Gliese 581 c and Earth compared. Computer artwork comparing the size of the extrasolar planet Gliese 581 c (right) with that of the Earth (left). Gliese 581 c is the most Earth-like alien planet yet discovered and lies about 20.3 light years away, in the constellation Libra. It is thought to be at least five times more massive than the Earth, with a radius about 1.5 times larger.