DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY
Extrasolar planet Gliese 581c, computer artwork. Gliese 581c (right) is one of three planets that orbit the star Gliese 581 (left). Another one of the planets is seen at far left crossing the parent star. 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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