Earth-Sized Planet Found Orbiting in a Habitable Zone

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, California, who spent a year analysing 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 fulfils a major goal of the $600-million Kepler mission.

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.

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