Gliese 581 is a main sequence red star with spectral type M3V, located 22 light years away from Earth, in the constellation Libra. Its estimated mass is about a third of that of the sun. Observations suggest that the star has a planetary system, containing at least free planets, possibly four, and, controversially, six planets. These planets are as follows: Gliese 581 b, Gliese 581 c, Gliese 581 d, Gliese 581 e, Gliese 581 d, Gliese 581 f, and Gliese 581 g. e, b, and c are confirmed to exist, d is unconfirmed but it is probable to exist, and f and g have been 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, then d and g.
The star had gained attention for Gliese 581 c, the first low-mass extrasolar planet to be found near a habitable zone. This planet was discovered in 2007. It has however been shown that under known terrestrial planet climate models, Gliese 581 c is likely to have a runaway greenhouse effect, similar to Venus, and as such may be too hot to be habitable. Gliese 581 d has been proposed as a habitable terrestrial planet, and it may be inside or just outside the outer boundary of the habitable zone (this depending on the greenhouse properties of its atmosphere), similar to Mars.
- Constellation: Libra
- Right ascension: 15h 19m 26.8250s
- Declination: -07° 43′ 20.209″
- Apparent magnitude (V): 10.56 to 10.58
- Absolute magnitude: 11.6
- Mass: 0.31 Solar mass
- Radius: 0.29 Solar radius
- Temperature: 3,480 ± 48 K (5804 °F, 3206 °C)
- Planets: 4 (Gliese 581 b, Gliese 581 c, Gliese 581 d, Gliese 581 e)