Water Worlds: Has NASA Found Mirror Earths?

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 planets—and more nearly Earthlike ones—all the time. What’s more, it’s 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 Science, William Borucki, Kepler’s 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 star’s just-right region. This, said Paul Hertz, director of NASA’s astrophysics division at a press conference, “is really cool.” In astronomer-speak, that’s 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, it’s 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 Germany’s Max Planck Institute of Astronomy, “would need a lot of greenhouse gases to keep it warm, so you wouldn’t 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.That’s a good thing, because the clouds would reflect some of the star’s heat, which might otherwise make the surface too hot.

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Neither planet, in short, would be exactly like Earth, either in size or in atmosphere, but astronomers are getting used to the idea that that isn’t really required for life anyway. “People have wanted to see an exact analog of Earth orbiting an exact analog of the Sun,” said Kaltenegger, and Kepler or another mission may yet find such a world. But if you’re looking for a place where life might have taken hold, he adds, a mirror Earth isn’t necessary.

For Borucki, who had to push NASA for more than a decade before finally getting Kepler approved in 2000, and had to work for nearly a decade more to get it built and launched, the discovery of the Kepler 62 system—which also has three non-habitable planets, dubbed 62b, c and d, and possibly more to be found—is an excuse to get just a little poetic.

Imagine standing on the outer of these two possibly habitable planets and gazing up into the sky, he says. The inner planet, 62e, would be about as far as Venus is from Earth—but it would be significantly larger. Venus is already the brightest object in our own night sky, after the Sun and the Moon, and 62e would be brighter still. “It would,” says Borucki, “be an amazing, beautiful jewel in the sky.”

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