It is the fourth closest known star to the Sun confirmed to possess a planetary system (after Alpha Centauri, Gliese 674 and Epsilon Eridani) and the closest such system known to consist of multiple planets. As of 2011, four extrasolar planets have been found to orbit the star.
The planetary system is also notable for the orbital properties of its planets. It is the only known system of orbital companions to exhibit a triple conjunction in the rare phenomenon of Laplace resonance (a type of resonance first noted in Jupiter's inner three Galilean moons). It is also the first extrasolar system around a normal star with measured coplanarity.
Distance and visibility 
Gliese 876 is located fairly close to the Solar System. According to astrometric measurements made by the Hipparcos satellite, the star shows a parallax of 213.28 milliarcseconds, which corresponds to a distance of 4.69 parsecs (15.3 ly), currently making it the fourth closest known star with orbiting planets, after Alpha Centauri, Epsilon Eridani, and Gliese 674. 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.
Stellar characteristics 
As a red dwarf star, Gliese 876 is much less massive than our Sun: estimates suggest it has only 32% of the mass of the Sun. 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 6.5 to 9.9 billion years old, depending on the theoretical model used. However, the low rotational period of the star as well as its membership among the young disk population suggest that the star is between 0.1–5 billion years old.
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.
Planetary system 
Observation history 
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.
Based on luminosity measurement, the circumstellar habitable zone (CHZ) is believed to be located between 0.116 to 0.227 AU.
On April 4, 2001, a second planet designated Gliese 876 c was detected, inside the orbit of the previously-discovered planet. The 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.
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.
On June 23, 2010, astronomers announced a fourth planet, designated Gliese 876 e. This discovery better constrained the mass and orbital properties of the other three planets, including the high eccentricity of the innermost planet. This also filled out the system inside e's orbit; additional planets there would be unstable at this system's age.
If this system has a comet disc, it is undetectable "brighter than the fractional dust luminosity 10−5" of a recent Herschel study.
Orbital arrangement 
Gliese 876 has a notable orbital arrangement. It is the first planetary system around a normal star to have mutual inclination between planets measured (previously the mutual inclination of the planets orbiting the pulsar PSR B1257+12 had been determined by measuring their gravitational interactions). Later measurements reduced the value of the mutual inclination, and in the latest four-planet models the incorporation mutual inclinations does not result in significant improvements relative to coplanar solutions.
The system has the second known example of a Laplace resonance with a 1:2:4 resonance of its planets. The first known example was Jupiter's closest Galilean moons - Ganymede, Europa and Io. Numerical integration indicates that the coplanar, four-planet system is stable for at least another billion years. This planetary system comes close to a triple conjunction between the three outer planets once per orbit of the outermost planet.
The outermost three of the known planets likely formed further away from the star, and migrated inward.
- Gliese 876 d
- Gliese 876 c
Gliese 876 c, discovered in 2001, is a giant planet at 0.62 Jupiter-mass planet. It is in a 1:2 orbital resonance with the outermost known planet, taking 30.340 days to orbit the star. The planet orbits within the habitable zone. Its mass makes it more likely to be a Class II planet in the Sudarsky extrasolar planet classification. The presence of surface liquid water is possible on sufficiently massive satellites should they exist.
- Gliese 876 b
Gliese 876 b, discovered in 1998 is around twice the mass of Jupiter and 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. Its mass makes it more likely to be a Class II or Class III planet in the Sudarsky model. The presence of surface liquid water is possible on sufficiently massive satellites should they exist.
- Gliese 876 e
Gliese 876 e, discovered in 2010, has a mass similar to that of the planet Uranus and its orbit takes 124 days to complete.
(in order from star)
|d||6.83 ± 0.40 M⊕||0.02080665 ± 0.00000015||1.937780 ± 0.000020||0.207 ± 0.055||—||—|
|c||0.7142 ± 0.0039 MJ||0.129590 ± 0.000024||30.0081 ± 0.0082||0.25591 ± 0.00093||—||—|
|b||2.2756 ± 0.0045 MJ||0.208317 ± 0.000020||61.1166 ± 0.0086||0.0324 ± 0.0013||—||—|
|e||14.6 ± 1.7 M⊕||0.3343 ± 0.0013||124.26 ± 0.70||0.055 ± 0.012||—||—|
See also 
- Uncertainties in the planetary masses and semimajor axes do not take into account the uncertainty in the mass of the star.
- Perryman, M. A. C. et al. (April 1997). "The HIPPARCOS Catalogue". Astronomy and Astrophysics 323: L49–L52. Bibcode:1997A&A...323L..49P.
- van Leeuwen, Floor (2007). Hipparcos, the new Reduction of the Raw data. Astrophysics and Space Science Library 350. Bibcode:2007A&A...474..653V. ISBN 978-1-4020-6341-1. Note: see VizieR catalogue I/311.
- Correia, A. C. M. et al. (February 2010). "The HARPS search for southern extra-solar planets. XIX. Characterization and dynamics of the GJ 876 planetary system". Astronomy and Astrophysics 511: A21. arXiv:1001.4774. Bibcode:2010A&A...511A..21C. doi:10.1051/0004-6361/200912700.
- Bean, Jacob L.; Benedict, G. Fritz; Endl, Michael (December 2006). "Metallicities of M Dwarf Planet Hosts from Spectral Synthesis". Astrophysical Journal Letters 653 (1): L65–L68. arXiv:astro-ph/0611060. Bibcode:2006ApJ...653L..65B. doi:10.1086/510527.
- Rojas-Ayala, Bárbara et al. (April 2012). "Metallicity and Temperature Indicators in M Dwarf K-band Spectra: Testing New and Updated Calibrations with Observations of 133 Solar Neighborhood M Dwarfs". The Astrophysical Journal 748 (2): 93. arXiv:1112.4567. Bibcode:2012ApJ...748...93R. doi:10.1088/0004-637X/748/2/93.
- Saffe, C.; Gómez, M.; Chavero, C. (November 2005). "On the Ages of Exoplanet Host Stars". Astronomy and Astrophysics 443 (2): 609–626. arXiv:astro-ph/0510092. Bibcode:2005A&A...443..609S. doi:10.1051/0004-6361:20053452.
- Marcy, Geoffrey W. et al. (October 1998). "A Planetary Companion to a Nearby M4 Dwarf, Gliese 876". The Astrophysical Journal Letters 505 (2): L147–L149. arXiv:astro-ph/9807307. Bibcode:1998ApJ...505L.147M. doi:10.1086/311623.
- Johnson, H. M.; Wright, C. D. (November 1983). "Predicted infrared brightness of stars within 25 parsecs of the sun". The Astrophysical Journal Supplement Series 53: 643–711. Bibcode:1983ApJS...53..643J. doi:10.1086/190905.
- Samus et al. (2007–2010). "IL Aqr". Combined General Catalogue of Variable Stars. Retrieved 2010-06-28.
- Bopp, B.; Evans, D. (1973). "The spotted flare stars BY Dra, CC Eri: a model for the spots, some astrophysical implications". Monthly Notices of the Royal Astronomical Society 164: 343–356. Bibcode:1973MNRAS.164..343B.
- Schmitt, Jürgen H. M. M.; Fleming, Thomas A.; Giampapa, Mark S. (September 1995). "The X-ray view of the low-mass stars in the solar neighborhood". The Astrophysical Journal 450: 392–400. Bibcode:1995ApJ...450..392S. doi:10.1086/176149.
- Delfosse, X. et al. (October 1998). "The closest extrasolar planet. A giant planet around the M4 dwarf GL 876". Astronomy and Astrophysics 338: L67–L70. arXiv:astro-ph/9808026. Bibcode:1998A&A...338L..67D.
- Jones, Barrie W.; Underwood, David R.; Sleep, P. Nick (April 2005). "Prospects for Habitable "Earths" in Known Exoplanetary Systems". The Astrophysical Journal 622 (2): 1091–1101. arXiv:astro-ph/0503178. Bibcode:2005ApJ...622.1091J. doi:10.1086/428108.
- Marcy, Geoffrey W. et al. (July 2001). "A Pair of Resonant Planets Orbiting GJ 876". The Astrophysical Journal 556 (1): 296–301. Bibcode:2001ApJ...556..296M. doi:10.1086/321552.
- Rivera, Eugenio J.; Lissauer, Jack J. (September 2001). "Dynamical Models of the Resonant Pair of Planets Orbiting the Star GJ 876". The Astrophysical Journal 558 (1): 392–402. Bibcode:2001ApJ...558..392R. doi:10.1086/322477.
- Rivera, Eugenio J. et al. (November 2005). "A ~7.5 M⊕ Planet Orbiting the Nearby Star, GJ 876". The Astrophysical Journal 634 (1): 625–640. arXiv:astro-ph/0510508. Bibcode:2005ApJ...634..625R. doi:10.1086/491669.
- Bean, J. L.; Seifahrt, Andreas (March 2009). "The architecture of the GJ876 planetary system. Masses and orbital coplanarity for planets b and c". Astronomy and Astrophysics 496 (1): 249–257. arXiv:0901.3144. Bibcode:2009A&A...496..249B. doi:10.1051/0004-6361/200811280.
- Rivera, Eugenio J. et al. (July 2010). "The Lick-Carnegie Exoplanet Survey: A Uranus-mass Fourth Planet for GJ 876 in an Extrasolar Laplace Configuration". The Astrophysical Journal 719 (1): 890–899. arXiv:1006.4244. Bibcode:2010ApJ...719..890R. doi:10.1088/0004-637X/719/1/890.
- Gerlach, Enrico; Haghighipour, Nader (2012). Can GJ 876 host four planets in resonance?. arXiv:1202.5865.
- B. C. Matthews; forthcoming study promised in J.-F. Lestrade et al. (2012). "A DEBRIS Disk Around The Planet Hosting M-star GJ581 Spatially Resolved with Herschel". Astronomy and Astrophysics. arXiv:1211.4898.
- As of 2006: Shankland et al, PD (2006). "On the search for transits of the planets orbiting Gliese 876". The Astrophysical Journal (653): 700–707.. No transit has been found as of 2012, either; so they are unlikely.
- Konacki, Maciej; Wolszczan, Alex (July 2003). "Masses and Orbital Inclinations of Planets in the PSR B1257+12 System". The Astrophysical Journal 591 (2): L147–L150. arXiv:astro-ph/0305536. Bibcode:2003ApJ...591L.147K. doi:10.1086/377093.
- Butler, R. P. et al. (December 2006). "Catalog of Nearby Exoplanets". The Astrophysical Journal 646 (1): 505–522. arXiv:astro-ph/0607493. Bibcode:2006ApJ...646..505B. doi:10.1086/504701.
|Wikimedia Commons has media related to: Gliese 876|
- "Artist renderings and data for the three planets orbiting GJ 876". Retrieved 2008-06-21.
- "A Dangerous Sunrise on Gliese 876d". NASA. Astronomy Picture of the Day. 2008-05-21. Retrieved 2008-06-21.
- "A planet for Gliese 876". NASA. Astronomy Picture of the Day. 1998-06-26. Retrieved 2008-06-21.
- "Gliese 876". Extrasolar Visions. Retrieved 2008-06-21.
- "Gliese 876 / Ross 780". SolStation. Retrieved 2008-06-21.
- "Gliese 876 : THE CLOSEST EXTRASOLAR PLANET". Observatoire de Haute Provence. Retrieved 2008-06-21.
- "Smallest extrasolar planet found". BBC News. 2005-06-13. Retrieved 2008-06-21.
- Image Gliese 876
- Extrasolar Planet Interactions by Rory Barnes & Richard Greenberg, Lunar and Planetary Lab, University of Arizona
- Sky map