Earth’s moon. According to Kipping, if Kepler can find moon-size planets, it should also be able to find planet-size moons. Yet even though Kipping is combing through Kepler’s data for signs of them, he is not a member of the Kepler team, nor is his project affiliated with the NASA mission. In fact, anyone could do what he is doing: the Kepler data are publicly available. Astronomers and hobbyists alike have already discovered new planets by sifting through the voluminous data set. Kipping’s everyman approach extends to fund-raising as well—he raised $12,000 on a crowdfunding Web site to buy CPUs, which are now part of the Michael Dodds Computing Facility, named for the most generous donor. Kipping’s search strategy is founded on a counterintuitive quirk of gravitational interactions: in a sense, moons orbit planets, but planets also orbit moons. More strictly, a planet and a moon actually orbit a shared center of mass, so that as a moon whips around a planet, the planet wobbles back and forth. Imagine that you are looking out at a distant moon-planet system. If the moon swings around to the right of the planet, the planet, orbiting the same center of mass, will shift a little bit to the left. Now imagine that moon-planet system transiting left to right across the face of the star. The planet will be left of where it would be if it did not have a lunar companion. This leftward shift, in a left-to-right-moving planet, will delay the onset of the transit by perhaps a few minutes. On the same system’s next transit, the moon may be on the other side of its orbit, slightly shifting the position of the planet to the right and advancing the planet’s transit a few minutes early. In addition to these shifts in the onsets of transits, a circling moon can alter the transit’s total duration. Unfolding over multiple orbits, this to-and-fro temporal waltz of fluctuating transit properties is an exomoon’s expected calling card. In addition to these timing effects, a sufficiently large moon could block a star’s light, adding its own minuscule dip to a transiting planet’s signal. The combined planet-moon dip would look much like the signal from an ordinary planet, except for the fact that occasionally the moon would pass directly in front of or behind the planet. The eclipsed moon-planet system would not block quite as much light. Astronomers could use this variation to infer the presence of the hidden moon. Yet searching for any of these subtle effects has its challenges. A small dip in starlight from a transiting exomoon could just as plausibly be caused by more prosaic phenomena. Every modulation of the light curves so far has been best explained by simple things such as star spots, stellar fluctuations and instrumental errors. Worse, a single timing signature could be produced by a wide range of possible planet-moon arrangements that varied in details such as the size of the moon and the period and inclination of its orbit. This inherent uncertainty makes it quite difficult to characterize any given exomoon through timing alone. Yet if astronomers manage to pin down a planet-moon system’s orbital configuration through timing effects, as well as the moon’s dip in a light curve, they can establish masses for the system’s moon, the

planet and the star. By pairing those masses with size estimates based on how much starlight a planet or moon blocks, astronomers can infer each object’s density, creating a window into the composition, formational history, and potential habitability of planets and their moons. With careful scrutiny of transit after transit for any given system, even more faint details can coalesce from those fluctuations of starlight. “It’s amazing how much can be packed into a light curve,” Kipping muses in his office, several floors above the subterranean computer room. “What happens if a transiting planet or moon is slightly oblate or if it has rings? What happens if a world’s atmosphere refracts and bends the starlight passing through? These sorts of effects can be salient in the data. It’s incredibly satisfying to look up at the stars, these twinkling pinpricks of light in the sky at night, and know that we’re able to take this simple measurement of brightness and turn it into all this more complex information.” To tease out the presence of a moon orbiting any particular transiting planet, Kipping’s HEK project first makes a guess. What would the light curve look like if a moon were orbiting this particular planet? The HEK algorithm generates a very large number of artificial light curves from hypothetical, virtual planet-moon systems that possess a wide variance of masses, radii and orbits. Next it sifts through the Kepler data for matches, gradually homing in on any statistically plausible lunar signals. This exhaustive trial-and-error process is why HEK requires so much computing power. It is also why Kipping prefers to carefully select just the very best targets from Kepler’s gargantuan hoard of planets and candidates. Most of those targets are low-mass, Neptune-size worlds that orbit fairly close to a sunlike host star, completing an orbital lap in six months or less. Such planets would manifest the clearest signals of an accompanying large moon. The project also plans to examine transiting planets around red dwarf stars, which are far smaller, dimmer and more numerous than stars like our sun. The small sizes mean that a transiting planet will block a higher percentage of the star’s total light. The relatively dim output moves the habitable zone close to the star; any planet orbiting at that radius would have to whip around quickly, giving astronomers more transits to work with. “For us, everything gets better with these stars,” Kipping says. “In the very best cases, we could probably detect a moon only a tenth or a fifth of an Earth-mass.” In perhaps the very worst case, HEK will detect no exomoons at all, a prospect that would at least allow Kipping and his colleagues to set upper limits on how many planets harbor large moons. Already we know something about what is not there. “If there were lots of really big moons, like a two-Earth-radius moon around a Jupiter-size transiting planet, you could just look by eye at the light curve and see the moon’s effect,” says University of Florida astronomer Eric Ford. “So there’s a good chance if that was in the Kepler field, someone would’ve found it by now or be hot on its trail.” After further analysis, Kipping’s team has ruled out the possibility that Kepler-22b, one target of the early investigations, has a moon larger than about half the size of Earth. Other astronomers, such as Eric Agol of the University of Washington, remain skeptical that Kepler’s current data set can deliver verifiable exomoons, particularly via temporal effects alone. “My opinion is that a believable detection is going to require actually seeing the transit of a moon,” Agol says. “But that’s at the very hairy edge of what Kepler can do. Of course, nature can always surprise us.” Despite his doubts, Agol acknowledges that he and a few other collaborators are pursuing an unofficial search of their own, one that, in comparison to HEK, uses less intensive computation to seek more obvious effects in a larger number of Kepler light curves. “My feeling is our search should be around every planet that’s been detected, within reason,” Agol says. Lunar Lenses Kipping points out that moons can increase the chance for life in more than one way. For example, he says, without the moon, Earth’s climate and seasons could be quite different because on astronomical timescales the moon helps to stabilize our planet’s tilt. What is more, before the moon spiraled out to its present orbital distance from our world, its enormous tidal effects on the early Earth could have played a vital role in the origin and flourishing of life. “When we find an Earth-size planet in the habitable zone, one of the first questions will be, ‘Well, does it have a moon?’” Kipping says. The answer to that question will help determine whether a planet is a true Earth twin or merely a cousin with a vague family resemblance. “I wonder if our own is a fluke or if things like it are really common,” he adds. “With a sample size of one, we can’t really know the answer. If we find some outside our solar system, we’ll get a better idea.” Through the right kind of telescopic eyes, ones well beyond Kepler’s capabilities, an exomoon could do far more than simply signpost a promising mirror Earth orbiting a nearby star. Whether observing an Earth-size transiting planet or an Earth-size transiting moon, Kipping says, a sufficiently large telescope on the ground or in space could investigate that distant world’s atmosphere, looking for markers of life, such as the oxygen that fills our own planet’s skies.

Kipping also thinks some exomoons could be used to map the surfaces of their host planets. Astronomers already use transiting planets to map the surfaces of stars by carefully monitoring the star’s brightness as the planet crosses its face. “When a moon passes in front of a planet as seen from Earth, you’re getting the same opportunity, but now you’re looking at the surface brightness of the planet,” he explains. “So, potentially, using something very sophisticated, you could begin mapping an Earth twin’s continents, its water distribution, all from how the light curve changes shape as the moon passes over. Sometimes I think that’s the most likely way we’ll ever get anything like a photograph of one of these potentially habitable planets. This could be the first, smallest slice of a very big pie.” --Originally published: Scientific American 310(1); 38-43( (January 2014).

Extraterrestrial Intelligence by John Matson Fifty years ago a young astronomer, indulging in a bit of interstellar voyeurism, turned a telescope on the neighbors to see what he could see. In April 1960 at the National Radio Astronomy Observatory in Green Bank, W.Va., Frank Drake, then 29, trained a 26-meter-wide radio telescope on two nearby stars to seek out transmissions from civilizations possibly in residence there. The search came up empty, but Drake’s Project Ozma began in earnest the ongoing search for extraterrestrial intelligence, or SETI. Drake, who turned 80 in May, is still at it, directing the Carl Sagan Center for the Study of Life in the Universe at the nonprofit SETI Institute in Mountain View, Calif. Instead of just borrowing time from other astronomical instruments, those in the field now have purpose-built tools at their disposal, such as the fledgling Allen Telescope Array (ATA) in Hat Creek, Calif. But funding is scarce—the ATA growth stalled at 42 dishes of a planned 350—and astronomers have not yet gathered enough data to make firm pronouncements about intelligent life in the universe. “Although we have ‘been doing it’ for 50 years, we have not been on a telescope very much of that time,” says Jill Tarter, director of the Center for SETI Research at the SETI Institute. “What we can say is that every star system in the galaxy isn’t populated by a technology that’s broadcasting radio signals at this time.” Theoretical astrophysicist Alan P. Boss of the Carnegie Institution for Science agrees. “The lack of a SETI signal to date simply means that civilizations that feel like broadcasting to us are not so common that the limited SETI searches would have found one,” Boss says. “There is still a lot of the galaxy that has not yet been searched.” One of the most extensive campaigns to date, Project Phoenix, surveyed nearby stars across a wide range of frequencies using some of the world’s largest radio telescopes. In nine years Phoenix sampled roughly 800 stars, less than one millionth of 1 percent of the Milky Way. Even for stars that have been scanned, the parameters for a possible signal are frustratingly numerous. Like those for terrestrial radio, they include frequency (what station does it broadcast on?), time (24/7 or midnight sign-off?), type of modulation (AM or FM?), and so on. “At the very least this search is ninedimensional,” Tarter says, “and we could guess right about what to look for and build the right instrument for eight of those dimensions, but we could still miss it because we got one wrong.” Arguments for SETI and for widespread life in general have been bolstered by the confirmation that planetary systems are common around other stars. Most of the more than 1,000 exoplanets are scalding giants inhospitable to life as we know it. But in the next few years NASA’s Kepler space telescope, now surveying more than 100,000 stars for planets, should settle the question of how common Earth-like planets are. Even on Earth-like worlds, however, technological, radio-broadcasting life may not be common. Many researchers hold out more hope for finding simpler life-forms, such as microbes or slime molds. Boss says that life of this kind should be widespread, but we will not have the technology to detect it for two decades at best. But what if someone does pick up a signal from an intelligent civilization? The SETI community has protocols in place, such as alerting observatories around the world for verification, but the same cannot be said of the world’s governments. A United Nations–level framework does not yet exist to guide the contentious next steps—if we hear a shout from a potentially hostile neighbor, do we dare shout back? It would not be an entirely new experience for Drake, who as a graduate student thought he had made a detection. “You feel a very special emotion if you think that has happened, because you realize everything is going to change,” he says, noting that we would soon be enriched with new knowledge of other worlds, species and cultures. “It’s an emotion you have to feel to understand, and I felt it.” --Originally published: Scientific American 302(6); 40 (June 2010).

Contact: The Day After by Tim Folger One day in 2010 Frank Drake returned to the observatory at Green Bank, W.Va., to repeat a search he first conducted there in 1960 as a 30-year-old astronomer. Green Bank has the largest steerable telescope in the world—a 100-meter-wide radio dish. Drake wanted to aim it at the same two sunlike stars he had observed more than 50 years ago, Tau Ceti and Epsilon Eridani, each a bit more than 10 light-years from Earth, to see if he could detect radio transmissions from any civilizations that might exist on planets orbiting either of the two stars. This encore observing run was largely ceremonial for the man who pioneered the worldwide collaborative effort known as SETI—the search for extraterrestrial intelligence. As a young man, Drake had half-expected to find a cosmos humming with the equivalent of ET ham radio chatter. The elder Drake did not expect any such surprises from Tau Ceti or Epsilon Eridani. The Great Silence, as some astronomers call the absence of alien communiqués, remains unbroken after five decades of searching. And yet so does Drake’s conviction that it is only a matter of time before SETI succeeds. “Fifty years ago, when I made the first search, it took two months—200 hours of observing time at Green Bank,” says Drake, who is now chairman emeritus at the SETI Institute in Mountain View, Calif. “When I went back, they gave me an hour to repeat the experiment. That turned out to be way too much time. It took eight tenths of a second—each star took four tenths of a second! And the search was better. I looked at the same two stars over a much wider frequency band with higher sensitivity and more channels, in eight tenths of a second. That shows how far we’ve come. And the rate of improvement hasn’t slowed down at all.” Computer-processing power has roughly doubled every two years for the past 50. Drake and other SETI scientists believe that within 30 years or so, computing advances will allow them to sift through enough frequencies from enough of the 200 billion stars in our galaxy to have a reasonable shot at finding a signal from an extraterrestrial civilization. “My guess—and ‘guess’ is the right word—is that the number of detectable civilizations in our galaxy right now is 10,000,” Drake says. “That means one of every some millions of stars has a detectable civilization.” His estimate, he adds, assumes an average life span of about 10,000 years for a technological civilization. “In 20 or 30 years we will be able to look at 10 million stars. That’s the challenge, even though it’s based on a guess.” Drake may be too conservative, says Seth Shostak, senior astronomer at the SETI Institute. “If this experiment has merit, it’s going to succeed within two or three decades,” he says. “If it doesn’t, then there’s something fundamentally wrong in our assumptions. If it’s going to happen, it’s going to happen soon.” Drake and Shostak could, of course, be wildly off base. It is not hard to find astronomers who would peg the number of civilizations in our galaxy at one—our own. But if Drake and Shostak are right—if we are within a few decades of discovering that we are not alone in the universe—what then? What happens after we detect a signal from an alien intelligence? Could we even translate the message? How likely is it that the message might contain knowledge that would transform our culture? Would it be dangerous to respond and reveal our existence to beings from other worlds? One thing that definitely won’t happen if SETI scientists discover such a signal is a government coverup or any sort of conspiratorial secrecy. The world will learn the news almost immediately. Shostak is certain of this. So is Jill Tarter, director of the SETI Institute’s research center. They know exactly how events will unfold when they finally find a signal because on a June morning 13 years ago, they thought they had received one. Dress Rehearsal It happened at about 6 a.m. Tarter was at the Green Bank observatory when the signal came in. It was bunch of signals at discrete frequencies, with uniform spacing between them, which looked on a graph like a comb. “It was clearly an engineered signal,” she says. Tarter and her colleagues at Green Bank followed their protocols to rule out false alarms. They swung the telescope away from the target star. The signal vanished. They aimed at the star again. The signal came back. Ordinarily they would have verified

the precise origin of the signal with a separate telescope at an observatory in Woodbury, Ga. But lightning had recently struck that telescope and fried its hard drive. “It was rural Georgia, and it took about three days to get FedEx in there with a replacement drive,” Tarter says. “In the meantime, we had telescope time in West Virginia”—SETI observations typically piggyback on other, mainstream astronomical research—“and we were going to use it. Without our second site, the only thing we could do was nod back and forth between two different stars.” Tarter, who had been scheduled to fly home to California at noon that day, canceled her flight and left a phone message for Chris Neller, her assistant in Mountain View, to tell her about the change in plans. By late afternoon the target star that was thought to be the source of the signal began to set below the horizon. That is when Tarter and her team realized something was wrong. Although the target star was setting, the source of the signal seemed to be climbing, its strength undiminished. The signal, they eventually determined, was coming from a NASA satellite, the Solar and Heliospheric Observatory, or SOHO. During all the excitement, no one remembered to call the Mountain View office to tell them the whole episode had been a false alarm. Meanwhile Ann Druyan, Carl Sagan’s widow, had by chance called Mountain View to talk with Tarter about something unrelated. Neller told Druyan that Tarter was delayed at the Green Bank observatory, studying what might be a signal from an extraterrestrial civilization. Druyan immediately called William J. Broad, a science reporter for the New York Times. Broad in turn called Shostak to confirm the story. “The beauty of a false alarm is that you see what really happens,” Shostak says. “It’s no longer theoretical. Not a false alarm that lasts five minutes, but where for the better part of a day you think, maybe this is it. You have these nifty protocols, but what really happens? People don’t follow protocols. It’s not that people do anything mischievous or malevolent—you’re so caught up in the excitement of the moment, the media are immediately calling you on the phone, people send e-mails to their friends.” In the event of a signal that survives initial scrutiny—one that is quickly verified by a second observatory—the astronomers who made the discovery would send an International Astronomical Union (IAU) telegram—now delivered as an e-mail—to observatories around the world. Astronomers use IAU telegrams to notify one another of time-sensitive observations: supernovas, comets or gamma-ray bursts. Tarter says a SETI observation would be treated like any other astronomical discovery. “If something like that happens, we’ll want everybody who can to look at it right away,” she says. “We’d like people to look in the signal’s direction, with different tools, checking different frequencies, and try to figure it out.” Verifying the Message SETI scientists think they know, in broad terms, what an ET signal will look like. To stand out as obviously artificial against a background of natural cosmic radio emissions, the signal would have to be narrow, with a lot of energy packed into a few frequencies. Natural phenomena, such as pulsars and interstellar gases, spew out radio emissions at many different frequencies. If an observatory ever receives a narrowband signal coming from an astronomical distance, the source would almost certainly be artificial. According to voluntary, nonbinding protocols adopted by SETI researchers around the world, if IAU astronomers confirmed a signal as genuine, they would then notify the United Nations and various world leaders. Tarter says that some generous SETI sponsors would also receive discreet thank-you calls. At that point the astronomers who made the discovery would be free to hold a press conference—if the story had not already been leaked. But even those modest constraints would probably be breached. “The protocols are just a nice idea,” Shostak says. “They’re like red lights in Naples, Italy,” he laughs. “They’re suggestions.” What happens next? A triumphant announcement, followed by headlines? Panic? New Age celebrations of galactic harmony? Probably none of the above, except for the headlines, if Douglas Vakoch is right. A psychologist by training, Vakoch has an office across the hall from Tarter at the SETI Institute and must have the world’s most unusual job title: director of interstellar message composition. “While we may be able to detect that there is a signal that at least initially appears distinctly artificial, I suspect even that claim would be questioned,” Vakoch says. “There would be a lot of people trying to come up with natural explanations. I think the assumption that one day someone is going to announce that we’ve discovered extraterrestrial intelligence, and now the world knows, is a fallacy, because there’s going to be much more ambiguity in the process. It might be similar to what was recently considered a plausible claim that there was evidence of fossils in Martian meteorites—interesting enough to consider, but now let’s look at this over the next few months.” Even if the signal is confirmed as an authentic transmission from an extraterrestrial civilization, it is unlikely that astronomers would be able to extract any information from it for many years. SETI’s

instruments are designed to search for steady, periodic narrowband radio pulses—carrier waves powerful enough to be detectable across many light-years. The pulse itself would yield no information, other than its artificial nature. Any message content would likely be in the form of changes in amplitude or frequency buried within the pulse. Even a large radio telescope would need to repeatedly scan a small patch of sky to build up the signal pulse above background radio noise. In doing so, it would average out modulations on finer time-scales that might contain a message. Resolving the message would require an antenna far more powerful than Earth’s largest, the 305-meter dish at Arecibo, Puerto Rico. “You would need something on the order of 10,000 times bigger than Arecibo,” Shostak says. Rather than a single enormous dish, such a telescope would probably consist of many smaller antennas spread across a large area and linked electronically. Constructing such an instrument would require international collaboration and funding, with no guarantee that the message—if the signal contained one—could ever be deciphered. “That’s not something you’d do overnight,” Shostak observes. “That’s a big project. I think we would do it, because—gosh darn it—we would want to know what they’re saying.” The Fallout Taking into account political debates and the time needed to build a telescope sensitive enough to analyze the signal, years would pass before astronomers or cryptographers could begin to attempt to decipher a message from the stars. So whereas that first contact with another intelligence would in itself be one of the most important scientific discoveries of all time, the lack of any further knowledge about the nature of that alien intelligence would limit the immediate cultural impact. The story of the discovery would monopolize headlines for a while, but our collective attention span would inevitably move on while scientists sought to translate the message. “I have no doubt that the receipt of such a message would be a huge and genuinely exciting moment,” says Charles T. Rubin, a political philosopher at Duquesne University who studies the social issues raised by SETI research. “But I don’t think it would cause a great cultural shift, because the notion of extraterrestrials is common both in popular culture and in scientific circles. It would confirm what many already suspect to be true.” If some nation or group of nations decides to build an instrument that would give us a shot at cracking an extraterrestrial message, how likely is it that we would succeed? Sagan, an early advocate of SETI, imagined that we might receive an Encyclopedia Galactica, filled with the accumulated wisdom of many advanced extraterrestrial civilizations. Some SETI researchers assumed—and still assume—that the language of science might provide common ground for communication. Kathryn Denning, an anthropologist at York University in Toronto and a member of the SETI Post-Detection Task group, is less sanguine. “We run into an irreducible problem with communication that isn’t face to face, and that is the problem of establishing a referent,” Denning says. “If you and I speak different languages, and we’re in the same room, I can point to a table, and I can say ‘table,’ and you infer that ‘table’ is my word for that thing, and then we can go from there. That’s the time-honored way of learning languages. If you’re not in direct contact, if you can’t do that kind of pointing exercise, there’s always this question of what you’re referring to in these initial communications. Scientists—physical scientists and mathematicians in particular—tend to be more prone to thinking that because we’ll be dealing with the same physical structures in the universe, we can use those as our Rosetta Stone, so to speak, and build up from there— send each other the value of pi, and then we’re off to the races. But anthropologists tend not to be so comfortable with that. Errors can take place right at the get-go. For example, if I give you a signal—beep, beep, beep—is that three or two? Are we counting the beeps or the spaces? We have fundamental assumptions built in.” John R. Elliott, a researcher at Leeds Metropolitan University in England who studies artificial intelligence and the structure of languages, is already preparing for the day we receive the first extraterrestrial message. Even if it proves impossible to directly translate the message, it might be possible to discover patterns that Elliott suspects are fundamental to all languages. Those patterns might reveal something about the nature of the beings who sent the message, particularly how their level of intelligence compares with our own. Elliott has devised a computer program that compares any unknown language with a database of 60 human languages. All languages, he says, share what he calls functional elements—words such as “if,” “and” and “but”—that break up the complexity of a language into manageable chunks. The length of those chunks—the nouns, verbs and other words contained between the functional elements—provides a measure of our cognitive abilities. “It opens a window on our way of embedding information, the way we structure our sentences,” Elliott notes. “It shows the constraints on us as intelligent authors.” Elliott says his computer program shows that the functional elements in all human languages are typically separated by no more than about nine words. Assuming an ET signal arrives as a binary stream of ones and zeros, his program would search for patterns in the message and attempt to identify the

occurrence of functional elements. The program would, ideally, give us a rough measure of alien IQ by comparing the average interval between our "ifs", "ands" and "buts" with theirs. “Anything above 10 means it would exceed human cognition,” he says. Elliott thinks he could determine whether a signal bears the characteristics of a language within a few days; he might be able to tell if it contains images. “For the semantic side? We might never interpret it.” Worth the Risk? Some SETI proponents suggest we should do more than passively wait for a signal. They believe we should transmit messages and let anyone who might be listening know that we are here. Last spring, in a Discovery Channel series, Stephen Hawking of the University of Cambridge said that transmitting messages without knowing what is out there could be dangerous. He warned of the possibility of predatory aliens ravaging the resources of world after world. “If aliens visit us,” he said, “the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans.” The SETI community seems to be divided about the wisdom of sending messages versus quietly biding our time. But in any case, it is probably too late. Radio and television signals have been leaking from our planet for decades now. “I Love Lucy has already passed 10,000 stars,” says Dan Wertheimer, a SETI researcher at the University of California, Berkeley, who helped to develop the SETI@Home project, which allows anyone to download software to a home computer to help process SETI data. Moreover, there is no reason why an extraterrestrial civilization couldn’t spot Earth using the same—or better— techniques that terrestrial astronomers are already using to find planets around other stars. Geoffrey W. Marcy, an astronomer at Berkeley who has played a leading role in the discovery of dozens of extrasolar planets, says that by the end of this century, space-based telescopes will enable us to map the continents and oceans of these worlds. And if we will soon be doing that, it is likely that extraterrestrial civilizations —if any exist—are doing it, too. “Aliens who have a mere 1,000-year head start on us could listen to our conversation right now,” Marcy says. “They could read our lips. So this passive versus active thing makes no sense to me. We can’t hide— that’s crazy! Any more than ants can hide from us humans. It would be like one ant talking to another ant, ‘Oh, we’d better not talk because the humans would know we’re here, and they might step on us.’ No, sorry, guys, you ants can’t hide from us!” Drake believes Hawking’s fears are unfounded, largely because interstellar travel may be practically impossible, which he believes also answers the Fermi paradox, named for Enrico Fermi, the Italian physicist who first posed it: If extraterrestrial civilizations exist, why haven’t we seen them yet? Given the age of the galaxy, and its 200 billion stars, surely at least one civilization should have colonized the galaxy by now. Drake demurs. “To give you an idea why even a very small mission won’t work, just imagine a spacecraft the size of a 737 airplane, with perhaps 50 passengers. Suppose the nearest star with a habitable planet is only 10 light-years away, which is quite close— there are only a few stars that close. And assume you can go 10 percent the speed of light. Why that number? It never gets mentioned in all these discussions about space travel, but if you’re going a little faster than that, about 12 percent the speed of light, if you impact a pebble, the energy in that impact is equal to what that same mass would release if it was used in a nuclear fusion bomb. It would blow up the spacecraft. One pebble in the whole trip ends the mission.” But Drake believes that SETI’s limited funds should go to searching, not broadcasting. Marcy says the Fermi paradox presents a genuine problem for SETI researchers, and he sees only three possible solutions. “The fact that aliens haven’t landed tells you they’re rare, or that space travel is very hard, or that it’s not just worth doing.” Perhaps Hawking’s fears say more about us than about any aliens we might encounter. Given the history of our own species, who would have more to fear from contact, humans or extraterrestrials? SETI, unavoidably, reflects our own dreams and night terrors about our place in the universe. In postulating the presence of civilizations on other worlds, we are extrapolating wildly from a single known example—our own fragile, remarkable existence. Realistically, though, the quest to make contact will be an endeavor that spans centuries—if our own civilization lasts that long. SETI is, perhaps, the strangest and most profound experiment in the history of our world. One of the founding fathers of SETI, the late Philip Morrison, a physicist at the Massachusetts Institute of Technology, likened the SETI project to the medieval and Renaissance recovery of the knowledge of classical antiquity, in which scholars labored for generations. The patient transcribing of ancient texts revealed a world that had been lost and eventually transformed the world the scholars thought they knew. One day we may learn that we are not alone and, indeed, that intelligence is common in the universe. “If SETI succeeds, then intelligence happened in at least one other place,” Shostak says. “So it probably

happened in lots of places. In astronomy, the only numbers are one, two and infinity. So if you get two, there are probably lots more. It’s like finding two elephants.” --Originally published: Scientific American, 304(1); 40-45 (January 2011).

The Color of Plants on Other Worlds by Nancy Y. Kiang The prospect of finding extraterrestrial life is no longer the domain of science fiction or UFO hunters. Rather than waiting for aliens to come to us, we are looking for them. We may not find technologically advanced civilizations, but we can look for the physical and chemical signs of fundamental life processes: “biosignatures.” Beyond the solar system, astronomers have discovered more than 200 worlds orbiting other stars, so-called extrasolar planets. Although we have not been able to tell whether these planets harbor life, it is only a matter of time now. Last July astronomers confirmed the presence of water vapor on an extrasolar planet by observing the passage of starlight through the planet’s atmosphere. The world’s space agencies are now developing telescopes that will search for signs of life on Earth-size planets by observing the planets’ light spectra. Photosynthesis, in particular, could produce very conspicuous biosignatures. How plausible is it for photosynthesis to arise on another planet? Very. On Earth, the process is so successful that it is the foundation for nearly all life. Although some organisms live off the heat and methane of oceanic hydrothermal vents, the rich ecosystems on the planet’s surface all depend on sunlight. Photosynthetic biosignatures could be of two kinds: biologically generated atmospheric gases such as oxygen and its product, ozone; and surface colors that indicate the presence of specialized pigments such as green chlorophyll. The idea of looking for such pigments has a long history. A century ago astronomers sought to attribute the seasonal darkening of Mars to the growth of vegetation. They studied the spectrum of light reflected off the surface for signs of green plants. One difficulty with this strategy was evident to writer H. G. Wells, who imagined a different scenario in The War of the Worlds: “The vegetable kingdom in Mars, instead of having green for a dominant colour, is of a vivid blood-red tint.” Although we now know that Mars has no surface vegetation (the darkening is caused by dust storms), Wells was prescient in speculating that photosynthetic organisms on another planet might not be green. Even Earth has a diversity of photosynthetic organisms besides green plants. Some land plants have red leaves, and underwater algae and photosynthetic bacteria come in a rainbow of colors. Purple bacteria soak up solar infrared radiation as well as visible light. So what will dominate on another planet? And how will we know when we see it? The answers depend on the details of how alien photosynthesis adapts to light from a parent star of a different type than our sun, filtered through an atmosphere that may not have the same composition as Earth’s. Harvesting Light In trying to figure out how photosynthesis might operate on other planets, the first step is to explain it on Earth. The energy spectrum of sunlight at Earth’s surface peaks in the blue-green, so scientists have long scratched their heads about why plants reflect green, thereby wasting what appears to be the best available light. The answer is that photosynthesis does not depend on the total amount of light energy but on the energy per photon and the number of photons that make up the light.

Credit: Lisa Applebacher

Whereas blue photons carry more energy than red ones, the sun emits more of the red kind. Plants use blue photons for their quality and red photons for their quantity. The green photons that lie in between have neither the energy nor the numbers, so plants have adapted to absorb fewer of them. The basic photosynthetic process, which fixes one carbon atom (obtained from carbon dioxide, CO2) into a simple sugar molecule, requires a minimum of eight photons. It takes one photon to split an oxygen-hydrogen bond in water (H2O) and thereby to obtain an electron for biochemical reactions. A total of four such bonds must be broken to create an oxygen molecule (O2). Each of those photons is matched by at least one additional photon for a second type of reaction to form the sugar. Each photon must have a minimum amount of energy to drive the reactions. The way plants harvest sunlight is a marvel of nature. Photosynthetic pigments such as chlorophyll are not isolated molecules. They operate in a network like an array of antennas, each tuned to pick out photons of particular wavelengths. Chlorophyll preferentially absorbs red and blue light, and carotenoid pigments (which produce the vibrant reds and yellows of fall foliage) pick up a slightly different shade of blue. All this energy gets funneled to a special chlorophyll molecule at a chemical reaction center, which splits water and releases oxygen. The funneling process is the key to which colors the pigments select. The complex of molecules at the reaction center can perform chemical reactions only if it receives a red photon or the equivalent amount of energy in some other form. To take advantage of blue photons, the antenna pigments work in concert to convert the high energy (from blue photons) to a lower energy (redder), like a series of step-down transformers that reduces the 100,000 volts of electric power lines to the 120 or 240 volts of a wall outlet. The process begins when a blue photon hits a blue-absorbing pigment and energizes one of the electrons in the molecule. When that electron drops back down to its original state, it releases this energy—but because of energy losses to heat and vibrations, it releases less energy than it absorbed. The pigment molecule releases its energy not in the form of another photon but in the form of an electrical interaction with another pigment molecule that is able to absorb energy at that lower level. This pigment, in turn, releases an even lower amount of energy, and so the process continues until the original blue photon energy has been downgraded to red. The array of pigments can also convert cyan, green or yellow to red. The reaction center, as the receiving end of the cascade, adapts to absorb the lowestenergy available photons. On our planet’s surface, red photons are both the most abundant and the lowest energy within the visible spectrum. For underwater photosynthesizers, red photons are not necessarily the most abundant. Light niches change with depth because of filtering of light by water, by dissolved substances and by overlying organisms themselves. The result is a clear stratification of life-forms according to their mix of pigments. Organisms in lower water layers have pigments adapted to absorb the light colors left over by the layers

above. For instance, algae and cyanobacteria have pigments known as phycobilins that harvest green and yellow photons. Nonoxygen-producing (anoxygenic) bacteria have bacteriochlorophylls that absorb farred and near-infrared light, which is all that penetrates to the murky depths. Organisms adapted to low-light conditions tend to be slower-growing, because they have to put more effort into harvesting whatever light is available to them. At the planet’s surface, where light is abundant, it would be disadvantageous for plants to manufacture extra pigments, so they are selective in their use of color. The same evolutionary principles would operate on other worlds. Just as aquatic creatures have adapted to light filtered by water, land dwellers have adapted to light filtered by atmospheric gases. At the top of Earth’s atmosphere, yellow photons (at wavelengths of 560 to 590 nanometers) are the most abundant kind. The number of photons drops off gradually with longer wavelength and steeply with shorter wavelength . As sunlight passes through the upper atmosphere, water vapor absorbs the infrared light in several wavelength ands beyond 700 nm. Oxygen produces absorption lines—narrow ranges of wavelengths that the gas blocks—at 687 and 761 nm. We all know that ozone (O3) in the stratosphere strongly absorbs the ultraviolet (UV). Less well known is that it also absorbs weakly across the visible range. Putting it all together, our atmosphere demarcates windows through which radiation can make it to the planet’s surface. The visible radiation window is defined at its blue edge by the drop-off in the intensity of short-wavelength photons emitted by the sun and by ozone absorption of UV. The red edge is defined by oxygen absorption lines. The peak in photon abundance is shifted from yellow to red (about 685 nm) by ozone’s broad absorbance across the visible. Plants are adapted to this spectrum, which is determined largely by oxygen—yet plants are what put the oxygen into the atmosphere to begin with. When early photosynthetic organisms first appeared on Earth, the atmosphere lacked oxygen, so they must have used different pigments from chlorophyll. Only over time, as photosynthesis altered the atmospheric composition, did chlorophyll emerge as optimal. The firm fossil evidence for photosynthesis dates to about 3.4 billion years ago (Ga), but earlier fossils exhibit signs of what could have been photosynthesis. Early photosynthesizers had to start out underwater, in part because water is a good solvent for biochemical reactions and in part because it provides protection against solar UV radiation—shielding that was essential in the absence of an atmospheric ozone layer. These earliest photosynthesizers were underwater bacteria that absorbed infrared photons. Their chemical reactions involved hydrogen, hydrogen sulfide or iron rather than water, so they did not produce oxygen gas. Oxygen-generating (oxygenic) photosynthesis by cyanobacteria in the oceans started 2.7 Ga . Oxygen levels and the ozone layer slowly built up, allowing red and brown algae to emerge. As shallower water became safe from UV, green algae evolved. They lacked phycobilins and were better adapted to the bright light in surface waters. Finally, plants descended from green algae emerged onto land— two billion years after oxygen had begun accumulating in the atmosphere. And then the complexity of plant life exploded, from mosses and liverworts on the ground to vascular plants with tall canopies that capture more light and have special adaptations to particular climates. Conifer trees have conical crowns that capture light efficiently at high latitudes with low sun angles; shade-adapted plants have anthocyanin as a sunscreen against too much light. Green chlorophyll not only is well suited to the present composition of the atmosphere but also helps to sustain that composition— a virtuous cycle that keeps our planet green. It may be that another step of evolution will favor an organism that takes advantage of the shade underneath tree canopies, using the phycobilins that absorb green and yellow light. But the organisms on top are still likely to stay green. Painting the World Red To look for photosynthetic pigments on another planet in another solar system, astronomers must be prepared to see the planet at any of the possible stages in its evolution. For instance, they may catch sight of a planet that looks like our Earth two billion years ago. They must also allow that extrasolar photosynthesizers may have evolved capabilities that their counterparts here have not, such as splitting water using longer-wavelength photons. The longest wavelength yet observed in photosynthesis on Earth is about 1,015 nm (in the infrared), in purple anoxygenic bacteria. The longest wavelength observed for oxygenic photosynthesis is about 720 nm, in a marine cyanobacterium. But the laws of physics set no strict upper limit. A large number of longwavelength photons could achieve the same purpose as a few short-wavelength ones. The limiting factor is not the feasibility of novel pigments but the light spectrum available at a planet’s surface, which depends mainly on the star type. Astronomers classify stars based on color, which relates to temperature, size and longevity. Only certain types are long-lived enough to allow for complex life to evolve. These are, in order from hottest to coolest, F, G, K and M stars. Our sun is a G star. F stars are larger, burn brighter and bluer, and take a couple of billion years to use up their fuel. K and M stars are smaller, dimmer, redder and longer-lived.

Around each of these stars is a habitable zone, a range of orbits where planets can maintain a temperature that allows for liquid water. In our solar system, the habitable zone is a ring encompassing Earth’s and Mars’s orbits. For an F star, the habitable zone for an Earth-size planet is farther out; for a K or M star, closer in. A planet in the habitable zone of an F or K star receives about as much visible radiation as Earth does. Such a planet could easily support oxygenic photosynthesis like that on Earth. The pigment color may simply be shifted within the visible band. M stars, also known as red dwarfs, are of special interest because they are the most abundant type in our galaxy. They emit much less visible radiation than our sun; their output peaks in the near-infrared. John Raven, a biologist at the University of Dundee in Scotland, and Ray Wolstencroft, an astronomer at the Royal Observatory, Edinburgh, have proposed that oxygenic photosynthesis is theoretically possible with near-infrared photons. An organism would have to use three or four near-infrared photons to split H2O, rather than the two that suffice for Earth’s plants. The photons work together like stages of a rocket to provide the necessary energy to an electron as it performs the chemical reactions. M stars pose an extra challenge to life: when young, they emit strong UV fl ares. Organisms could avoid the damaging UV radiation deep underwater, but would they then be starved for light? If so, photosynthesis might not arise. As M stars age, though, they cease producing flares, at which point they give off even less UV radiation than our sun does. Organisms would not need a UV-absorbing ozone layer to protect them; they could thrive on land even if they did not produce oxygen. In sum, astronomers must consider four scenarios depending on the age and type of star: Anaerobic, ocean life. The parent star is a young star of any type. Organisms do not necessarily produce oxygen; the atmosphere may be mostly other gases such as methane. Aerobic, ocean life. The parent star is an older star of any type. Enough time has elapsed for oxygenic photosynthesis to evolve and begin to build up atmospheric oxygen. Aerobic, land life. The parent star is a mature star of any type. Plants cover the land. Life on Earth is now at this stage. Anaerobic, land life. The star is a quiescent M star, so the UV radiation is negligible. Plants cover the land but may not produce oxygen. Photosynthetic biosignatures for these different cases would clearly not be the same. From experience with satellite imagery of Earth, astronomers expect that any life in the ocean would be too sparsely distributed for telescopes to see. So the first two scenarios would not produce strong pigment biosignatures; life would reveal itself to us only by the atmospheric gases it produced. Therefore, researchers studying alien plant colors focus on land plants, either on planets around F, G and K stars with oxygenic photosynthesis or on planets around M stars with any type of photosynthesis. Black Is the New Green Regardless of the specific situation, photosynthetic pigments must still satisfy the same rules as on Earth: pigments tend to absorb photons that are either the most abundant, the shortest available wavelength (most energetic) or the longest available wavelength (where the reaction center absorbs). To tackle the question of how star type determines plant color, it took researchers from many disciplines to put together all the stellar, planetary and biological pieces. Martin Cohen, a stellar astronomer at the University of California, Berkeley, collected data for an F star (sigma Bootis), a K star (epsilon Eridani), an actively flaring M star (AD Leo), and a hypothetical quiescent M star with a temperature of 3,100 kelvins. Antígona Segura, an astronomer at the National Autonomous University of Mexico, ran computer simulations of Earth-like planets in the habitable zone of these stars. Using models developed by Alexander Pavlov, now at the University of Arizona, and James Kasting of Pennsylvania State University, Segura studied the interaction between the stellar radiation and the atmosphere’s likely constituents (assuming that volcanoes on these worlds emit the same gases they do on Earth) to deduce the planets’ atmospheric chemistry, both for negligible oxygen and for Earth-like oxygen levels. Using Segura’s results, Giovanna Tinetti, a physicist at University College London, calculated the filtering of radiation by applying a model developed by David Crisp of the Jet Propulsion Laboratory in Pasadena, Calif. (This is one of the models enlisted to calculate how much light reaches the solar panels of the Mars rovers.) Interpreting these calculations required the combined knowledge of five of us: microbial biologist Janet Siefert of Rice University, biochemists Robert Blankenship of Washington University in St. Louis and Govindjee of the University of Illinois at Urbana-Champaign, planetary scientist Victoria Meadows of the University of Washington, and me, a biometeorologist at the NASA Goddard Institute for Space Studies.

We found that the photons reaching the surface of planets around F stars tend to be blue, with the greatest abundance at 451 nm. Around K stars, the peak is in the red at 667 nm, nearly the same as on Earth. Ozone plays a strong role, making the F starlight bluer than it otherwise would be and the K starlight redder. The useful radiation for photosynthesis would be in the visible range, as on Earth. Thus, plants on both F-and K-star planets could have colors just like those on Earth but with subtle variations. For F stars, the flood of energetic blue photons is so intense that plants might need to reflect it using a screening pigment similar to anthocyanin, giving them a blue tint. Alternatively, plants might need to harvest only the blue, discarding the lower-quality green through red light. That would produce a distinctive blue edge in the spectrum of reflected light, which would stand out to telescope observers. The range of M-star temperatures makes possible a very wide variation in alien plant colors. A planet around a quiescent M star would receive about half the energy that Earth receives from our sun. Although that is plenty for living things to harvest—about 60 times more than the minimum needed for shade-adapted Earth plants—most of the photons are near-infrared. Evolution might favor a greater variety of photosynthetic pigments to pick out the full range of visible and infrared light. With little light reflected, plants might even look black to our eyes. Pale Purple Dot The experience of life on Earth indicates that early ocean photosynthesizers on planets around F, G and K stars could survive the initial oxygen-free atmosphere and develop the oxygenic photosynthesis that would lead ultimately to land plants. For M stars, the situation is trickier. We calculated a “sweet spot” about nine meters underwater where early photosynthesizers could both survive UV fl ares and still have enough light to be productive. Although we might not see them through telescopes, these organisms could set the stage for life at the planet’s surface. On worlds around M stars, land plants that exploited a wider range of colors would be nearly as productive as plants on Earth. For all star types, an important question will be whether a planet’s land area is large enough for upcoming space telescopes to see. The first generation of these telescopes will see the planet as a single dot; they will lack the resolution to make maps of the surface. All scientists will have is a globally averaged spectrum. Tinetti calculates that for land plants to show up in this spectrum, at least 20 percent of the surface must be land that is both covered in vegetation and free from clouds. On the other hand, oceanic photosynthesis releases more oxygen to the atmosphere. Therefore, the more prominent the pigment biosignature, the weaker the oxygen biosignature, and vice versa. Astronomers might see one or the other, but not both. If a space telescope sees a dark band in a planet’s reflected light spectrum at one of the predicted colors, then someone monitoring the observations from a computer may be the first person to see signs of life on another world. Other false interpretations have to be ruled out, of course, such as whether minerals could have the same signature. Right now we can identify a plausible palette of colors that indicate plant life on another planet; for instance, we predict another Earth to have green, yellow or orange plants. But it is currently hard to make finer predictions. On Earth, we have been able to determine that the signature of chlorophyll is unique to plants, which is why we can detect plants and ocean phytoplankton with satellites. We will have to figure out unique signatures of vegetation for other planets. Finding life on other planets—abundant life, not just fossils or microbes eking out a meager living under extreme conditions—is a fast approaching reality. Which stars shall we target, given there are so many out there? Will we be able to measure the spectra of M-star planets, which tend to be very close to their stars? What wavelength range and resolution do the new telescopes need? Our understanding of photosynthesis will be key to designing these missions and interpreting their data. Such questions drive a synthesis of the sciences in a way that is only beginning. Our very ability to search for life elsewhere in the universe ultimately requires our deepest understanding of life here on Earth. --Originally published: Scientific American 298(4); 48-55 (April 2008).

SECTION 1 Going the Distance: Interstellar Travel

Reaching for the Stars by Stephanie D. Leifer The notion of traveling to the stars is a concept compelling enough to recur in countless cultural artifacts, from Roman poetry to 20th-century popular music. So ingrained has the concept become that when novelists, poets or lyricists write of reaching for the stars, it is instantly understood as a kind of cultural shorthand for striving for the unattainable. Although interstellar travel remains a glorious if futuristic dream, a small group of engineers and scientists is already exploring concepts and conducting experiments that may lead to technologies capable of propelling spacecraft to speeds high enough to travel far beyond the edge of our solar system. A propulsion system based on nuclear fusion could carry humans to the outer planets and could propel robotic spacecraft thousands of astronomical units into interstellar space (an astronomical unit, at 150 million kilometers, or 93 million miles, is the average distance from Earth to the sun). Such a system might be built in the next several decades. Eventually, even more powerful engines fueled by the mutual annihilation of matter and antimatter might carry spacecraft to nearby stars, the closest of which is Proxima Centauri, some 270,000 astronomical units distant. The attraction of these exotic modes of propulsion lies in the fantastic amounts of energy they could release from a given mass of fuel. A fusion-based propulsion system, for example, could in theory produce about 100 trillion joules per kilogram of fuel—an energy density that is more than 10 million times greater than the corresponding figure for the chemical rockets that propel today’s spacecraft. Matterantimatter reactions would be even more difficult to exploit but would be capable of generating an astounding 20 quadrillion joules from a single kilogram of fuel—enough to supply the entire energy needs of the world for about 26 minutes. In nuclear fusion, very light atoms are brought together at temperatures and pressures high enough, and for long enough, to fuse them into more massive atoms. The difference in mass between the reactants and the products of the reaction corresponds to the amount of energy released, according to Albert Einstein’s famous formula E = mc2. The obstacles to exploiting fusion, much less antimatter, are daunting. Controlled fusion concepts, whether for rocket propulsion or terrestrial power generation, can be divided into two general classes. These categories indicate the technique used to confine the extremely hot, electrically charged gas, called a plasma, within which fusion occurs. In magnetic confinement fusion, strong magnetic fields contain the plasma. Inertial confinement fusion, on the other hand, relies on laser or ion beams to heat and compress a tiny pellet of fusion fuel. In November 1997 researchers exploiting the magnetic confinement approach created a fusion reaction that produced 65 percent as much energy as was fed into it to initiate the reaction. This milestone was achieved in England at the Joint European Torus, a tokamak facility—a doughnut-shaped vessel in which the plasma is magnetically confined. A commercial fusion reactor would have to produce far more energy than went into it to start or maintain the reaction. But even if commercial fusion power becomes a reality here on Earth, there will be several problems unique to developing fusion rockets. A key one will be directing the energetic charged particles created by the reaction to produce usable thrust. Other important challenges include acquiring and storing enough fusion fuel and maximizing the amount of power produced in relation to the mass of the spacecraft. Since the late 1950s, scientists have proposed dozens of fusion rocket concepts. Although fusion produces enormous amounts of very energetic particles, the reaction will accelerate a spacecraft only if these particles can be directed so as to produce thrust. In fusion systems based on magnetic confinement, the strategy would be to feed in fuel to sustain the reaction while allowing a portion of the plasma to escape to generate thrust. Because the plasma would destroy any material vessel it touched, strong magnetic fields, generated by an assembly that researchers call a magnetic nozzle, would direct the charged particles out of the rocket.

In an engine based on the inertial confinement approach, high-power lasers or ion beams would ignite tiny fusion fuel capsules at a rate of perhaps 30 per second. A magnetic nozzle might also suffice to direct the plasma out of the engine to create thrust. The particles created in a fusion reaction depend on the fuels used. The easiest reaction to initiate is between deuterium and tritium, two heavy isotopes of hydrogen whose atomic nuclei include one and two neutrons, respectively, besides a proton. The reaction products are neutrons and helium nuclei (also known as alpha particles). For thrust, the positively charged alpha particles are desirable, whereas the neutrons are not. Neutrons cannot be directed; they carry no charge. Their kinetic energy can be harnessed for propulsion, but not directly—to do so would involve stopping them in a material and making use of the heat generated by their capture. Neutron radiation also poses a danger to a human crew and would necessitate a large amount of shielding for piloted missions. These facts lead to a key difficulty in fusion fuel selection. Although it is easiest to initiate fusion between deuterium and tritium, for many propulsion concepts it would be more desirable to use deuterium and the isotope helium 3 (two protons, one neutron). Fusion of these nuclei produces an alpha particle and a proton, both of which can be manipulated by magnetic fields. The problem is that helium 3 is exceedingly rare on Earth. In addition, the deuterium–helium 3 reaction is more difficult to ignite than the deuterium-tritium reaction. But regardless of the fusion fuel selected, a spacecraft of thousands of tons—much of it fuel—would be necessary to carry humans to the outer reaches of the solar system or deep into interstellar space (for comparison, the International Space Station will have a mass of about 500 tons). Even individually, the key obstacles to fusion propulsion—getting higher levels of power out of a controlled reaction, building effective containment devices and magnetic nozzles, and finding enough fuel —seem overwhelming. Still, for each of them, there is at least a glimmer of a future solution. In the first place, there is every reason to believe that fusion reactors will go far beyond the break-even point, at which a reactor produces as much energy as is fed into it. Inertial confinement work in the U.S. is enjoying robust funding as part of the stockpile stewardship program, in which researchers are working on methods of assuring the safety and reliability of thermonuclear weapons without actually testfiring them. The research is centered at the National Ignition Facility, now under construction at Lawrence Livermore National Laboratory. The facility is expected to start up in 2001, with full laser energy of 1.8 million joules—for four billionths of a second—available in 2003. With that kind of power, researchers anticipate liberating up to 10 times the energy required to initiate the reaction. There are indications, too, that the tokamak, which has dominated magnetic confinement research, may someday be supplanted by more compact technologies more amenable to rocket propulsion. In 1996 the Fusion Energy Sciences Advisory Committee of the U.S. Department of Energy endorsed investigation of such promising magnetic confinement schemes as reverse-field pinches, the field-reversed configuration and the spherical tokamak. In the meantime, workers have begun preliminary work on magnetic nozzles. The largest research effort at present is a collaboration among the National Aeronautics and Space Administration, Ohio State University and Los Alamos National Laboratory. Researchers from the three organizations are using extremely high electric currents to create a plasma, which in the experiments stands in for a fusion plasma, and to study its interactions with a magnetic field. Even the fusion fuel problem may be tractable. Although there is very little helium 3 on Earth, there are larger quantities of it in the lunar soil and in Jupiter’s atmosphere as well. Also, other elements found on Earth, such as boron, may figure in alternative fusion reactions that are difficult to ignite but that yield alpha particles. For all the promise of fusion propulsion, there is one known physical phenomenon—matter-antimatter annihilation—that releases far more energy for a given mass of reactants. A space propulsion system based on this principle would exploit the mutual annihilation of protons and antiprotons. This annihilation results in a succession of reactions. The first of these is the production of pions—shortlived particles, some of which may be manipulated by magnetic fields to produce thrust. The pions resulting from matter-antimatter annihilation move at speeds close to that of light. Here again, though, one of the key problems is scarcity: the number of antiprotons produced at highenergy particle accelerators all over the world adds up to only a few tens of nanograms a year. To carry humans on a rendezvous mission to the nearest star, Proxima Centauri, a matter-antimatter drive system would need tons of antiprotons. Trapping, storing and manipulating antiprotons present other major challenges because the particles annihilate on contact with ordinary protons. Nevertheless, it may be possible to exploit, albeit to a lesser extent, antimatter’s high energy content while requiring much smaller numbers of antiprotons—amounts that are most likely to be available in the

next decade. Such a system would use antiprotons to trigger inertial confinement fusion. The antiprotons would penetrate the nuclei of heavy atoms, annihilating with protons and causing the heavy nuclei to fission. The energetic fission fragments would heat the fusion fuel, initiating the fusion reaction. The first steps toward determining the feasibility of such a propulsion system are already being taken under NASA sponsorship. One research activity is the design and construction, at Pennsylvania State University, of a device in which antiprotons could be trapped and transported. At this very early stage, the challenges to building fusion—let alone antimatter—propulsion systems may seem insurmountable. Yet humankind has achieved the seemingly impossible in the past. The Apollo program and the Manhattan Project, among other large undertakings, demonstrated what can be accomplished when focused, concerted efforts and plenty of capital are brought to bear. With fusion and antimatter propulsion, the stakes could not be higher. For these will be the technologies with which humanity will finally and truly reach for the stars. --Originally published: Scientific American 280(2); 94-95 (February 1999).

Starship Humanity by Cameron M. Smith When space shuttle Atlantis rolled to a stop in 2011, it did not mark, as some worried, the end of human spaceflight. Rather, as the extinction of the dinosaurs allowed early mammals to flourish, retiring the shuttle signals the opening of far grander opportunities for space exploration. Led by ambitious private companies, we are entering the early stages of the migration of our species away from Earth and our adaptation to entire new worlds. Mars is the stated goal of Elon Musk of PayPal fortune; polar explorers Tom and Tina Sjogren, who are designing a private venture to Mars; and Europe’s privately funded MarsOne project, which would establish a human colony by 2023. The colonization of space is beginning now. But technology is not enough. If space colonization is to succeed in the long run, we must consider biology and culture as carefully as engineering. Colonization cannot be about rockets and robots alone— it will have to embrace bodies, people, families, communities and cultures. We must begin to build an anthropology of space colonization to grapple with the fuzzy, messy, dynamic and often infuriating world of human biocultural adaptation. And we must plan this new venture while remembering the clearest fact of all regarding living things: they change through time, by evolution. Three main concepts shape current thought about space colonization. First is the colonization of Mars. Widely publicized by the peppery space engineer and president of the Mars Society, Robert Zubrin, Martian colonies would be self sufficient, using local resources to generate water and oxygen as well as to make construction materials. Next is the concept of free-floating colonies—enormous habitats built from lunar or asteroid metals. Popularized by physicist Gerard K. O’Neill in the 1970s, these would house thousands of people, could rotate to provide an Earth-like gravity (as beautifully envisioned in the 1968 film 2001: A Space Odyssey), and could either orbit Earth or hang motionless at so-called Lagrangian points, spots where an object’s orbital motion balances the gravitational pull of the sun, moon and Earth. Finally, we might also consider the concept of the Space Ark, a giant craft carrying thousands of space colonists on a oneway, multigenerational voyage far from Earth. I have been working with the nonprofit foundation Icarus Interstellar to design just such a mission. Each of these approaches has its merits, and I think they are all technologically inevitable. But we must never confuse space colonization with the conquest of space. The world beyond ours is unimaginably vast; it will be what it has always been. When humankind begins to make its home in space, it is we who will change. THE PIONEERS Wow will be the space colonists? Here we must ditch the old concept of crew selection and the comically diabolical tests of chisel-chinned space heroes depicted in The Right Stuff. Space colonists will be ordinary families and communities who will not be on a mission but who are intending to live out their lifetimes. We will need a few Captain Picards, although most early colonists will most likely be farmers and construction workers. Still, early colonists will have to be genetically healthy. In smallish populations, individuals carrying genetic maladies could threaten the future in ways that do not play out in a population of billions. In a Space Ark, the biological fate of the colony is strongly conditioned by the genetic constitution of the founding population—if just a few travelers carry the genes for inherited disease, these genes will spread much more thoroughly. We now know the details of hundreds of genes that cause disorders, from cancers to deafness. (Recently researchers announced that they could screen for more than 3,500 such traits in human fetuses.) A genetic screening program seems clear—if you are carrying certain genes, you remain Earth-bound— but life is not so simple. Many maladies are polygenic— that is, the result of complex interactions among myriad genes. And even though one might carry the gene or genes for a certain disorder, environmental factors encountered during the course of life can determine whether or not those genes are activated in a healthy or unhealthy way.

For example, the human ATRX gene helps to regulate processes related to oxygen transportation. But ATRX activity can be altered by environmental influences as diverse as nutrient intake or a person’s state of mind. When ATRX function is significantly modified, oxygen transport is impeded, resulting in seizures, mental disabilities and stunted growth. Thus, one cannot simply screen out people carrying ATRX: everyone has it. In some people, though, based on poorly understood environmental factors, ATRX will go haywire. Can we deselect someone for space colonization for something that might happen? Complicating matters, we must also ensure broad genetic diversity of the gene pool. If all members of a population are genetically identical, a single sweep of disease could wipe everyone out. (This consideration demolishes the concept of a genetically engineered superrace of space travelers, as depicted in the 1997 film Gattaca.) Once screened, what should be the population of space colonies? In a Mars colony, populations can grow and expand into new territory. But in a Space Ark, the population will be relatively low, and inbreeding becomes a concern. For example, in a study of Amish, Indian, Swedish and Utah populations, infant mortality was roughly double when matings occurred between first cousins than when they occurred between unrelated people. To avoid these issues, we will have to consider the minimum population needed to maintain a healthy gene pool. Our minimum viable population has been much debated, but several anthropologists have suggested a figure of about 500. Because small populations are always at greater risk of collapse, I would suggest beginning with a population at least four times that at minimum—2,000, or about half the size of a well-staffed aircraft carrier—in a spacecraft that gives this population ample room to grow. For humans away from Earth, safety will indeed be found in numbers. (Even interstellar voyages will focus on reaching another solar system and inhabiting its planets, where populations can grow again.) We will also have to carefully consider the crew’s demographic structure—the age and sex of colonial populations. Simulations by my colleague William Gardner-O’Kearney show that over a few centuries, populations that begin with certain ratios of young to old and males to females persist better than others. Early colonial populations, then, should be individually healthy and collectively diverse to give future populations the best chance of having genes on hand that might be adaptive in new environments. But we cannot control everything. At some point we will have to roll the genetic dice—which we already do every time we choose to have children on Earth—and set out from cradle Earth. Space-Based Selection No matter how carefully we prepare our colonial populations, life off planet Earth, at least at first, will be more dangerous and perhaps shorter than life here. Away from Earth, people will be exposed to forces of natural selection that we have removed from modern life. Little of this selection will play out in the dramatic ways we might expect from science fiction movies, which tend to focus on the lives of adults. Instead it will occur during critical periods of tissue development in embryos and infants, when life is most delicate. How could such selection play out? For one example, consider that the human body has evolved close to sea level under an atmospheric pressure of roughly 15 pounds per square inch (psi) for the past several million years, breathing a mix of roughly 80 percent nitrogen and 20 percent oxygen. Yet space travel requires pressurized habitats that grow more expensive and laborious to build the more pressure they need to hold. To ease the engineering requirements, atmospheric pressure in any off-Earth structure will be lower than on Earth. Fair enough—Apollo astronauts survived just fine at 5 psi—but if you lower atmospheric pressure, you must increase oxygen as a percentage of what you are breathing. (Those same astronauts breathed 100 percent oxygen on their lunar voyages.) Unfortunately, lower atmospheric pressure and elevated oxygen levels both interfere in vertebrate embryo development. Miscarriages and infant mortality will rise—at least for a time. Inevitably, selection will preserve the genes suitable for extraterrestrial conditions and remove those that are less suitable. Infectious disease—to which small, dense populations such as space colonies are particularly vulnerable —will return as a significant concern, imposing new selection pressures as well. However careful we are with immunization and quarantine, plagues will eventually sweep through colonies, resulting in selection for people more capable of surviving the disease and selection against those less capable. Finally, we must remember that we bring with us thousands of domesticates—plants and animals for food and materials—and that selective pressures will act on them as well. Ditto the millions of microbial species that ride on and in human bodies—invisible genetic hitchhikers that are critical to our health. Based on a few calculations, I think it is reasonable that within five 30-year generations—about 150 years—such changes will be apparent in the extraterrestrial human body.

Exactly what biological adaptations evolve will depend heavily on the atmospheric and chemical environments of the habitats we build. We can control these to a large extent. Yet we cannot easily control two other important factors that will shape humanity in space: gravity and radiation. Mars travelers will feel just a third of Earth’s gravity. Those conditions will select for a more lithe body stature that can move with less effort than the bulky, relatively muscular builds we use to counteract Earth’s gravity. In Space Ark and other free-floating scenarios, gravitation might remain about Earth normal, so Earth-normal statures might persist. Radiation causes mutations, and any space colony will be unlikely to provide the protection from radiation that Earth’s atmosphere and magnetic field provide. Will increased mutations create physical errors—repeated parts like an extra finger or malformed parts like a cleft palate? Certainly, but we cannot know what kind. The only thing we can predict with confidence is selection for increased resistance to radiation damage. Some people have better and more active DNA-repair mechanisms than others, and they will be more likely to pass their genes on. Could more efficient DNA-repair mechanisms have any visible correlate—such as, say, a particular hair color? Again, we do not know. But it is also possible for beneficial genetics to spread when they have no such visible correlates. Among Hutterites of South Dakota, who interbreed among a relatively small number of small communities, anthropologists have found that people appear to be strongly influenced in their mate choice by body aroma—and the better the person’s immune system, fascinatingly, the better the aroma. On a moderate, five-generation timescale, then, human bodies will be subtly reshaped by their environment. We will see adaptations on the order of those of the natives of the high Andes and Tibet, where more efficient oxygen-transport physiology has evolved, resulting in broader and deeper chests. Each alteration is a compromise, however, and these high-altitude populations also sustain higher infant mortality when giving birth at altitude. One cultural adaptation to this biological change has been for mothers to descend to oxygen-richer altitudes to deliver children. We can expect similar biocultural shifts off of Earth, and we should plan for the most likely of them. For example, on Mars, birthing mothers might shuttle to an orbiting station where delivery could happen in a rotating, 1-g facility with a more Earth-normal atmosphere, but I bet that eventually they would not bother and that distinctive Martian human characteristics would evolve. A Space-Based Culture Cultural change will be more apparent than biological change on a 150-year time span. Studies of human migrations have taught us that while migrating peoples tend to carry on some traditions to maintain identity, they also devise novel traditions and customs as needed in new environments. For example, the Scandinavians who first colonized Iceland after A.D. 800 continued to worship Norse gods and speak the Viking language but quickly developed a distinctive cuisine—heavy on meat (whereas rye and oats were grown in Scandinavia) and on preserved foods to survive the harsh winters—as they explored the resources of an unknown land. On Mars, this acculturation will play out in innumerable ways. There, in low-pressure, oxygen-rich atmospheres contained in unique architectural materials and arrangements, sound might propagate differently— even if subtly—perhaps affecting pronunciation and even the pacing of speech, resulting in novel accents and dialects. The lighter gravity could influence body language, an important element of human communication, and would influence performance arts of all kinds. Cultural divergence occurs as just such small, innumerable differences accumulate. More profound cultural change could occur in Space Ark scenarios, where life would have less to do with Earth at each moment that the starship speeds away. Here basic concepts of space and time could well be transformed rather quickly. For example, how long would Space Ark cultures use Earth timekeeping? Without Earth’s days and nights and years, civilizations might invent a base-10 timekeeping scale. Or they might decide to count time down until a distant solar system is reached rather than up from some event in the past (such as the departure from an Earth to which they will never return). Long-Term Genetic Change Significant genetic change occurs when new genes become widespread in a population. An example from prehistory is the spread of genes that resulted in lactose tolerance in adults, which appeared independently in both Africa and Europe not long after the domestication of cattle. This genetic equipment allowed more energy to be derived from cattle, and in these populations, it quickly became nearly universal, or “fixed.” Although we cannot predict which mutations will arise, population genetics enables us to estimate how long it would take mutations to become fixed in the genome of space-based explorers. My calculations— based on model Mars populations of 2,000 people of certain age and sex structures—indicate that it could

occur in just a few generations and certainly within 300 years; we can expect significant original off-Earth physical characteristics in human populations on this timescale. These changes will be on the order of the broad geographical variation we see in humans today—a spectrum of different statures, skin colors, hair textures and other features. On Mars, there might be further, internal divergence as some populations elect to live most of their lives sheltered in underground habitats, while others prefer to take the increased radiation risks to live in surface habitats offering greater mobility. In the limited-population, closed-system Space Ark scenario, gene fixation could happen much more rapidly, perhaps driving a greater uniformity than on Mars. Whereas there will be some biological change, long-term cultural change will be more profound. Consider that in the three centuries from the early 1600s to the early 1900s, the English language changed so much that comprehending 17th-century English texts today requires special training. Three centuries hence, the language spoken on a Space Ark might be profoundly different. Larger-scale cultural change is also quite likely. Exactly what divides one culture from another is a topic of tremendous debate in anthropology, but I believe that anthropologist Roy Rappaport made the distinction clear. Different cultures have different “ultimate sacred postulates”—core concepts, usually unquestionable and unquestioned, ingrained by tradition and ritual, that shape a population’s essential philosophical and moral codes. For Christianity, for example, one such postulate is that “In the beginning, God created the heaven and the Earth.” How long it will take for such foundation beliefs to change off of Earth—and in what direction—is impossible to say, but several centuries is certainly enough time to allow new cultures to arise. The Rise of Homo Extraterrestrialis When will we see even more fundamental biological change—that is, speciation? Small populations can change quickly, as evidenced by the unusually large mice that roam the Faroe Islands 1,200 years after Viking ships dropped off ordinary house mice. But anatomically modern humans have gone more than 100,000 years—migrating from Africa into a wide variety of environments, from desert to open ocean— apparently without biological speciation. (Our nearest hominin relatives, such as the cold-adapted Neandertals and the apparently miniaturized “hobbit” humans of the island of Flores in the western Pacific, split from our common ancestor substantially earlier.) This is largely because we use culture and technology to adapt more than biology alone. It would take, then, significant natural and cultural selection to reshape extraterrestrial humans to such a degree that they could no longer productively mate with earthlings. Unless, of course, humans devise their own speciation. It seems inevitable that off-Earthers will eventually harness the staggering power of DNA to tailor their own bodies for many conditions. Perhaps the people of Mars will biologically engineer gill-like structures to split the oxygen from atmospheric carbon dioxide or toughened skin and tissues to endure low pressure. They might make themselves into a new species, Homo extraterrestrialis, by conscious choice. Where to Begin? Human space colonization will require plenty of engineering and technical advances. We must also improve our understanding of how human biology and culture adapt to new conditions and use that knowledge to help space colonization succeed. I suggest beginning immediately with three courses of action. First, we must abandon the technocrat’s essential revulsion of humanity and begin procreating off of Earth, giving birth there and raising children there, to understand critical issues of human reproduction, development, and growth in new radiation, pressure, atmospheric and gravity environments. Bureaucrats will recoil at the risks involved—children exposed to risk beyond that of a bicycle-helmeted, First World suburbanite!—but concerns will diminish as space access is privatized. Still, at times the adaptation to space will be painful—but so is birth. Second, we must experiment with growing and maintaining the health of domesticated species off of Earth. We are going nowhere without our microbes, plants and other animals. And to promote these first two goals, an X-Prize should be awarded for the first functional, livable human habitat off of Earth: not a sterile orbiting laboratory (as important as those are), but a home where people can grow plants, raise animals and even have children. Many would shudder at the prospect of staying in such a place, but at the same time, there will be no shortage of volunteers. Finally, we must reengage the proactive approach that has made human survival possible up to the present and use that capacity to shape our own evolution beyond our home planet. We must be immensely bolder than our bureaucracies. Failing that, in time we will become extinct, like everything else on Earth. As H. G. Wells wrote about the human future in 1936, it is “all the universe or nothing.”

-Originally published: Scientific American, 308(1); 38-43 (January 2013).