|Designation|| HO Librae|
|Right ascension||15h 19m 26.8250s|
|Declination||−07o 43′ 20.209″|
|Type of object||Red dwarf|
|Magnitude|| Apparent Mag: 10,56|
Absolute Mag: 11.6
|Distance from Earth||20.3 ±0.3 ly|
|Radial velocity||–−9.5 ±0.5 km/s|
|Proper motion|| RA: −1233.51 mas/yr|
Dec.: −94.52 mas/yr
|Parallax||160.91 ± 2.62 mas|
Gliese 581, also known as HO Librae, is a red dwarf star around 20.4 light years away. It is most famous for the discovery of the first low mass planet within a star's habitable zone. In total, four planets have found thus far orbiting the star, two of which may sit within the habitable zone.
Despite being only 20 light years distant Gliese 581 is invisible to the unaided eye. This is due to Gliese 581 being a red dwarf, and thus dim, with an apparent magnitude of only 10.56.
Gliese 581 is a cool, dim, red dwarf in the main sequence with the spectral classification of M3 V. The star is one-third as massive as our own Sun, with around 38 percent of its diameter. The star is only one percent as luminous as our Sun. However, much of the light emanating from Gliese 581 is in the infrared, therefore visually it is only 0.2 percent as bright. The star’s metallicity appears to be between 38 and 62 percent as enriched as our Sun, according to its abundance of iron.
There are four known planets in orbit around Gliese 581, with two inside the star’s habitable zone. The first, Gliese 581 c, is the most widely reported. However Gliese 581 d, discovered later, is a much better candidate for a terrestrial, possibly life supporting world under current terrestrial climate models.
Gliese 581 b
Gliese 581 b is the first planet discovered and the second closest known to its parent star. The planet was first discovered by a team of French and Swiss astronomers using the HARPS spectrograph and was announced on November 30, 2005. At the time, it was one of the smallest extrasolar worlds found and the fifth such potential terrestrial world to be found around a red dwarf star.
The planet has a minimum estimated mass of 15.65 times that of Earth. It orbits Gliese 581 in what is commonly called a "torch orbit", being only 0.04 AU (6 million kilometers) away, the orbit is so close that it only takes a little more than 5 days to complete.
On April 25, 2007 the same team of astronomers, again using the HARPS spectrograp, announced the discovery of Gliese 581 c (along with Gliese 581 d). When announced, Gliese 581 c was the smallest known extrasolar planet around a main sequence star (until Gliese 581 e was found). It is the third closest of the four known worlds.
The planet is calculated to have at least 5.36 times Earth’s mass (maximum of 10.4 times), and is estimated to be 1.5 times the Earth’s diameter. The planet orbits with a mean distance of only 0.07 AU, which takes just under 13 days to complete. At this distance, Gliese 581 c is tidally locked to its parent star.
Initially the planet was thought to be inside Gliese 581’s habitable zone, making it the first such terrestrial world outside of our Solar System to be found. This generate speculation about the possibility of the planet being suitable for life as we know it. However, further research indicated that the planet may be too close to the star, thus causing a Venus-like runaway greenhouse effect in its atmosphere that would boil off any water. Gliese 581 d instead is the better candidate for habitability.
Gliese 581 d
Gliese 581 d was discovered alongside and announced at the same time as Gliese 581 c, also sitting within the habitable zone of Gliese 581, and is now the prime candidate for habitability among the known worlds of the Gliese 581 system. It is the fourth of the four known worlds in distance from its parent star.
The planet has a mass between 7.09 and 13.8 times that of Earth. Gliese 581 d has a mean orbit of 0.22 AU, which it completes in 66.8 days. In April 2009, after further studies, the planet was confirmed to be within the habitable zone where liquid water could exist with a greenhouse effect under the right atmospheric conditions. The planet is thought to be too massive to be made only of rock and may have been an ice world at one time, with the abundant ices melting into deep worldwide oceans as the planet migrated closer to Gliese 581.
Gliese 581 e
The discovery of Gliese 581 e was announced on April 21, 2009. The planet is estimated to have a minimum mass only 1.9 times that of Earth making it the smallest extrasolar planet in the system and the smallest extrasolar planet known overall. Of the four known worlds it is nearest to its parent star.
The planet has a mean orbit of only 0.03 AU, which it completes in a mere 3 days. Because of its short distance, the planet is too close to be within Gliese 581’s habitable zone. It is unlikely to have an atmosphere because of the high temperatures and strong radiation it would receive at such a close distance to even a low mass star.