Gliese 876 c

Planetbox begin
name=Gliese 876cPlanetbox 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=0.1303 ± 0.0075
eccentricity=0.2243 ± 0.0013
period=30.340 ± 0.013
ang_dist=27.542
long_peri=198.30 ± 0.90
t_peri=2,452,464.0 ± 0.1
semi-amp=88.36 ± 0.72
semimajor_gigameter =
periastron =0.10107371
periastron_gigameter =
apastron = 0.15952629
apastron_gigameter =
period_year =0.083
period_megasecond = Planetbox character
mass=>0.619 ± 0.088
radius=0.952
density=
gravity=
temperature=251
mass_earth = 196.7182
gravity_earth =
insolation =
insolation_earth =
radius_megameter =
mass_wekagram =
radius_earth =10.65738476Planetbox discovery
discovery_date = 4 April 2001
discovery_method = Radial velocity
discoverers = Marcy "et al."
discovery_site = California and
Carnegie Planet Search
flag|United States
discovery_status = PublishedPlanetbox catalog
names=GJ 876 c Planetbox reference
star=Gliese+876
planet=c

Gliese 876c is an extrasolar planet orbiting the red dwarf star Gliese 876, taking 30.340 days to complete an orbit. The planet was discovered in April 2001 and is the second planet in order of distance from its star.

Discovery

At the time of discovery, Gliese 876 was already known to host an extrasolar planet designated Gliese 876 b. In 2001, further analysis of the star's radial velocity revealed the existence of a second planet in the system, which was designated 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|doi=10.1086/321552] The orbital period of Gliese 876c was found to be exactly half that of the outer planet, which meant that the radial velocity signature of the second planet was initially interpreted as a higher eccentricity of the orbit of Gliese 876 b.

Orbit and mass

Gliese 876c is in a 1:2 orbital resonance with the outer planet Gliese 876 b. This 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|doi=10.1086/322477] causing the orbital elements to change 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|doi=10.1086/504701 ( [http://exoplanets.org/planets.shtml web version] )] The orbit is more eccentric than any of the major planets in our solar system. The semimajor axis is only 0.1303 AU, around a third of the average distance between Mercury and the Sun. Despite this, it is located in the inner regions of the system's habitable zone, since Gliese 876 is such an intrinsically faint star. [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|doi=10.1086/428108]

A limitation of the radial velocity method used to detect Gliese 876c is that only a lower limit on the planet's mass can be obtained. In the case of Gliese 876c, this lower limit is 62% of the mass of Jupiter. The true mass of the planet depends on the inclination of the orbit, which in general is unknown. However in a resonant system such as Gliese 876, gravitational interactions between the planets can be used to determine the true masses. Using this method, the orbital inclination is estimated to be around 50° with respect 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|doi=10.1086/491669] In this case, the planet is around 30% more massive than the lower limit: around 0.81 Jupiter masses. On the other hand, astrometric measurements of the outer planet suggest that the inclination is around 84°. Assuming the orbits are coplanar, this would mean the true mass is close to 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|doi=10.1086/346073]

Characteristics

Based on its high mass, Gliese 876c is likely to be a gas giant with no solid surface. Since it was 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, the planet is predicted to have a cloudless upper atmosphere. [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|doi=10.1086/374331]

Gliese 876c lies at the inner edge of the system's habitable zone. While the prospects for life on gas giants are unknown, it might be possible for a large moon of the planet to provide a habitable environment. Unfortunately tidal interactions between a hypothetical moon, the planet and the star could destroy moons massive enough to be habitable over 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|doi=10.1086/341477 (paper incorrectly refers to Gliese 876 b as GJ876c)] In addition 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|doi=10.1038/nature04860]

See also

* Appearance of extrasolar planets
* Eccentric Jupiter

References

External links

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* [http://www.extrasolar.net/planettour.asp?StarCatID=normal&PlanetID=156 www.extrasolar.net]