Residents of the Pacific Northwest sometimes refer to the region as “God’s Country,” not for the ceaseless rain that soaks the land from October until May, but for those few glorious summer months when the sun emerges from behind the clouds and the world bursts forth with life. On one such morning—August 3, 2010—dozens of the world’s top planetary scientists met in the back room of the Talaris Conference Center to contemplate the origins of life on Earth and elsewhere.
Talaris lies just a half-mile east of the University of Washington, where Victoria Meadows serves as director of the astrobiology program. The conference center is situated amid 18 acres of rolling lawns dotted with Douglas firs and veined by meandering streams, and as Meadows drove to the conference that morning, she was surrounded by evidence of her planet’s lush habitability. She didn’t need a telescope to see it; it all was right there.
This was the first day of a conference that had come to be called “Revisiting the Habitable Zone,” which Meadows had spent the last several months organizing. Many of her guests were members of the Virtual Planetary Laboratory, known by its acronym VPL, the project Meadows founded at the turn of the millennium and for which she has since secured more than $13 million in NASA funding. VPL’s members hail from places as far flung as Sydney and Mexico City, and conferences like these offer them a rare opportunity for them to convene in physical space. It is an interdisciplinary team, with astronomers and physicists, oceanographers and geologists, chemists and biologists. Diverse though their specialties may be, they have all dedicated themselves to understanding the delicate and complex mixture of factors that can make or break a planet’s habitability. It is a cryptic recipe, and much remains to be deciphered, but the essential ingredient, they would all agree, is water.
On Earth, the seeds of life were sown beneath the seas, some three-and-a-half billion years ago, shortly after the seas themselves had settled and pooled. Even in the planet’s driest deserts, not a single living thing has been found that can survive without water. So when the 38 scientists gathered in Seattle to answer the question, “What makes a planet habitable?” the riddle they really sought to solve was, “What makes a planet’s surface suitable for water?”
The answer is complicated, but perhaps the simplest variable determining whether water will accumulate on a planet is distance—specifically, the distance between a planet and the star it orbits. Our own solar system is a case in point. Venus is a planetary Icarus, a cautionary illustration of the perils of orbiting too close to the Sun, where torrid heat long ago dissolved any liquid water that may have once been present. And while water has been detected in the atmosphere of distant Neptune, it is frozen solid, preventing organic compounds from intermingling and giving rise to life.
And then there is Earth, our home, traveling around the Sun within a range of space that is neither too hot nor too cold, a Goldilocks zone where water flows and life thrives. Around virtually every star in the sky, there is a ring of temperate space, and its borders and breadth vary in accordance with the size and brightness of the star. Scientists refer to this area as the “habitable zone.”
It was precisely this habitable zone that Meadows and her colleagues wanted to revisit when they gathered on that warm summer day in Seattle. They were particularly interested in mapping the habitable zones of distant stars, the best places to look for planets with life.
In one sense, the question “Revisiting the Habitable Zone” sought to answer—Are we alone?—is as ancient as humanity itself; humans have likely been asking it ever since our prehistoric ancestors first gazed at the evening sky. But the conference’s scientific heritage can be directly traced to October 6, 1995, a landmark date in the annals of human space exploration. If our descendants ever succeed in settling distant planets and building a future for themselves among the stars, schoolchildren may well learn that on that autumn day, two Swiss astronomers announced the discovery of 51 Pegasi b, a planet orbiting a star 50 light-years away. It was the first planet ever found orbiting a star beyond our Sun.
The search for exoplanets—shorthand for extra-solar planets, those that orbit stars beyond our Sun—has gathered momentum in the intervening years. Space-based telescopes have been launched into orbit, designed to detect exoplanets hundreds of light-years away. Giant telescopes on the surface of the Earth have also joined the search. In February, NASA announced that its Kepler Space Telescope had verified the existence of an additional 715 new exoplanets, bringing the total to 1,768. Of those, 20 have been found in the habitable zone. By April, scientists had found Kepler-186f, a planet-so Earth-like they described it as a “first cousin.”
The harder we look, the more familiar the galaxy grows. The discoveries have rendered science writers dizzy. From The New York Times last year: “The known odds of something—or someone—living far, far away from Earth improved beyond astronomers’ boldest dreams.”
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The boldest dream of all—to find life on other planets—has been the driving force behind much of Meadows’s scientific career. But the splashy announcements about Earth-like exoplanets have her concerned. “There is this danger,” she says, “that we might cry wolf.” A problem both simple and profound undermines the big talk about habitable planets: No one has seen them.
Perhaps it is reasonable to expect actual sightings to accompany the discoveries of exoplanets, as if they were sunken ships or strange toads. But exoplanets are light-years away and, compared with the stars around which they orbit, exoplanets are small, cold and dark. The chances of spotting an exoplanet through the world’s most powerful telescope are about as good as photographing a speck of dust floating next to a floodlight from 300 miles away.
Astronomers searching for exoplanets will observe a star for months or even years and measure subtle changes in its light that reveal the presence of orbiting planets. Then they try to determine whether any of those planets are in the boundaries of the habitable zone. This requires measuring a planet's distance from its star and determining the brightness of the star itself. If the planet is too far away or too close, or if the star is too dim or too bright, the planet may not be habitable.
But even if a planet is located in the habitable zone, it doesn’t guarantee habitability, which is instead a “multifaceted process,” Meadows told her colleagues at the conference last year. “We don’t want people to see habitability as a one-dimensional thing.” Rory Barnes was one of a handful of Meadows’s protégés in attendance that day. He had already earned the nickname “the Destroyer of Worlds,” recognition from Meadows of his knack for identifying seemingly insignificant factors that can make a planet inhospitable to life. Barnes excels at calming optimistic speculation about potentially habitable planets. Sometimes, the first step is deflating the hype.
“People get very excited when they find these planets, and people often tend to overstate the possibilities as far as having life,” Barnes told me. “I often try and figure out, what would it actually take to make this thing habitable? Because it turns out, for almost any planet that we find, it’s actually pretty easy to make it uninhabitable.”
He offers the example of Kepler 22b, a planet discovered orbiting in the habitable zone around a star that’s just about as bright and energetic as the Sun, somewhere in the Cygnus constellation 600 light-years away. The Kepler Space Telescope had detected it there using a technique known as the “transit method.” A transit occurs when an exoplanet passes between its parent star and a telescope, and the star’s light can be seen to dim. Kepler 22b was first seen transiting in front of its star on May 15, 2009. Though a single transit is interesting, it isn’t enough to rule out the possibility that the star dimmed for some other reason. Just as the Pope requires two postmortem miracles before he’ll canonize a saint, astronomers demand at least three transits before announcing a discovery. It’s a necessary standard of proof, but it makes detecting planets with long orbits a time-consuming endeavor.
Two-hundred-ninety days after its first transit was observed, Kepler 22b’s star dimmed a second time, and to the same extent that it had before, suggesting that the same object had passed in front of it again. NASA would have to wait another 290 days—a year on Kepler 22b—and on December 15, 2010, exactly 290 days later, it was seen, like clockwork, making another transit.
Kepler 22b was real.
The next step was to learn about it. From the time it took to orbit its star, scientists were able to calculate Keppler 22b’s average orbital distance, and when they compared that to where they expected the habitable zone to be, they were thrilled to find that there it was, right on the inner edge of the habitable zone.
The discovery seemed to justify the $600 million NASA spent to launch the Kepler telescope, a project of extraordinary ambition designed specifically to discover Earth-size planets in or near the habitable zone. NASA had a beautiful artist’s conception drawn up:
Scientists described what this world might look like, even imagining clouds in its atmosphere. But, says Barnes, there was a problem: “It was obviously too big.”
“Rory was like, ‘No way,’” remembers Meadows. “He killed it pretty quick.” At more than eight times Earth’s mass and around two and a half times its radius, Kepler 22b’s gravitational field would have accreted large amounts of hydrogen and helium from the surrounding space. In other words, it was a gas planet, like Neptune or Jupiter, with no rocky surface to gather water. It was just too big to be habitable.
Planets can also be too small. Mars, for example, is the right distance from our Sun, but it’s the wrong size. By most estimates, Mars’s orbit is within the outer boundary of the habitable zone, and the dried-up riverbeds and empty ocean floors that mark its surface indicate where ancient water may have once flowed. But the planet’s gravity was weak, and before long, one theory goes, its atmosphere blew off into space, never to return.
Astronomers are better at finding planets than they are at characterizing them, and their optimism about the habitability prospects of the planets they discover is at times unwarranted. To be sure, astronomers are aware that planets can be too big to be habitable, and the announcements they make about their exciting discoveries always carry caveats. In private, they are even more circumspect. When he goes to conferences, Barnes will often go out for drinks with astronomers in the evenings. He has a game he likes to play. “If I find myself out at a bar with a bunch of people I respect, I ask them, 'Who has the balls to stand in front of the world and tell them you've found an inhabited planet?'” he says. “And everybody just looks soberly into their beer and keeps drinking."
Barnes gives NASA credit for acknowledging that Kepler 22b “might possibly be too big. But,” he says, “they certainly trumpeted it up as quite an exciting discovery, and, I mean, it was,” but not because it was habitable. The fact that the Kepler telescope had been operational for just three days when it spotted the planet was a sign that almost-habitable planets ought to be common. But Kepler 22b would not be it. Scientists would have to keep looking.
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When Meadows left Australia to work for NASA in the United States in 1994, the only planets anywhere in the universe known to orbit stars were the ones in our solar system orbiting the same star we do. Exoplanets were a little like the Divine; believing was a matter of faith.
Fresh off her Ph.D. from the University of Sydney, Meadows had secured a position at NASA’s Jet Propulsion Laboratory, the team famous for putting rovers on Mars and sending probes to the outer reaches of the solar system. She was put to work observing Venus, but the discovery of 51 Pegasi b would change her life. Before the exoplanet era began, planetary scientists were something like linguists condemned to study only their own language group, their horizons so limited as to make any drastic grammatical deviations unimaginable. Until 1995, it was believed that any planetary system would be something like ours, where the small, rocky planets orbit close to the sun, and the large, gaseous planets orbit farther away.
Then came 51 Pegasi b.
At about 150 times the size of the Earth, it had to be a gas giant. But unlike the gas giants in our solar system, 51 Pegasi b appeared to be incredibly close to its star, closer even than Mercury is to the Sun. It completed an orbit every four days. “We didn’t believe it at first. We thought it was crazy,” Meadows said. “I mean, how the heck can you get a gas giant that close to its parent star? It makes no sense.”
Prevailing theories of planetary formation continue to suggest that it is physically impossible for gas giants to form so close to their stars. The existence of 51 Pegasi b and the dozens of other hot Jupiters that have been discovered have forced scientists to conclude that they must have formed far away and were somehow pulled into very close orbit. The conclusion seemed inescapable, yet scientists found it difficult to embrace; they had previously believed that planets did not travel far from their point of formation. They had stumbled upon a planetary language unlike any they had known before.
51 Pegasi b would not be the last exoplanet to shock and puzzle. There was 55 Cancri e, the Diamond Planet, a world so dense that a third of its surface may be strewn with diamonds; and PSR B1620-26 b, dubbed “Methuselah” by astronomers because, like the Biblical geezer for whom it was named, it seems impossibly old, having formed shortly after the Big Bang, a time when scientists thought planets couldn’t form because there weren’t enough materials floating around to create a planet’s core. By the late 1990s, as the initial trickle of exoplanet discoveries grew to a steady current, it became increasingly clear to Meadows and her colleagues at NASA that, if they ever discovered a habitable planet, it might be very different from Earth. In 1998, NASA established the Astrobiology Institute, which promptly set about distributing its $9 million to fund studies of life in the universe.
Meadows’s colleagues at the Jet Propulsion Laboratory urged her to write a proposal, which to her seemed a daunting task. “But I did it anyway,” she says, “and we won, so that’s kind of how we started.” In 2001, Meadows secured a five-year, multimillion-dollar grant from NASA to bring together the Virtual Planetary Laboratory. The idea was to compile a list of all the conceivable atmospheres that might be found on exoplanets, and to determine which were likely to be habitable and which were not. It was a forward-looking proposal, anticipating a time when telescopes could do more than simply detect a planet’s existence and estimate its size and orbit. When Meadows wrote her proposal, analyzing exoplanetary atmospheres, especially on planets small enough to be habitable, was outside the technological capabilities of even the most modern telescopes.
Still, measuring the atmospheric composition of a faraway planet is not as difficult as it sounds. One need not land on it, clad in a protective space suit and equipped with special instruments, or even fly by it, searching for clouds and oceans. All that is needed is a direct image, no larger than a pixel or two, and signs of life would reveal themselves. This atmospheric soothsaying is made possible by spectroscopy, a technique that takes advantage of the fact that light alters its color according to the chemical composition of the matter that emits, reflects or absorbs it. Breaking down light into a spectrum can reveal the molecules present in any type of matter, including the atmospheres of exoplanets.
Some atmospheric compositions are thought to be telltale signs of life, “biosignatures” as illuminating as flower blossoms in spring. Oxygen and methane, for example, cannot coexist in an atmosphere without eliminating each other unless something at the surface continues producing them. If an atmosphere were tinted by a combination of oxygen and methane, it would be likely that oxygenic photosynthesis was taking place down below. The duo of oxygen and methane is virtually a sure-fire biosignature. Meadows’s grand ambition for VPL was to compile a catalogue of potential biosignatures that could be used to judge the habitability of any conceivable planet. According to Geoff Marcy, the American astronomer credited with discovering more exoplanets than anyone else, VPL could do for planetary habitability what the human genome project did for genetics. “VPL” he says, “is an absolutely unique and precious research project.”
In the 13 years since its creation, VPL has helped foster some of astrobiology’s most visually scintillating research, including that of the biogeochemist Nancy Kiang, another of Meadows’s protégés. Kiang now works at NASA’s Goddard Institute of Space Studies in New York City, but when Meadows was getting VPL up and running, Kiang was a Ph.D. candidate at the University of California, Berkeley.
Kiang first learned about VPL on May 1, 2002, at a talk Meadows gave at Berkeley’s Space Sciences Laboratory. Kiang was doing her doctorate work on global warming, but the prospect of entering the contentious field of climate change science, where diligent research is often warped and exploited for political ends, had left her unenthused. For a young Ph.D. candidate with an uncertain professional future, watching Meadows wax optimistic about finding habitable exoplanets was an epiphany.
“I went to her talk, and I thought, ‘Oh, yeah!’” says Kiang. “All of a sudden everything congealed and crystallized for me, and why I was studying biogeochemistry and Earth systems sciences. And it was about searching for life on other planets.”
Kiang immediately wrote to Meadows, begging to be accepted as a postdoctoral fellow at VPL When Meadows replied with her regrets that there was no funding available for postdocs, Kiang volunteered her spare time. Finally, in 2007, NASA renewed the project’s funding, and Kiang was brought on as a full-time member.
Few people are better suited to envisioning alien life on invisible planets than Kiang. “I always wanted to be an artist,” says Kiang, “but I’m Chinese-American. I have Chinese parents, and you have to get a Ph.D. if you have Chinese parents.” Her father, a former aerospace engineer at Rockwell International, nixed her youthful artistic ambitions. “Very strongly,” she says. “Vehemently.”
But Kiang remained interested in art, and after she came to work at NASA she began carving out weekends and evenings to direct the film “Solidarity,” a short comedy adapted from a story by the Italian writer Italo Calvino. The film won the Lord Mayor’s Award at the Heart of Gold International Film Festival in Queensland, Australia. (Her next short, “Chip,” is based on an Anton Chekhov story and will be released this year.) Kiang is cautious about drawing connections between her art and her science. “Visualizing pictures is not the same thing as analyzing spectra,” she observes. Still, her flair for visually alluring science is undeniable.
Kiang is interested in how life might look on planets that orbit red dwarfs, the small and relatively cool stars that are by far the most common in the galaxy. In scientific vernacular, red dwarfs are known as M-dwarfs, and yellow stars like our sun are called G-stars. It is estimated that some 70 percent of the stars in the Milky Way galaxy are M-dwarfs, so the first habitable planet we find may well orbit a star much dimmer, smaller and less energetic than our G-star sun.
What Kiang has sought to understand is whether photosynthetic life forms, which produce oxygen and would therefore provide a useful biosignature, could emerge and survive on planets orbiting M-dwarfs, which emit more infrared light than G-stars. Unlike light in the visible spectrum, infrared light nurtures very little life on Earth. On another planet, however, that could change. “The competitive advantage would be for an organism that can use that near infrared light,” explains Kiang. “So the question is, well, around an M-star, could you have oxygenic photosynthesis surviving okay?”
Kiang pointed to a rare species of Cyanobacterium called Acarychloris marina, the only organism on Earth known to have adapted its photosystem to perform photosynthesis using the Sun’s leftover, low-energy, red-shifted light. She proposed that the existence of Acarychloris marina was evidence that photosynthetic organisms could emerge on a planet orbiting a red dwarf. Kiang’s finding, published in a 2007 paper for the journal Astrobiology, might have gone largely unnoticed had it not been for one tantalizing implication of Acarychloris marina’s unique photosynthetic adaptability. Trees, shrubs and plants on planets orbiting red dwarfs, Kiang pointed out, might be able to use photosynthetic pigments that would cause them to appear red, orange or even black—an image straight out of science fiction.
NASA was so excited about the notion of crazy-colored plants that it commissioned an artist to make a digital rendering of just such an alien world. Of course, as long as exoplanets remained invisible to telescopes, the plants Kiang described would exist only in the realms of science and art, never to be found in nature. The signature tints of oxygen and methane in an atmosphere, moreover, would forever be bound by the pages of scientific journals. If VPL’s work were ever to be anything more than academic, stronger telescopes would have to be built.
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The little dome on the roof of Columbia University’s Pupin Hall is home to a Meade SCT telescope with a 14-inch aperture. Caleb Scharf chuckles when asked about it. “Yeah, so that has historical interest,” he says. “These days it’s not really used for anything more than training or public outreach.” The rooftop telescope is beginner stuff, the sort of thing you can use on a clear night to see Venus and Mars. These days, Scharf is more interested in big telescopes, ones that can find Earth-sized exoplanets in the habitable zone.
Scharf, like Meadows, became a scientist before the exoplanet boom. He is a convert to exoplanetology. In 2003, he made a name for himself when he pointed the world’s most sensitive X-ray telescope into the far reaches of the universe and uncovered evidence of supermassive black holes at the centers of galaxies. It was a significant discovery, and Scharf was soon offered a permanent university position, a chance to study the secrets of the cosmos until he retired. But Scharf had grown weary of black holes. He felt that the field had matured, and the opportunity for great discoveries had faded. Meanwhile, Scharf recalls, “the search for exoplanets was really getting going full-steam.” The future, it seemed, lay in exoplanets. He turned down the job offer.
“It sounds kind of crazy to have left that field just when I was doing good stuff, but at the same time, you know, you’ve got to do what you’re interested in,” Scharf says. “It was one of those little epiphanies of, do I want to be excited every day, or just once a week? And so I kind of decided, okay, I’m going to give this a shot.”
For Scharf, exoplanetology is science’s next great adventure. “There really is something profound going on here,” he says. If searches had come up empty, or had shown that planets were sparse, “you’d have a much harder time ever asking the question: is there life on another planet?” Instead, we find planets everywhere; five have even been detected orbiting Tau Ceti, a star in our galactic backyard, just 12 light-years away. “We’ve already kind of discovered that, hey, the universe is actually making it a lot easier than it might have been,” says Scharf.
The technology needed to find life, however, remains maddeningly out of reach. Finding life would require a telescope of incredible power, capable of capturing direct images of tiny planets orbiting blazing stars. Such a telescope has never been built, but a decade ago, most exoplanet scientists were certain one would be in orbit by now. “If you went back 10 years ago,” says Scharf, “even NASA had at least somewhere on a drawing board plans for space-based telescopes that would give you that blue marble image of an exoplanet.”
When Meadows gave her talk at Berkeley in 2002, she brought with her a PowerPoint presentation, the fourth slide of which may have helped persuade Kiang, awestruck in the audience, to shift her focus from global warming to exoplanets. “Terrestrial Planet Finder,” the slide is titled. “Direct detection of planets,” it says. “Launch 2011-2015.”
Instead of one space-based telescope, like Kepler, Terrestrial Planet Finder was to be a suite of telescopes equipped with instruments designed to suppress the light from a star and reveal the small, dim planets in orbit. The telescopes would fly through space in precise formation, like an armada, pooling light to construct images of other worlds. This would enable scientists on Earth to see faraway planets clearly, rather than simply detecting the presence of a planet based on the star it orbits. As Scharf puts it, “you could see our solar system if it were 20 light-years away.” Terrestrial Planet Finder was and still is the Holy Grail for exoplanetary astronomers. In its 2001 Decadal Survey, the National Academy of Physics recommended that NASA immediately begin development of a version of Terrestrial Planet Finder, describing it as “the most ambitious science mission ever attempted by NASA.”
The PowerPoint presentation from Meadows’s Berkeley talk is a window to the past, revealing how certain the Terrestrial Planet Finder seemed at the time and how severely NASA has since curtailed its exoplanetary ambitions.
The 2001 Decadal Survey had estimated the mission would require a budget of $200 million, a figure that in retrospect seems outrageously conservative. Congress had supported the mission at first, but the soaring costs soon grew onerous. NASA didn’t help matters when in 2004 it opted to develop two missions that would essentially accomplish the same goal but compete with each other for funds. In 2007, Congress cut NASA’s budget by some $400 million, and both projects were shelved indefinitely.
Last May, during a hearing on exoplanets before the House Committee on Science, Space and Technology, Chairman Lamar Smith declared his support for further exoplanet research. “Imagine,” he said during his opening remarks, “how the discovery of life outside our solar system would alter our priorities for space exploration and how we view our place in the universe.” But Smith made no mention of Terrestrial Planet Finder, the congressman was frank about its prospects. “These are financially challenging times for the nation,” he told me in an email. For the foreseeable future, Terrestrial Planet Finder would have little hope of being resurrected.
NASA has not abandoned the search for life on other planets. The agency’s pipeline includes the James Webb Space Telescope, which will collect spectra from the backlit atmospheres of exoplanets as they transit in front of their stars. It is set to launch in 2018. Ground-based telescopes of unprecedented strength are also in the works, but neither they nor James Webb are likely to capture direct images of planets small enough to be habitable. With Terrestrial Planet Finder indefinitely postponed, many astronomers have been left disappointed, wondering what might have been.
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Mike Rampino is not an astronomer, and in many ways he’s an unlikely defender of life, in space or on Earth. A biology professor at New York University, Rampino specializes in mass extinctions, catastrophic events in the Earth’s history that annihilated a substantial percentage of the planet’s species. The important thing about mass extinctions, says Rampino, is not how many species are destroyed but that, no matter how extraordinary the challenges, some always manage to survive. “I can’t imagine a catastrophe, either from volcanism or from impact, that would kill off life,” he says. “Because certainly the bacteria, the one-celled things, are pretty resilient.”
It’s fitting, then, that those whose curiosity compels them to probe the galaxy for signs of life must, too, be sustained by resilience. Even if Terrestrial Planet Finder is never built, Meadows plans to continue the work of VPL, knowing that, if humans ever do glimpse life in space, they will likely be aided by VPL’s contributions to the understanding of biosignatures. For Meadows, the pixel-sized image of a habitable exoplanet may never come. “On a human scale, in my career, it now looks like we probably won’t have the mission that will do that until after I’m retired,” she says. “So, you know, that’s frustrating for me on a personal level.”
But she’s confident, in her own studiously rational way, that scientists will find a habitable planet someday. “It seems like the characteristics that the Earth has, with liquid water on the surface and an atmosphere and a star, those are all common characteristics that these planets we’re finding potentially have.”
“You know,” she adds, “it’s just a matter of time.” And she’s right, of course. For on a long enough scale, what isn’t?
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