Gliese 581

Gliese 581 ( /ˈɡliːzə/, also known as HO Librae) is a red dwarf with spectral type M3V, located about 22 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star to the Sun. Observations suggest that the star has a planetary system consisting of at least three, maybe four, and controversially six planets: Gliese 581 e, b, and c are certain; d is probable; and f and g are disputed.

Gliese 581 has been the subject of a "huge amount of attention" in the quest to discover the first habitable extrasolar planet; first for c, and then d and g.

The star first gained attention after Gliese 581 c, the first low-mass extrasolar planet found near a 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 too hot to be habitable, analogous to Venus. A subsequently proposed planet Gliese 581 d, may be just inside or just outside the outer boundary of the habitable zone (depending in part on the greenhouse properties of its atmosphere), analogous to Mars.

Gliese 581 e was announced in April 2009; this was at the time the least-massive planet found around a normal star.

Excitement spiked again in September 2010 with the claimed discovery of Gliese 581 g, orbiting between c and d, believed to be the planet with the greatest likelihood of having conditions suitable for liquid water at its surface found to date because it is within the middle of the habitable zone. Its existence remains in dispute.

On 27 November 2012, the European Space Agency announced a comet belt, with at least ten times as many comets as the Solar system. This put constraints on possible planets beyond 0.75 AU.