Looking for Life? Try Gliese 581c.

Finding 227 planets beyond our solar system was fun, but astronomers were getting a little edgy that all these orbs were duds, biologically speaking. Some were too hot for life and some were too cold; some were big bags of gas where life would have trouble getting a toehold, and some orbited so close to their star that solar radiation would fry any organic molecules with the temerity to organize themselves into life. So when astronomers using the 141-inch telescope at the European Southern Observatory in Chile detected Gliese 581c, there was more relief than wonder: they had found the first "habitable" planet. London bookies immediately lowered the odds on extraterrestrial life from 1,000-1 to 100-1.

The astronomers believe Gliese 581c is habitable because it has a radius just 50 percent larger than Earth's, a mass five times greater, and orbits its star (in the constellation Libra) at a distance of 7 million miles. With many stars, that would be the torrid zone where liquid water, thought to be a requirement for life, could not exist. But this star is a red dwarf, a class that is smaller and 50 times fainter than our Sun. That means that even at 7 million miles (Earth is 93 million miles from our Sun), planetary temperatures likely range from 0 to 40 degrees Celsius, calculates Stéphane Udry of the Geneva Observatory, one of the discoverers. Just right.

Astronomers not associated with the discovery are withholding judgment on whether Gliese 581c indeed harbors liquid water. More observations will be needed to confirm that, and that's what the ESO astronomers plan. They detected the planet indirectly, by observing a wobble in the orbit of the host star that could have been caused only by the gravitational tug of a circling planet. From that wobble they inferred the planet's orbital distance and mass. The next step is to watch the planet pass in front of its star, letting them pin down its size and therefore its density. That will suggest what it's made of. Models of planetary formation predict that a planet of roughly this size and orbital distance "should be either rocky, like our Earth, or covered with oceans," says Udry. More observation should also reveal if the planet has an atmosphere, and if so, its composition and weight. A crushing atmosphere such as Venus's would make liquid water on the surface impossible.

But it's a big galaxy, and an even bigger universe. If Gliese 581c comes up short on the checklist for life, it's likely that some other planet will make the cut. Even those 100-1 odds look pessimistic.