Gliese 581 is an M2.5V red dwarf star located 20.40 light years away from Earth. Gliese 581
It is 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 with respect to Earth, is the 87th closest star system.
The star system recently gained attention after Gliese 581 c, the first extrasolar planet found to be in its star's habitable zone, was discovered in April 2007.
Observations suggest that the star has at least three planets.

The name Gliese 581 refers to the Star catalogue of nearby stars by Wilhelm Gliese.
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.
Gl 581 is classified as a BY Draconis variable, which is a star that shows a fluctuating luminosity with periods ranging from hours to several months.
These light changes are caused by variations in the surface brightness of the photosphere combined with the axial rotation of the star.
Gl 581 undergoes long-term changes in luminosity, with the brightness varying by up to half a magnitude.
(Short-term variability is at most 0.006 magnitudes.)
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. As a result, this star has only 1.3% of the Sun's total luminosity and 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-accurately, the habitable zone.

At least three planets are believed to be orbiting this star. One, about Neptune-sized, was discovered in 2005.
Another, having an estimated radius 1.5 times that of Earth, was discovered in 2007.
The latter is notable as it is the smallest planet yet discovered in the habitable zone of another star,
making it the most "earthlike" exoplanet found to date.
Observations of the star also revealed the possibility of a third planet with a mass of roughly 8 Earths and an orbit of 84 Earth days.
Gliese 581 b was the fifth planet to be discovered around a red dwarf star. This inner planet is 16 times as massive as Earth (similar to Neptune's mass)
 and completes a full orbit of Gliese 581 in only 5.4 days.
Gliese 581 c is believed to be a rocky planet of 50% greater radius than Earth and about five times Earth's mass,
orbiting within the habitable zone of its parent star.
The mean surface temperature has been estimated to lie between -3 °C (for a Venus-like albedo) and 40 °C (for an Earth-like albedo),
suggesting the possibility of liquid water on a solid surface.
 However, given the closer-orbiting Neptune-mass planet, the system has 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. Gliese 581 c orbits one of the hundred closest stars to Earth,
which has led some to suggest that Earth-like planets are much more common than previously speculated.
Gliese 581 d is about eight times as massive as the Earth and orbits its sun in 84 days.

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Image credit : ESA

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Gliese 581

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