Gliese 581, also known as GJ 581, is a red dwarf star approximately 20 light years from Earth. It is estimated to have a mass approximately 1/3rd the Sun, meaning the habitable zone around Gliese 581 is much closer than around the Sun. (The habitable zone is the region in which a planet orbiting a star can sustain liquid water on its surface). Gliese 581 has been the subject of much interest over the past few years as it is believed to have a number of orbiting planets including some that are in the habitable zone and could possibly support extraterrestrial life. In order, starting with the closest to the star, these planets are Gliese 581e, b, c, g, d, and f, although there is currently significant debate as to the existence of Gliese 581g and Gliese 581f (also known as GJ 581g and GJ 581f).
The history of observations and publications about the Gliese 581 system is a good example of the recent search for exoplanets. The first planet, Gliese 581b, was reported in 2005 in an article by Xavier Bonfils and others of Observatoire de Genève, Universit´e de Gen`eve using data gathered by the High Accuracy Radial velocity Planet Searcher (HARPS) instrument on the 3.6 meter telescope at La Silla Observatory in Chile. They reported a Nepune-sized planet (16 times Earth's mass) orbiting at 0.04 AU - too close to sustain liquid water on its surface.
Gliese 581c and Gliese 581d were identified in 2007 in an article by Stephane Udry and others of Observatoire de Gen`eve. Gliese 581c was calculated to have a minimum mass 5 times Earth and to orbit at 0.07 AU - near the hotter, inner boundary of the habitable zone. However, it was subsequently proposed that GJ 581c may have a runaway greenhouse effect similar to the planet Venus. Gliese 581d was estimated to have a minimum mass equal to 8 times Earth and to orbit at 0.25 AU - closer to the colder, outer boundary of the habitable zone, similar to planet Mars.
In 2009, Michael Mayor and others of Observatoire de Gen`eve reported a fourth planet, Gliese 581e (or GJ 581e). This planet was calculated to have a mass just 2 times Earth but orbited even closer to its red dwarf star than GJ 581b.
Gliese 581f and 581g were identified in 2010 by a team led by Steven Vogt of the University of California, Santa Cruz. This team developed a 6 planet solution to fit their extended set of observations that were composed of the previously published data supplemented with data drawn from the HIRES spectrometer at the W.M. Keck Observatory in Mauna Kea, Hawaii. Gliese 581f was estimated to be a planet with a mass 7 times Earth but well outside the habitable zone. Gliese 581g was calculated to have 3 times the mass of the Earth and be in the middle of the habitable zone, between Gliese 581c and Gliese 581d.
The picture on this page illustrates the 4 inner planets of the 6 planet version of the Gliese 581 system. In the upper left is the red dwarf star at the center of the system with orbiting planets GJ 581d, c, and b. On the right side is GJ 581g with liquid water on its surface.
Since publication of the 6 planet version by Vogt et al, there have been a number of articles debating whether GJ 581f or GJ 581g exist or are just artifacts of complex inferential modeling applied to data at the limit of accurate measurement. It would seem that more data and some time will provide a resolution to the issue. For example, in 2011, a team led by Robin D. Wordsworth of Laboratoire de Météorologie Dynamique, Institut Pierre Simon Laplace produced a model in which GJ 581d has a stable atmosphere and liquid water on its surface. Their climate model predicted that due to Rayleigh scattering the redder light from the red dwarf star was able to penetrate the atmosphere better than previously assumed. So this group would argue that GJ 581d is actually a good candidate for an exoplanet in the habitable zone of its star.