The star Gliese 581.
Digital Sky Survey photo.
Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||15h 19m 26.8250s|
|Declination||−07° 43′ 20.209″|
|Apparent magnitude (V)||10.56 to 10.58a|
|B-V color index||1.61|
|Radial velocity (Rv)||−9.5 ± 0.5 km/s|
|Proper motion (μ)||RA: −1233.51
Dec.: −94.52 mas/yr
|Parallax (π)||160.91 ± 2.62 mas|
|Distance||20.3 ± 0.3 ly
(6.2 ± 0.1 pc)
|Absolute magnitude (MV)||11.6|
|Surface gravity (log g)||4.92 ± 0.10|
|Luminosity (bolometric)||0.013 L☉|
|Temperature||3,480 ± 48 K|
|Metallicity||[M/H] = −0.33 ± 0.12|
|Age||7 to 11 × 109 years|
Gliese 581 (pronounced /ˈɡliːzə/) 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.
The name Gliese 581 refers to the catalog number from the Gliese Catalogue of Nearby Stars. Other names of this star include BD-07° 4003 (BD catalogue, first known publication) and HO Librae (variable star designation). It does not have an individual name such as Sirius or Procyon. The star is a red dwarf with spectral type M3V, located 20.3 light years away from Earth. It is located about two degrees north of Beta Librae, the brightest star in the constellation Libra. Its mass is estimated to be approximately a third that of the Sun, and it is the 87th closest known star system to the Sun.
An M-class dwarf star such as Gliese 581 has a much lower mass than the Sun, causing the core region of the star to burn hydrogen at a significantly lower rate. From the apparent magnitude and distance, we can estimate an effective temperature of 3200 kelvins and a visual luminosity of 0.2% of that of the Sun. However, a red dwarf such as Gliese 581 radiates primarily in the near infrared, with peak emission at a wavelength of roughly 830 nanometres (estimated using Wien's displacement law, which assumes the star radiates as a black body), so such an estimate will underestimate the star's total luminosity. (For comparison, the peak emission of the Sun is roughly 530 nanometres, in the middle of the visible part of the spectrum). When radiation over the entire spectrum is taken into account (not just the part that humans are able to see), something known as the bolometric correction, this star has a bolometric luminosity 1.3% of the Sun's total luminosity. A planet would need to be situated much closer to this star in order to receive a comparable amount of energy as the Earth. The region of space around a star where a planet would receive roughly the same energy as the Earth is sometimes termed the "Goldilocks Zone", or, more prosaically, the habitable zone. The extent of such a zone is not fixed and is highly specific for each planetary system. Gliese 581 appears too massive to be a Flare star which makes life more probable. 
At least four planets are believed to be orbiting Gliese 581. Gliese 581 b, approximately Neptune-sized, was discovered in August 2005 and was the fifth planet to be discovered around a red dwarf star. This inner planet is at least 16 times as massive as Earth (similar to Neptune's mass) and completes a full orbit of Gliese 581 in only 5.4 days.
Another planet, Gliese 581 c, was discovered in April 2007. Gliese 581 c is believed by some to be a rocky planet with a radius 1.5 times that of Earth. A direct measurement of the radius cannot be taken because, viewed from Earth, the planet does not transit its sun. With a minimum mass of roughly five times Earth—or one third that of Neptune—Gliese 581 c orbits just inside of the habitable zone of its parent star. It is notable as it is the planet with lowest minimum mass yet discovered in the habitable zone of another star, making it the most earthlike exoplanet found to date. The mean blackbody surface temperature has been estimated to lie between -3 °C (for a Venus-like albedo) and 40 °C (for an Earth-like albedo), however, the temperatures could be much higher (about 500 degrees Celsius) due to a runaway greenhouse effect akin to that of Venus. Some astronomers believe the system may have undergone planetary migration and Gliese 581 c may have formed beyond the frost line, with a composition similar to icy bodies like Ganymede. Gliese 581 c completes a full orbit in just under 13 days.
Observations of the star also revealed a third planet, Gliese 581 d, with a mass of roughly 7 Earths, or half a Uranus, and an orbit of 66.8 Earth days. It orbits just inside of the habitable zone of its star, which makes it a potential candidate for being able to support life.
Discovery of a fourth planet, Gliese 581 e was announced on 21 April 2009. This planet, at an estimated minimum mass of 1.9 Earths, is currently the lowest mass exoplanet identified around a normal star. It takes 3.15 days to orbit Gliese 581.
Dynamical simulations of the Gliese 581 system assuming that the orbits of planets b to d are coplanar show that the system becomes unstable if its component masses are more than 1.6 – 2 times the minimum mass. The upper limits on the masses of the planets are found to be 3.1, 30.4, 10.4 and 13.8 Earth masses for planets e, b, c and d respectively.
(in order from star)
|e||3.1 ≥ m ≥ 1.94 M⊕||0.03||3.14942 ± 0.00045||0|
|b||30.4 ≥ m ≥ 15.65 M⊕||0.04||5.36874 ± 0.00019||0|
|c||10.4 ≥ m ≥ 5.36 M⊕||0.07||12.9292 ± 0.0047||0.17 ± 0.07|
|d||13.8 ≥ m ≥ 7.09 M⊕||0.22||66.80 ± 0.14||0.38 ± 0.09|
In October 2008, members of the networking website Bebo beamed A Message From Earth, a high-power transmission at Gliese 581 c, using the RT-70 radio telescope belonging to the National Space Agency of Ukraine. This transmission is due to arrive in the Gliese 581 system's vicinity by the year 2029; the earliest possible arrival for a response, should there be one, would be in 2049.