Gliese 876 b

Planetbox begin
name=Gliese 876 bPlanetbox image


caption = An artist's impression of Gliese 876b.Planetbox star
star = Gliese 876
constell = Aquarius
RA = RA|22|53|16.73
DEC = DEC|−14|15|49.3
dist_ly = 15.3
dist_pc = 4.7
class = M3.5VPlanetbox orbit
semimajor=0.208 ± 0.012
eccentricity=0.029 ± 0.005cite journal| author=G. Laughlin "et al." | journal=Astrophysical Journal | issue=622| page=1182-1190 |year=2005]
period=60.940 ± 0.013
ang_dist=44.032
long_peri=175.7 ± 6.0
t_peri=2,452,460.3 ± 1.0
semi-amp=212.6 ± 0.76
semimajor_gigameter =
periastron = 0.201968
periastron_gigameter =
apastron = 0.214
apastron_gigameter =
period_year =0.1668446
period_megasecond = Planetbox character
mass=>1.93-2.4 [G. Laughlin (2005) m sin i = 1.935, i = 84±6 degrees per Benedict, G. F. "et al." ApJL, 581, L115 (2002)]
radius=
density=
gravity=
temperature=
mass_earth = 613.354
gravity_earth =
insolation =
insolation_earth =
radius_megameter =
mass_wekagram =
radius_earth =Planetbox discovery
discovery_date = 23 June 1998
discoverers = Marcy "et al."
discovery_method = Radial velocity
discovery_site = California and
Carnegie Planet Search
flag|United States
discovery_status = PublishedPlanetbox catalog
names=GJ 876 b, G 156-57B, 8pc 212.69B Planetbox reference
star=Gliese+876
planet=b

Gliese 876b is an extrasolar planet orbiting the red dwarf star Gliese 876 every 60.940 days. Discovered in June 1998, Gliese 876b was the first planet to be discovered orbiting a red dwarf. It is the outermost known planet in its planetary system.

Discovery

Gliese 876b was discovered independently by two different teams, one led by Geoffrey Marcycite journal|url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?1998ApJ...505L.147M&db_key=AST&nosetcookie=1|author=Marcy, G. et al.|title=A Planetary Companion to a Nearby M4 Dwarf, Gliese 876|journal=The Astrophysical Journal|volume=505|issue=2|pages=L147–L149|year=1998] and the other by Xavier Delfosse. [cite journal|url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?1998A%26A...338L..67D&db_key=AST&nosetcookie=1|author=Delfosse, X. et al.|title=The closest extrasolar planet. A giant planet around the M4 dwarf GL 876|journal=Astronomy and Astrophysics|volume=338|pages=L67–L70|year=1998] Like the majority of known extrasolar planets, it was discovered by detecting variations in its star's radial velocity as a result of the planet's gravity. This was done by making sensitive measurements of the Doppler shift of the spectral lines of Gliese 876.

Orbit and mass

Gliese 876b is in a 2:1 orbital resonance with the inner planet Gliese 876c, [cite journal|url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2001ApJ...556..296M&db_key=AST&nosetcookie=1|author=Marcy, G. et al.|title=A Pair of Resonant Planets Orbiting GJ 876|journal=The Astrophysical Journal|volume=556|issue=1|pages=296 – 301|year=2001] which leads to strong gravitational interactions between the two planets. [cite journal|url=http://adsabs.harvard.edu/abs/2001ApJ...558..392R|author=Rivera, E., Lissauer, J.|title=Dynamical Models of the Resonant Pair of Planets Orbiting the Star GJ 876|journal=The Astrophysical Journal|volume=558|issue=1|pages=392 – 402|year=2001] As a result, the orbital elements of the planet change fairly rapidly as the orbits precess. [cite journal|url=http://www.journals.uchicago.edu/doi/abs/10.1086/504701|author=Butler, R. et al.|title=Catalog of Nearby Exoplanets|journal=The Astrophysical Journal|volume=646|pages=505 – 522|year=2006 ( [http://exoplanets.org/planets.shtml web version] )] The planet's orbit has a low eccentricity, similar to the planets in our solar system. The semimajor axis of the orbit is only 0.208 AU, less than that of Mercury in our solar system. However Gliese 876 is such a faint star that this puts it in the outer part of the habitable zone. [cite journal|url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2005ApJ...622.1091J&db_key=AST&nosetcookie=1|author=Jones, B. et al.|title=Prospects for Habitable "Earths" in Known Exoplanetary Systems|journal=The Astrophysical Journal|volume=622|issue=2|pages=1091 – 1101|year=2005]

A limitation of the radial velocity method used to detect Gliese 876b is that only a lower limit on the planet's mass can be obtained. This lower limit is around 1.93 times the mass of Jupiter. The true mass depends on the inclination of the orbit, which in general is unknown. In the case of a resonant system like Gliese 876, gravitational interactions between the planets can be used to find the true masses: the best fit to the radial velocity data is for an inclination of around 50° to the plane of the sky. [cite journal|url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2005ApJ...634..625R&db_key=AST&nosetcookie=1|author=Rivera, E. et al.|title=A ~7.5 M_⊕ Planet Orbiting the Nearby Star, GJ 876|journal=The Astrophysical Journal|volume=634|issue=1|pages=625 – 640|year=2005] If this is the case, the true mass is approximately 30% greater than this lower limit, at around 2.5 Jupiter masses. On the other hand, astrometric measurements suggest the orbital inclination is around 84°, which suggests the mass is only slightly greater than the lower limit. [cite journal|url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2002ApJ...581L.115B&db_key=AST&nosetcookie=1|author=Benedict, G. et al.|title=A mass for the extrasolar planet Gliese 876b determined from Hubble Space Telescope fine guidance sensor 3 astrometry and high-precision radial velocities|journal=The Astrophysical Journal|volume=581|issue=2|pages=L115 – L118|year=2002]

Physical characteristics

Given the planet's high mass, it is likely that Gliese 876b is a gas giant with no solid surface. Since the planet has only been detected indirectly through its gravitational effects on the star, properties such as its radius, composition and temperature are unknown. Assuming a composition similar to Jupiter and an environment close to chemical equilibrium, it is predicted that the atmosphere of Gliese 876b is cloudless, though cooler regions of the planet may be able to form water clouds. [cite journal|url=http://adsabs.harvard.edu/abs/2003ApJ...588.1121S|author=Sudarsky, D. et al.|title=Theoretical Spectra and Atmospheres of Extrasolar Giant Planets|journal=The Astrophysical Journal|volume=588|issue=2|pages=1121 – 1148|year=2003]

Gliese 876b lies within the habitable zone of Gliese 876 as defined by the ability of an Earth-mass planet to retain liquid water at its surface. While the prospects for life on a gas giant are unknown, large moons may be able to support a habitable environment. Models of tidal interactions between a hypothetical moon, the planet and the star suggest that large moons should be able to survive in orbit around Gliese 876b for the lifetime of the system. [cite journal|url=http://adsabs.harvard.edu/abs/2002ApJ...575.1087B|author=Barnes, J., O'Brien, D.|title=Stability of Satellites around Close-in Extrasolar Giant Planets|journal=The Astrophysical Journal|volume=575|issue=2|pages=1087 – 1093|year=2002 (paper incorrectly refers to Gliese 876b as GJ876c)] On the other hand, it is unclear whether such moons could form in the first place. [cite journal|url=http://www.nature.com/nature/journal/v441/n7095/abs/nature04860.html|author=Canup, R., Ward, W.|title=A common mass scaling for satellite systems of gaseous planets|journal=Nature|volume=441|year=2006|pages=834 – 839]

See also

* Appearance of extrasolar planets

References

External links

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* [http://www.extrasolar.net/planettour.asp?StarCatID=normal&PlanetID=2 www.extrasolar.net]