This artist's conception shows the inner four planets of the Gliese 581 system and their host star, a red dwarf star only 20 light-years away from Earth. The large planet in the foreground is the newly discovered Gliese 581g, which has a 37-day orbit right in the middle of the star's habitable zone and is only 3 to 4 times the mass of Earth, with a diameter 1.2 to 1.4 times that of Earth.