The Gliese 581 planetary system is the gravitationally bound system comprising the star Gliese 581 and the objects that orbit it. The system is known to consist of at least three planets discovered using the
radial velocity method, along with a
debris disk. The system's notability is due primarily to early
exoplanetology discoveries, between 2008 and 2010, of possible
terrestrial planets orbiting within its
habitable zone and the system's relatively close proximity to the
Solar System at 20 light years away. 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 confirmed planets are believed to be located close to the star with near-circular orbits. In order of distance from the star, these are
Gliese 581e,
Gliese 581b, and
Gliese 581c. The letters represent the discovery order, with b being the first planet to be discovered around the star.
Observation history 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. Detected in August 2005 and using extensive data from the ESO/
HARPS spectrometer it was the fifth planet to be discovered around a red dwarf. The orbital period of Gliese 581d was originally thought to be 83 days but was later revised to a lower value of 67 days. 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. 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. Vogt was optimistic: "I'm not a biologist, nor do I want to play one on TV. Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say that ... the chances of life on this planet are 100%. I have almost no doubt about it." and the relevant measurements were included in a paper uploaded to the arXiv preprint server, though still unpublished in a refereed journal. The non-existence of Gliese 581f was accepted relatively quickly: it was shown that the radial velocity variations that led to the claimed discovery of Gliese 581f were instead associated with the stellar activity cycle rather than an orbiting planet. Nevertheless, the existence of planet g remained controversial: Vogt responded in the media that he stood by the discovery and questions arose as to whether the effect was due to the assumption of circular rather than eccentric orbits or the statistical methods used. Bayesian analysis found no clear evidence for a fifth planetary signal in the combined HIRES/HARPS data set, 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. On 27 November 2012, the European Space Agency announced that the Herschel space observatory had discovered a
comet belt "at 25 ± 12 AU to more than 60 AU". However another (undiscovered) planet further out, say a Neptune-mass planet at 5 AU, might be required to keep the comet belt replenished. This result was further supported by a 2014 study, whose authors argued that
Gliese 581d is "an artifact of stellar activity which, when incompletely corrected, causes the false detection of the planet
g." all subsequent studies of the radial velocity data have confirmed the stellar, rather than planetary, origin of the signal corresponding to Gliese 581d, Most of these models predict, however, that the inner planets are close in with circular orbits, while outer planets, particularly Gliese 581d, should it exist, are on more elliptical orbits. Models of the habitable zone of Gliese 581 show that it extends from about 0.1 to 0.25 AU. The three confirmed planets orbit closer to the star than the inner edge of the habitable zone, while planets g and d would have orbited within it. 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), It has been hypothesized that 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. Gliese 581c completes a full orbit in just under 13 days. Using optical SETI,
Ragbir Bhathal claimed to have detected an unexplained pulse of light from the direction of the Gliese 581 system in 2008. 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.
Debris disk At the outer edge of the system is a massive debris disk containing more comets than the Solar System. The debris disc has an inclination between 30° and 70°. This is supported by a 2024 study, which found an inclination for the planetary orbits of about 47°. ==See also==