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'Gliese 876' is a red dwarf star located approximately 15 light-years away in the constellation Aquarius. It has the variable star designation IL Aquarii. As of 2006 the star is known to host three extrasolar planets, including one with a mass less than half that of Neptune.

Contents

Distance and visibility

Stellar characteristics

Planetary system

See also

References

External links

Distance and visibility

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,^[1] which corresponds to a distance of 4.70 parsecs (15.3 light years). Despite being located so close to us, 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 our local star.^[2] The surface temperature of Gliese 876 is cooler than our Sun and the star has a smaller radius.^[3] These factors combine to make the star only 1.24% as luminous as the Sun, though 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).^[4] Based on chromospheric activity the star is likely to be around 6,520 or 9,900 million years old, depending on the theoretical model used.^[5]
Like many low-mass stars, Gliese 876 is a variable star. It is classified as a BY Draconis variable and its brightness fluctuates by around 0.04 magnitudes.^[6] This type of variability is thought to be caused by large starspots moving in and out of view as the star rotates.^[7]

Planetary system

In 1998 an extrasolar planet was announced in orbit around Gliese 876 by two independent teams led by Geoffrey Marcy² and Xavier Delfosse.^[8] 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.^[9]
In 2001 a second planet was detected in the system, inside the orbit of the previously-discovered planet.^[10] 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. The two planets undergo strong gravitational interactions as they orbit the star, causing the orbital elements to change rapidly.^[11]
In 2005, further observations by a team led by Eugenio Rivera revealed a third planet in the system, inside the orbits of the two Jupiter-size planets.^[12] The planet, designated Gliese 876 d, has a minimum mass only 5.88 times that of the Earth and may be a terrestrial planet. Based on the radial velocity measurements and modelling the interactions between the two giant planets, the system's inclination is estimated to be around 50° to the plane of the sky. If this is the case and the system is assumed to be coplanar, the planetary masses are around 30% greater than the lower limits established by the radial velocity method. This would make the inner planet have a true mass around 7.5 times that of Earth. On the other hand, astrometric methods suggest an inclination around 84° for the outermost planet, which would mean the true masses are only slightly greater than the lower limit.^[13]. Another investigation led by Paul Shankland (which included Rivera and others), reveals a lack of any astronomical transits of the planets across the face of the star -- along with a radial velocity 'tilt' away from 90° (caused by the Rossiter-McLaughlin effect) -- and so indicates the notional ~90° inclination is further unlikely.^[14]
Both of the system's Jupiter-mass planets are located in the 'traditional' habitable zone (HZ) of Gliese 876, which extends between 0.116 to 0.227 AU from the star.^[15] This leaves little room for an additional habitable Earth-size planet in that part of the system. On the other hand, large moons of the gas giants, if they exist, may be able to support life. Furthermore, the habitable zone for planets whose rotation is synchronous with their orbital motion may be wider than the traditional view, which may enable the existence of habitable planets elsewhere in the system.^[16].

See also

★ Planetary system

★ Wilhelm Gliese

★ List of nearest stars

★ List of stars with confirmed extrasolar planets

References

1. HIP 113020
2. A Planetary Companion to a Nearby M4 Dwarf, Gliese 876, Marcy, G. ''et al.'', , , The Astrophysical Journal, 1998
3. Predicted infrared brightness of stars within 25 parsecs of the sun, Johnson, H., Wright, C., , , The Astrophysical Journal Supplement Series, 1983
4. Metallicities of M Dwarf Planet Hosts from Spectral Synthesis, Bean, J.L. ''et al.'', , , Astrophysical Journal Letters, 2006
5. On the Ages of Exoplanet Host Stars, Saffe, C. ''et al.'', , , Astronomy and Astrophysics, 2005
6. IL Aqr Samus ''et al.''
7. The spotted flare stars BY Dra, CC Eri: a model for the spots, some astrophysical implications, Bopp, B., Evans, D., , , Monthly Notices of the Royal Astronomical Society, 1973
8. The closest extrasolar planet. A giant planet around the M4 dwarf GL 876, Delfosse, X. ''et al.'', , , Astronomy and Astrophysics, 1998
9. Catalog of Nearby Exoplanets, Butler, R. ''et al.'', , , The Astrophysical Journal, 2006 (web version)
10. A Pair of Resonant Planets Orbiting GJ 876, Marcy, G. ''et al.'', , , The Astrophysical Journal, 2001
11. Dynamical Models of the Resonant Pair of Planets Orbiting the Star GJ 876, Rivera, E., Lissauer, J., , , The Astrophysical Journal, 2001
12. A ~7.5 M_⊕ Planet Orbiting the Nearby Star, GJ 876, Rivera, E. ''et al.'', , , The Astrophysical Journal, 2005
13. A mass for the extrasolar planet Gliese 876b determined from Hubble Space Telescope fine guidance sensor 3 astrometry and high-precision radial velocities, Benedict, G. ''et al.'', , , The Astrophysical Journal, 2002
14. On the Search for Transits of the Planets Orbiting Gliese 876, Shankland, P. D. ''et al.'', , , The Astrophysical Journal, 2006
15. Prospects for Habitable "Earths" in Known Exoplanetary Systems, Jones, B. ''et al.'', , , The Astrophysical Journal, 2005
16. Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability, Joshi, M.M., ''et al.'', , , Icarus, 1997

External links

★ ★ class=wikiexternal target=_blank>+IL+Aqr 'SIMBAD': V
★ IL Aqr -- High proper-motion Star

★ 'The Extrasolar Planets Encyclopaedia': Notes for star Gliese 876

★ 'SolStation': Gliese 876 / Ross 780

★ 'ARICNS': 4C01870

★ 'BBC': Smallest extrasolar planet found

★ 'Extrasolar Visions': Gliese 876

★ 'APOD': A planet for Gliese 876

★ 'Science News Online': Two teams find planet orbiting nearby star

★ Gliese 876 : THE CLOSEST EXTRASOLAR PLANET

★ Gliese 876d Pictures

★ Extrasolar.net page for GJ 876