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Earth-like alien planet. Computer illustration of a view across the outer atmosphere of an Earth-like alien planet towards another planet and an arm of its galaxy in the distance. Earth-like planets capable of supporting life are found only in a star's habitable zone, a region around a star where the surface temperature of a planet is suitable for liquid water. (KEYSTONE/SCIENCE PHOTO LIBRARY/DETLEV VAN RAVENSWAAY/SCIENCE PHOTO LIBRARY)