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.