French scientists have confirmed with computer models that Gliese 581d, a planet orbiting a red dwarf star about 20 light years from here, has a stable atmosphere, comfortable temperatures, and a surface covered in liquid water. It's the first planet orbiting another star that could definitely support life, and it's basically next door.
While Gliese 581d is too small and far away to observe directly, we can infer some things about it from the gravitational effects that it has on its parent star and fellow planets. We know that Gliese 581d is about twice the size of Earth (and six times the mass), we know that it's rocky (not a gas giant like Jupiter or Saturn). This means that it's large enough and dense enough to be able to hold on to a substantial atmosphere. We can also estimate about how much energy Gliese 581d receives from its red dwarf star, and based on all of this information, French scientists have been able to model a range of potential climates showing that "GJ581d will have a stable atmosphere and surface liquid water for a wide range of plausible cases."
"Will have" is a pretty strong language when you're talking about a planet some 117,569,996 million miles away, but based on the models, it sounds like it's a sure thing. That's not to say it would necessarily be a pleasant place to live, though. Gliese 581d probably depends on a significant greenhouse effect to keep itself warm since it gets relatively little energy from its star. The atmosphere is mostly CO2, and while you'd get clouds and warm rain and oceans and stuff, the surface itself would be "in a perpetual murky red twilight." The planet also may be tidally locked (meaning that one side perpetually faces its sun), and at double Earth's gravity, it's not exactly a vacation spot.
Despite all this, it would be an ideal place to find some extraterrestrial plant life, and where there are plants there might be animals, specifically animals which have adapted to high gravity, low light, low oxygen environment. So think small and low to the ground with big eyes. And of course, there's lots of potential for animal life in warm oceans, too.
While 20 light years is extremely close on the galactic scale, using current technology it would still take us humans about 300,000 years to reach the Gliese system. A better bet, at least for now, might be to just send an interstellar probe, which might be able to reach Gliese 581d in just a century or two.
(The original headline as assigned by me read: "Habitable exoplanet CONFIRMED! Warm and wet, scientists say." It has been changed to better reflect the tone of the post. -Ed)