Sifting through observations from more than 100,000 distant stars, astronomers say they have discovered the first definitive Earth-sized planet that orbits in a habitable zone where water could exist in liquid form — a necessary condition for life as we know it.
Scientists don't know whether the planet has water or a protective atmosphere. They don't even know its mass. But they said the landmark discovery gives astronomers great hope that a bumper crop of Earth-like planets is waiting to be found much closer to home, including around temperamental stars that until recently were considered inhospitable to life.
"This is really a tip-of-the-iceberg discovery," said Jason Rowe, an astronomer at the SETI Institute in Mountain View, Calif., who spent a year analyzing data gathered by NASA's Kepler space telescope and was part of a team that described the planet in Friday's edition of the journal Science. After finding the planet known as Kepler-186f, "we can infer that other ones are likely to exist." Locating them, he said, will be "the job of future missions."
It's not quite an "Earth twin" — its parent star is a red dwarf, smaller and dimmer than the sun — but it's clearly a close cousin, said study leader Elisa Quintana, a planetary scientist at NASA Ames Research Center at Moffett Field. Finding it fulfills a major goal of the $600-million Kepler mission, which was rejected four times before it launched in 2009.
Scientists who were not involved in the new study lauded the find.
"This is an historic discovery," said UC Berkeley astronomer Geoffrey Marcy. "This is the best case for a habitable planet yet found. The results are absolutely rock solid."
If such exoplanets do turn out to be common among the distant stars Kepler studies, astronomers should be able to find plenty of them closer to home, the thinking goes. Future NASA missions will enable scientists to determine which planets have the strongest signs of water and life-friendly atmospheres.
The discovery marks a milestone in the quest to find planets that are not just Earth-sized, but truly Earth-like, said Doug Hudgins, NASA's program scientist for the Kepler mission in Washington.
Out of 1,800 or so confirmed planets outside the solar system, fewer than two dozen sit in a habitable zone, where it's not so hot that water would boil off into space and not so cold that it would remain permanently locked in ice. And none of them are as close in size to Earth as Kepler-186f, which has a diameter only 10% larger.
Size is a critically important factor, scientists said: If a planet is about 50% wider than Earth, and if it's packing a lot of mass, its gravity could attract an envelope of hydrogen and helium, shrouding the surface in a gassy atmosphere that's too thick for Earth-like life.
Kepler-186f may be close to Earth's size, but it's hardly close by: It sits about 490 light-years away. It circles its home star, Kepler-186 in the constellation Cygnus, in just 130 days.
The orbit of Kepler-186f would fit inside that of sun-seared Mercury. But since its star gives off less energy than our sun, it is still safely ensconced in a habitable zone.
Scientists have argued that M-dwarf stars such as Kepler-186 may not be particularly hospitable to life, since they tend to produce more flares and damaging radiation than larger and brighter G-type stars such as our sun. But this particular planet appears to sit out of harm's way, and its star is relatively quiet. As a result, Kepler-186f highlights the diversity of potentially habitable planets, expanding the definition beyond worlds circling stars like our sun.
"I believe that planets are very diverse and a whole range of them could be habitable," said Sara Seager, an astrophysicist at MIT who was not involved in the study.
That's an encouraging sentiment, given that planets like Kepler-186f could be easier to find than planets exactly like Earth. Before it was hobbled last year by a broken gyroscope that robbed it of its precision-pointing ability, the Kepler telescope stared at a patch of roughly 150,000 stars and watched for dips in the starlight as planets passed in front.
Based on how frequently such dips in starlight appear, scientists can calculate how quickly a planet is circling — and thus, how close to its star it must be. They can estimate a planet's size by measuring the depth of the shadow it casts on its star.
Close-in planets with shorter orbits complete these transits more often, which makes them easier to find. Around our sun, those planets would be baked. But around a red dwarf like Kepler-186, the climate could still be mild.
Such planets are also easier to find because they block relatively more of their stars' light. And given that M-dwarf stars account for 70% of the stars in the Milky Way, there could be billions of Earth-sized planets in the galaxy waiting to be discovered.
Assuming that 10% of M-dwarfs within 100 light-years of us have an Earth-sized planet in a habitable zone, there could be 10 to 20 in that relatively close range, Rowe said.
NASA's Transiting Exoplanet Survey Satellite, set for launch in 2017, could look for such planets closer to home. And the infrared-sensitive James Webb Space Telescope, NASA's successor to the Hubble Space Telescope, will be able to isolate the signature of water and other molecules necessary for life in the atmospheres of nearby planets.
In that context, Kepler-186f is a sign that scientists are homing in on the answers to fundamental questions about life in the universe.
"Whether we are an extremely rare fluke — a phenomenon that only happens once in a universe — or in a galaxy teeming with life is a very basic question not only of science, but of our existence," said Dimitar Sasselov, a planetary astrophysicist at the Harvard-Smithsonian Center for Astrophysics who was not involved in the study. It's "the first time in human history we have a good shot at answering that question, and that's very exciting."
The SETI Institute is dedicated to the search for intelligent life on other planets, and identifying potentially Earth-like planets closer to home would mean that the radio signals our civilization sends into the universe could reach our theoretical neighbors in a matter of decades, Rowe said.
If humans ever develop high-speed interstellar travel — a big if, to be sure — those Earth-like planets would probably be the best ones to visit, Rowe said.
"I think everybody has their own end goal," Seager said.
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