Check out the details in Wired:
After years of saying habitable exoplanets are just around the corner, planet hunters have finally found one. Gliese 581g is the first planet found to lie squarely in its star's habitable zone, where the conditions are right for liquid water.
''The threshold has now been crossed,'' said astronomer R. Paul Butler of the Carnegie Institution of Washington, one of the planet's discoverers, in a press briefing September 29. ''The data says this planet is at the right distance for liquid water, and the right mass to hold on to a substantial atmosphere.''
While you're at Wired, check out this entry from Jonah Lehrer's blog The Frontal Cortex about just how much practice makes perfect.