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A little wobble spurs hope for finding life on distant worlds: extrasolar planet is in the right location to be habitable.

Scientists have spotted an Earth doppelganger that may have the right specs to harbor life, just 20 light-years distant in the direction of the constellation Libra.

Although details about conditions on the planet's surface remain a mystery, the find suggests that many more potentially habitable worlds are likely to be found. The discovery was reported online September 29 at arXiv.org and will be described in an upcoming Astrophysical Journal.

Finding the planet so nearby and so soon after extrasolar planets were first discovered suggests that the galaxy is teeming with Earthlike worlds, said coauthor Paul Butler of the Carnegie Institution for Science in Washington, D.C. He noted that it has been only 15 years since astronomers first located a planet orbiting a sunlike star beyond the solar system.

"This is the first one, but the threshold has now been crossed," Butler said in a September 29 press briefing. "Over the next 10 years I would be shocked if there weren't many tens of these things." The body is one of two newly discovered planets orbiting the red dwarf star Gliese 581--setting the current record at six for the most planets circling a star other than the sun.

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"It's kind of a mini-version of our own solar system," said study coauthor Steven Vogt of the University of California, Santa Cruz.

Vogt and his colleagues used more than 200 nights' worth of data to track tiny wobbles of Gliese 581 caused by the gravitational tug of orbiting planets. Astronomers can use these wobbles not only to detect unseen planets, but also to determine their masses and orbital paths.

The researchers found that the planet Gliese 581g, estimated to be about three times more massive than Earth, orbits its star about once every 37 days. The average surface temperature is estimated to range from -24 degrees to 10 degrees Fahrenheit, but may vary greatly.

Just as the moon always keeps the same face toward Earth, one side of the planet always faces its parent star. But the planet's temperature range may not be as extreme as the moon's, where the difference between the dark and illuminated sides can be more than 500 degrees Fahrenheit. Modeling experiments suggest that winds could help distribute heat on Gliese 581g, moderating extreme temperatures. "It would be quite a benign, comfortable place to live," Vogt said, and one with "a lot of different niches for different kinds of life to evolve stably."

The result is "fantastic news," said Franck Selsis, an astrophysicist at the Laboratoire d'Astrophysique de Bordeaux in France. Selsis and his colleagues have proposed that two other Gliese-orbiting planets might be habitable. Recent studies, Selsis said, show that the habitability zone of Gliese 581 extends farther than originally thought, placing planet Gliese 581d, which flanks Gliese 581g on the side farther from the star, in the habitability zone, too. More extensive modeling of Gliese 581d suggests that its bigger mass (about 7.1 times more massive than Earth) could have accumulated an atmosphere large enough to keep the planet warm, making liquid water a possibility there too.

A surface temperature that allows liquid water is considered necessary for life. "At this point, we can't say anything about the physical conditions on the planet," Butler said of Gliese 581g. "We can't say anything for sure about the atmosphere. We can't say anything sure about water."

Vogt points out that water is abundant in the galaxy, so it is hard to imagine that Gliese 581g wouldn't have any. "Given the ubiquity and propensity of life to flourish wherever it can ... my own personal feeling is that the chances of life on this planet are 100 percent," Vogt said. "I have almost no doubt about it."

But Selsis cautions that being in the all-important "habitability zone" where temperatures are right for life as we know it is "necessary, but certainly not sufficient." The planet could have formed without any water. It could be regularly bombarded with objects capable of wiping out any life that develops. Or it could have such a heavy atmosphere that its surface would be extremely hot.

Astrophysicist and planetary scientist Sara Seager of MIT said that the authors don't yet know if the greenhouse effect would be too strong on Gliese 581g for life to survive. For astronomers to get accurate atmospheric data, the planet would have to pass between its star and Earth, which Gliese 581g doesn't do. Another option would be to launch equipment into space to image the planet directly, but even a nuclear-powered probe would take more than 200 years to get close enough.

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On the small side

All but one of Gliese 581's planets (top) hug the parent star closer than Mercury orbits the sun (bottom). But Gliese 581 is substantially dimmer than the sun, so planet g could still have liquid water on its surface--considered a requisite for life.
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Title Annotation:STORY ONE
Author:Sanders, Laura
Publication:Science News
Geographic Code:1USA
Date:Oct 23, 2010
Words:816
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