DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY
Surface of extrasolar planet Gliese 581c, computer artwork. Gliese 581c orbits its parent star Gliese 581, which is seen setting on the horizon. It is one of three planets orbiting this star; the other two (Gliese 581b and Gliese 581d) are also seen in the sky at upper left. Gliese 581c is the most Earth-like alien planet yet discovered. It is 1.5 times Earth's diameter and 5 times its mass, and it lies in the star's habitable zone. This is a region around a star where the surface temperature of a planet is suitable for liquid water. Gliese 581c is fourteen times closer than Earth is to the Sun, but the low heat output of the red dwarf makes its habitable zone much closer to it than the Sun's. Gliese 581 lies about 20.5 light years from Earth in the constellation Libra.
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