Kepler Space Probe: A Shot at Finding New Earths

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NASAAmes / JPL-Caltech

A portion of the Milky Way galaxy appears in an image made by the Kepler satellite

William Borucki was more than a little bit nervous as he watched the Kepler spacecraft lift off from Cape Canaveral last March, and no wonder. It was way back in 1984 that he had first proposed sending up a telescope to search for Earth-like planets orbiting distant stars — and after more than two decades of tireless effort, it was finally happening. "In the back of my mind," says the NASA astronomer, "I was imagining this thing going up ... and then going plunk! into the ocean."

It didn't go plunk, though. In fact, Kepler is working better than anyone could have expected, NASA says. "There's not a flaw, not the tiniest fly in the Kepler ointment," says Geoffrey Marcy, a Berkeley astronomer and the world's premier planet hunter, who helped report the first results of the space probe for the Aug. 7 issue of Science. "I've never seen anything like it."

Kepler has not found any Earth-size planets yet, Borucki hastens to explain — nobody has. And the only new planet Kepler did see, inelegantly named HAT-P-7b, wasn't exactly new. Other telescopes had already spotted the silhouette of the Jupiter-size orb as it repeatedly passed in front of its parent star.

While Kepler saw these so-called transits too, its vision is so extraordinary that it glimpsed something more: a tiny dip in the overall light as HAT-P-7b passed behind its star. That's because the planet itself is glowing, which isn't surprising considering its incredible proximity to the star — the planet's year is just three days long. "The temperature is more than 4,000 degrees Fahrenheit," says Marcy, "which means it's hotter than the filament in an incandescent lightbulb." And while such a dip in light was predicted, says Borucki, "Nobody's ever seen it before."

The discovery was enough to get the Kepler team published in Science, but Marcy is more excited by its implications. For at least the next 3½ years, Kepler will be staring, unblinking, at 100,000 sunlike stars, watching for the telltale dimming as planets pass in front of them. Something as small as Earth will cause a dip in starlight of only 1/100 of one percent. That's what Kepler has just proven it can do. "And that," Marcy says, "was just a test observation, right out of the box."

Astronomers know from the past 14 years of observations that planets like Jupiter and Saturn and even Neptune are out there, and we have a pretty good idea of how common they are. But the ultimate goal is to find planets like Earth because those are considered the most likely places to find some sort of extraterrestrial life. "The reason I get up in the morning," says Marcy, "is to find Earths." If they're out there — and there's really no reason to think otherwise — Kepler will find them in time.

Not only that: by looking at so many stars, the probe is bound to find out not only whether Earth-type planets exist, but also how common they are. "We're on the verge of answering a question that was posed by the ancient Greeks more than 2,000 years ago," says Marcy. "I'm not joking when I say it makes the hairs stand up on the back of my neck."