Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth. Its mass is estimated to be approximately a third of that of the Sun, and it is the 87th closest known star system to the Sun. Observations suggest that the star has at least four planets: Gliese 581 b, c, d, e.
The star system gained attention after Gliese 581 c, the first low mass extrasolar planet found to be near its star's habitable zone, was discovered in April 2007. It has since been shown that under known terrestrial planet climate models, Gliese 581 c is likely to have a runaway greenhouse effect, and hence is probably not habitable. However, the subsequently discovered outermost planet Gliese 581 d is firmly within the habitable zone. In April 2009, the discovery of exoplanet Gliese 581 e, at that time the closest-known in mass to Earth, was announced.