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Insanecowposse · 10-04-2010, 11:52 AM

An Earth-size planet has been spotted orbiting a nearby star at a distance that would makes it not too hot and not too cold -- comfortable enough for life to exist, researchers announced Wednesday.

If confirmed, the exoplanet, named Gliese 581g, would be the first Earth-like world found residing in a star's habitable zone -- a region where a planet's temperature could sustain liquid water on its surface.[Illustration of planet Gliese 581g.]

And the planet's discoverers are optimistic about the prospects for finding life there.

"Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent," said Steven Vogt, a professor of astronomy and astrophysics at the University of California, Santa Cruz, during a press briefing today.

"I have almost no doubt about it."

This artist's conception shows the inner four planets of the Gliese 581 system and their host star, a red dwarf star only 20 light years away from Earth. The large planet in the foreground is the newly discovered GJ 581g, which has a 37-day orbit right in the middle of the star's habitable zone and is only three to four times the mass of Earth, with a diameter 1.2 to 1.4 times that of Earth.

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"It's both an incremental and monumental discovery," Sara Seager, an astrophysicist at the Massachusetts Institute of Technology, told SPACE.com. Incremental because the method used to find Gliese 581g already has found several planets (all super-Earths, more massive than our own world) outside their stars' habitable zone, along with non-Earth-like planets within the habitable zone.

"It really is monumental if you accept this as the first Earth-like planet ever found in the star's habitable zone," said Seager, who was not directly involved in the discovery.

Vogt, Butler and their colleagues will detail the planet finding in the Astrophysical Journal.

The newfound planet joins more than 400 other alien worlds known to date. Most are huge gas giants, though several are just a few times the mass of Earth.