I became very excited when it was announced last fall that a planet known as Gliese 581g, the sixth planet found to orbit red dwarf star Gliese 581 a mere 20 light years away, might harbor life. The tidally locked rocky planet similar to Earth's size was within the "Goldilocks zone" for supporting life: not too hot, not too cold for liquid water. I wrote a whole post gushing about it, and how the description of the planet reminded me of my days of exciting space exploration in Mass Effect.
It wasn't long, however, before some scientists were challenging not the notion that this planet might be habitable, but whether the planet even exists in the first place*. The trouble is that we can't observe these faraway planets directly using telescopes (yet)--we basically have to observe the wobble of the star to determine how many planets it has, how far they are from the star, what their masses and volumes might be, etc. It's still not clear whether the planet does or does not exist, but it seems that this celebration over the discovery of another habitable planet--especially one so close to home--was premature.
If it exists*, Gliese 581g would lie comfortably in the middle of Gliese 581's Goldilocks zone, but it is not the only planet in the range of the star's habitable region. Gliese 581c and Gliese 581d are on the inside edge and outside edge of the Goldilocks zone, respectively, and the existence of both planets is well accepted and supported. However, the temperature (and thus inferred habitability) of a planet depends on more than mere distance from the sun. After all, it has been suggested that Mars is within the outside edge of our sun's Goldilocks region. There is evidence that Mars once had liquid water. It was kept warm in spite of its distance from the sun by high atmospheric concentrations of carbon dioxide, which, as we are now all too aware here on Earth, is a greenhouse gas. But now its remaining water is locked up in ice, its atmospheric carbon dioxide minuscule. What happened? Mars is small so its interior cooled much faster than Earth's, causing it to lose its ability to cycle carbon dioxide into the atmosphere (a process that likely required volcanic activity and burial of carbonate sediments at great pressure and heat). It lost its greenhouse effect, and thus its liquid water. Had Mars been the same size as the Earth, it might have avoided this fate and retained its water long enough to support life. (Kasting et al. 1988)
As for the planets around Gliese 581, Gliese 581c is thought to be too close to the star for liquid water, having surface temperatures over 1000 deg C. Gliese 581d was initially thought to be far enough from the star to be too cold for liquid water. But simulations of the planet's atmosphere have suggested that it may contain high enough concentrations of carbon dioxide to keep the planet warm enough to maintain liquid water. (See BBC News article, and Astrophysical Journal Letters.) Perhaps it has succeeded where Mars did not.
Gliese 581d, which may be tidally locked (with one side permanently facing its sun), receives less stellar energy than Mars but is more massive than Earth. 3D climate simulations predict that it will have a stable atmosphere that could support liquid water oceans, clouds, and rainfall. With sufficient concentrations of carbon dioxide and other gases, global mean temperatures could be above 0 deg C. But we shouldn't plan on moving in--the dense air on Gliese 581d would be toxic to humans.
This is all still far from a sure thing. The predictions are based on simulations which may or may not be accurate, and there are many "maybes" in the predictions. Just because it could support liquid water doesn't mean it does. And of course just because it has liquid water does not mean it has life. But a sci-fi geek can dream, right?
* Update 7/20/12: Looks like scientists have confirmed the existence of Gliese 581g aka Zarmina. Yay!