Epoch J2000.0 Equinox J2000.0
|Right ascension||22h 53m 16.7s|
|Declination||-14° 15′ 49″|
|Apparent magnitude (V)||10.17|
|U-B color index||1.15|
|B-V color index||1.59|
|R-I color index||1.22|
|Variable type||BY Draconis|
|Radial velocity (Rv)||-1.7 ± 2 km/s|
|Proper motion (μ)||RA: 960.33 ± 3.77 mas/yr
Dec.: -675.64 ± 1.58 mas/yr
|Parallax (π)||212.59 ± 1.96 mas|
|Distance||15.3 ± 0.1 ly
(4.7 ± 0.04 pc)
|Absolute magnitude (MV)||11.81|
|Mass||0.334 ± 0.03 M☉|
|Temperature||3350 ± 300 K|
|Rotational velocity (v sin i)||1.38 km/s|
Bright Star Catalogue (5th rev. ed.)
Gliese 876 (pronounced /ˈɡliːzə/) is a red dwarf star approximately 15 light-years away from Earth in the constellation of Aquarius (the Water-bearer). As of 2010, it has been confirmed that three extrasolar planets orbit the star, and may contain more. Two of the planets are similar to Jupiter, while the closest planet is thought to be similar to a small Neptune or a large Earth.
Gliese 876 is located fairly close to our solar system. According to astrometric measurements made by the Hipparcos satellite, the star shows a parallax of 212.59 milliarcseconds, which corresponds to a distance of 4.70 parsecs (15.3 light years). Despite being located so close to Earth, the star is so faint that it is invisible to the naked eye and can only be seen using a telescope.
As a red dwarf star, Gliese 876 is much less massive than our Sun: estimates suggest it has only 32% of the mass of our local star. The surface temperature of Gliese 876 is cooler than our Sun and the star has a smaller radius. These factors combine to make the star only 1.24% as luminous as the Sun, and most of this is at infrared wavelengths.
Estimating the age and metallicity of cool stars is difficult due to the formation of diatomic molecules in their atmospheres, which makes the spectrum extremely complex. By fitting the observed spectrum to model spectra, it is estimated that Gliese 876 has a slightly lower abundance of heavy elements compared to the Sun (around 75% the solar abundance of iron). Based on chromospheric activity the star is likely to be around 6.52 or 9.9 billion years old, depending on the theoretical model used.
Like many low-mass stars, Gliese 876 is a variable star. Its variable star designation is IL Aquarii and it is classified as a BY Draconis variable. Its brightness fluctuates by around 0.04 magnitudes. This type of variability is thought to be caused by large starspots moving in and out of view as the star rotates. Gliese 876 emits X-rays.
On June 23, 1998, an extrasolar planet was announced in orbit around Gliese 876 by two independent teams led by Geoffrey Marcy and Xavier Delfosse. The planet was designated Gliese 876 b and was detected by making measurements of the star's radial velocity as the planet's gravity pulled it around. The planet, around twice the mass of Jupiter, revolves around its star in an orbit taking approximately 61 days to complete, at a distance of only 0.208 AU, less than the distance from the Sun to Mercury.
On April 4, 2001, a second planet was detected in the system, inside the orbit of the previously-discovered planet. The 0.62 Jupiter-mass planet, designated Gliese 876 c is in a 1:2 orbital resonance with the outer planet, taking 30.340 days to orbit the star. This relationship between the orbital periods initially disguised the planet's radial velocity signature as an increased orbital eccentricity of the outer planet. Eugenio Rivera and J. Lissauer found that the two planets undergo strong gravitational interactions as they orbit the star, causing the orbital elements to change rapidly.
On June 13, 2005, further observations by a team led by Rivera revealed a third planet in the system, inside the orbits of the two Jupiter-size planets. The planet, designated Gliese 876 d, was estimated to have a minimum mass only 5.88 times that of the Earth and may be a terrestrial planet.
In 2008, the system was used as a test case for the migration of 5 Earth-mass planets which had formed inside the orbit of the innermost gas giant of the system. If it formed at (in this test) 0.07 AU from the star, b's gravity would have pulled d into an eccentric orbit. That orbit then would have restabilised to its current location.
In January 2009, the mutual inclination between planets b and c was determined using a combination of radial velocity and astrometric measurements. The planets were found to be almost coplanar, with an angle of only 5.0+3.9−2.3° between their orbital planes. This angle is comparable to the mutual inclination between the orbits of the Earth and Mercury (approximately 7°). It is the first planetary system around a normal star to have mutual inclination between planets determined. This reveals the true masses of the planets: planets b and c have masses 2.64 and 0.78 times that of Jupiter respectively. While the astrometric measurements could not detect Gliese 876 d, on the assumption that its orbit is aligned with that of Gliese 876 c, the true mass would be 8.41 times that of Earth.
However in January 2010, almost every parameters from Bean's observations are altered by Correia's observations. The planet Gliese 876 d is now believed to be mildly eccentric and undergo intense tidal heating by a star. The study of the dynamical stability of this system indicated that the fourth planet would be stable in a 4:1 mean-motion resonance with the outermost planet "b", with a 15-day period. The radial velocity observations constrain the mass of any planet in such an orbit to be less than about one Earth mass.
(in order from star)
|d||6.3 M⊕||0.021||1.93785 ± 0.00002||0.139 ± 0.032|
|c||0.83 MJ||0.132||30.258 ± 0.009||0.266 ± 0.003|
|b||2.64 MJ||0.211||61.067||0.029 ± 0.001|