The planet, called Gliese 581g, is quite close at 20 light years from Earth's solar system. It is considered to be in the habitable zone because of its distance from its sun and its size.We don't know yet if there is any water, or any atmosphere. Gliese 581g differs from Earth in one important way. Like our moon, and like Mercury, the time it takes to rotate on its axis is synchronous with its orbital period, which means that it keeps one side facing its sun (or, in the case of the moon, Earth) at all times, while the other side stays in darkness. The side permanently facing the sun would be too hot to support life, while the dark side would be too cold. Nevertheless, there would be a band between the bright and dark sides--a "twilight zone", if you will--where temperatures could be moderate enough to accommodate life. It strikes my layman's mind, however, that if Gliese 581g had an earth-like atmosphere, such a tremendous temperature difference would result in a huge difference in atmospheric pressure, which in turn would give rise to some ferocious weather, perhaps severe enough to preclude life, at least on the surface.
Together, those two measurements tell scientists that any water on the planet will be in liquid form, and that the planet is large enough to have the gravitational pull to hold an atmosphere around it.
What's really exciting to me about this discovery, apart from its being the first of a planet similar in important respects to earth, is that it's so close to us. Since, so far as I know, there's no reason to believe that our immediate galactic neighborhood is more likely than any other (except, perhaps, near the galactic center) to include solar systems that have earth-like planets, the fact that there are two separated by only twenty light years suggests that such systems are fairly common.