Gliese 581 (/ˈɡliːzə/) is a red dwarf star of spectral type M3V which hosts a planetary system, 20.5 light-years (6.3 parsecs) away from Earth in the constellation Libra.

From the apparent magnitude and distance, astronomers have estimated an effective temperature of 3200 K and a visual luminosity of 0.2% of that of the Sun.

[18] However, a red dwarf such as Gliese 581 radiates primarily in the near infrared, with peak emission at a wavelength of roughly 830 nm (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.

[19] A planet would need to be situated much closer to this star in order to receive a comparable amount of energy as the Earth.

However, its observation history has been controversial due to false detections, and the radial velocity method yields little information about the planets themselves beyond their orbit and mass.

The first announcement of a planet around the star was Gliese 581b discovered by astronomers at the Observatory of Geneva in Switzerland and Grenoble University in France.

By comparison, sunlight on Mars has about 40% of the intensity of that on Earth, though if high levels of carbon dioxide are present in the planetary atmosphere, the greenhouse effect could keep temperatures above freezing.

[27] The next discovery was the inner planet Gliese 581e, also by the Observatory of Geneva and using data from the HARPS instrument, was announced on 21 April 2009.

[26] This planet, at a minimum mass of 1.9 Earths, was at the time the least massive confirmed exoplanet identified around a main-sequence star.

[28][29] In an interview with Lisa-Joy Zgorski of the National Science Foundation, Steven Vogt was asked what he thought about the chances of life existing on Gliese 581g.

"[30] Two weeks after the announcement of the discovery of Gliese 581f and Gliese 581g, astronomer Francesco Pepe of the Geneva Observatory reported that in a new analysis of 179 measurements taken by the HARPS spectrograph over 6.5 years, neither planet g nor planet f was detectable,[31][32] and the relevant measurements were included in a paper uploaded to the arXiv preprint server, though still unpublished in a refereed journal.

[34] Nevertheless, the existence of planet g remained controversial: Vogt responded in the media that he stood by the discovery[35][36] and questions arose as to whether the effect was due to the assumption of circular rather than eccentric orbits[37] or the statistical methods used.

[38] Bayesian analysis found no clear evidence for a fifth planetary signal in the combined HIRES/HARPS data set,[39][40] though other studies led to the conclusion that the data did support the existence of planet g, albeit with strong degeneracies in the parameters as a result of the first eccentric harmonic with the outer planet Gliese 581d.

[18] In their 2007 paper, Udry et al. asserted that if Gliese 581c has an Earth-type composition, it would have a radius of 1.5R🜨, which would have made it at the time "the most Earth-like of all known exoplanets".

[56][57] Some astronomers believe the system may have undergone planetary migration and Gliese 581c may have formed beyond the frost line, with a composition similar to icy bodies like Ganymede.

[45] It was thought to orbit with a period of 36.6 days at a distance of 0.146 AU, placing it within the habitable zone, and to have a minimum mass of 3.1 M🜨.

Previous analyses suggested that the planet (if existing) orbits within the star's habitable zone, where the temperatures are just right to support life.

The signal is a digital time capsule containing 501 messages that were selected through a competition on the social networking site Bebo.

[59] Using optical SETI, Ragbir Bhathal claimed to have detected an unexplained pulse of light from the direction of the Gliese 581 system in 2008.

[60] In 2012, the International Centre for Radio Astronomy Research at Curtin University in Perth, Gliese 581 was precisely targeted by Australian Long Baseline Array using three radio telescope facilities across Australia and the Very Long Baseline Interferometry technique, however no candidate signals were found.