What kind of place is the fourth planet from the star Gliese 581? It's at least three times as massive as the Earth. It orbits its dim red dwarf once every 36.6 days, at a distance of about 14 million miles. And one more thing: It just might be the first truly habitable planet discovered outside our solar system. (For more on how scientists spot these alien worlds, check out NOVA scienceNOW's Hunt for Alien Earths. )
The fourth planet--also known as Gliese 581g--isn't the first exoplanet to lay a claim on that title. Two of Gliese 581g's neighboring planets have also temporarily held the mantle; one was eventually determined to be too hot, and the other likely too cold, to maintain the liquid water that astrobiologists believe is a critical ingredient for life. Figuring out the temperature of an alien world is tricky business. Without much hard data to go on, scientists must make educated guesses about how much sunlight an exoplanet reflects and how much it absorbs; how much extra warmth it gets from the greenhouse effect; and whether other heat sources, like a roiling interior or tidal friction, nudge up the mercury. (Of course, astronomers would love to be able to get this information--here's how they would use it to probe an exoplanet's atmosphere from afar.)
All of which is to say that scientists don't know exactly what a thermometer on Gliese 581g might read, but that there's a good chance that--at least somewhere on the planet--it would hover in the magical range between water's freezing and boiling points.
The planet's gravitational fingerprint was detected in data collected over 11 years at the Keck Observatory in Hawaii. For astronomers searching for "living planets," a star like Gliese 581 is low-hanging fruit: Close-in planets are typically easier to detect than more distant ones, and around a dim bulb like Gliese 581, tightly-orbiting planets won't be scorched dry, as they would be around a brighter star. The fact that this potentially habitable world turned up so quickly gives astronomers reason to believe that, as the team wrote right in their abstract, "our Milky Way could be teeming with potentially habitable planets."
And that's what's really remarkable about Gliese 581g: The fact that, one day soon, it could be utterly unremarkable, just another pale blue dot in a sky that's full of them.