Gliese 581 - Planetary System

Planetary System

Gliese 581's ecliptic, as defined by the plane of its dust disc, inclines between 30◦ and 70◦.

Three to six planets are believed to be orbiting this star. Gliese 581 b, approximately Neptune-sized, was discovered in August 2005 and was the fifth planet to be discovered around a red dwarf. 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. In their 2007 paper, Udry et al. asserted that if Gliese 581 c 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".

A direct measurement of the radius cannot be taken because, viewed from Earth, the planet does not transit its star. 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. 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 posited 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. Its orbit would be just within the outer limit of the habitable zone of its star, which makes it a candidate for carbon-based life.

Discovery of a then-fourth planet, Gliese 581 e, was announced on 21 April 2009 and confirmed in September 2012. This planet, at a minimum mass of 1.9 Earths, was up to then the least massive confirmed exoplanet identified around a main-sequence star. It takes just under 3.15 days to orbit Gliese 581.

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". It must have "at least 10 times" as many comets as does the Solar system. This likely rules out Saturn-mass planets beyond 0.75 AU. However another (undiscovered) planet further out, say a Neptune-mass planet at 5 AU, might be required to keep the comet belt replenished.