NASA

An artist's impression of Kepler 62f provided by NASA on April 18, 2013.

The search for Earthlike, habitable planets beyond the Sun has been something like a boulder rolling downhill ever since the Kepler space telescope went into orbit in 2009. Before that, ground-based astronomers had been finding so-called exoplanets one or two at a time, here and there in the cosmos, and pretty much all of them were far too large to be hospitable, or much close to the fires of their parent stars, or, usually, both.

But ever since Kepler soared into space and turned its relentless, unblinking eye on a single patch of stars and never looked away, it began notching discoveries at an ever-accelerating pace, finding more planetsand more nearly Earthlike onesall the time. Whats more, its finding them in the so-called habitable zone, just the right distance from their stars to allow life-sustaining liquid water to exist.

Nobody quite imagined what the Kepler team has just announced, however. Writing in Nature, William Borucki, Keplers principal scientist, along with dozens of collaborators, reports the discovery of not one, but two potentially life-sustaining planets, orbiting a star some 1,200 light-years away, in the constellation Lyra. One, named Kepler-62e, is about 60 percent larger than Earth, and lies at the inner, hotter edge of the habitable zone, where water might be awfully hot but still avoid boiling away. The second, Kepler 62f, is 40 percent larger than Earth and is more comfortably within the stars just-right region. This, said Paul Hertz, director of NASAs astrophysics division at a press conference, is really cool. In astronomer-speak, thats huge.

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Borucki and the other Kepler scientists were quick to say they had no direct evidence that either planet actually has liquid water on its surface. All they know for sure is the planets size, and their distance from the star: 33 million mi. (53 million km) out for the larger 62e; 65 million mi. (105 million km) for the smaller 62f.

In our solar system, that would make both planets too hot for water to stay liquid. But the star, Kepler 62, is only about two-thirds as large as the Sun, and significantly dimmer, so a planet can approach much closer and still be hospitable. Even so, its not just water that matters; the atmosphere has to be just right too. The outer [planet], said Lisa Kaltenegger, who has joint appointments at Harvard and at Germanys Max Planck Institute of Astronomy, would need a lot of greenhouse gases to keep it warm, so you wouldnt want to take off your face mask.

The inner world, she said, could well be covered with a planet-wide ocean, if it has the same volume of water as Earth does relative to its size. That means it might be perpetually cloudy as well, since so much water so close to the star would result in a lot of evaporation.Thats a good thing, because the clouds would reflect some of the stars heat, which might otherwise make the surface too hot.

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