"A rocky alien planet called Gliese 581d may be the first known world beyond Earth capable of supporting life as we know it, a new study suggests.

Astronomers performing a new atmospheric-modeling study have found that the planet likely lies in the "habitable zone" of its host star — that just-right range of distances that allow liquid water to exist. The alien world could be Earth-like in key ways, harboring oceans, clouds and rainfall, according to the research.

This conclusion is consistent with several other recent modeling studies. But it does not definitively establish that life-sustaining water flows across the planet's surface.

The new study assumes that Gliese 581d, which is about seven times as massive as Earth, has a thick, carbon-dioxide-based atmosphere. That's very possible on a planet so large, researchers said, but it's not a given.

Gliese 581d's parent star, known as Gliese 581, is a red dwarf located 20 light-years from Earth, just a stone's throw in the cosmic scheme of things. So far, astronomers have detected six planets orbiting the star, and Gliese 581d is not the only one intriguing to scientists thinking about the possibility of life beyond Earth.

Another planet in the system, called Gliese 581g, is about three times as massive as Earth, and it's also most likely a rocky world. This planet received a lot of attention when its discovery was announced in September 2010, because it's located right in the middle of the habitable zone. That makes 581g a prime candidate for liquid water and life as we know it — if the planet exists..."

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