Gliese 876 b

Gliese 876 b

Exoplanet		List of exoplanets
An artist's impression of Gliese 876 b as an enormous Jupiter-like planet with a system of natural satellites.
Parent star
Star		Gliese 876
Constellation		Aquarius
Right ascension	(α)	22h 53m 16.734s
Declination	(δ)	−14° 15′ 49.32″
Apparent magnitude	(mV)	10.15
Distance		15.3 ± 0.2 ly (4.69 ± 0.05 pc)
Spectral type		M4V
Mass	(m)	0.334 ± 0.030 M☉
Radius	(r)	0.36 R☉
Temperature	(T)	3350 ± 300 K
Metallicity	[Fe/H]	0.05 ± 0.20
Age		0.1–5.0 Gyr
Orbital elements Epoch HJD 2,450,602.093
Semi-major axis	(a)	0.208317 ± 0.000020 AU
Eccentricity	(e)	0.0324 ± 0.0013
Orbital period	(P)	61.1166 ± 0.0086 d
Inclination	(i)	59°
Argument of periastron	(ω)	50.3 ± 3.2°
Mean anomaly	(M)	325.7 ± 3.2°
Semi-amplitude	(K)	214.00 ± 0.42 m/s
Physical characteristics
Mass	(m)	2.2756 ± 0.0045 MJ
Stellar flux	(F⊙)	0.3 ⊕
Maximum stellar flux	(F⊙max)	0.32 ⊕
Minimum stellar flux	(F⊙min)	0.28 ⊕
Discovery information
Discovery date		June 23, 1998
Discoverer(s)		California and Carnegie Planet Search Team and independently by the Geneva Extrasolar Planet Search Team
Discovery method		Radial velocity
Other detection methods		Astrometry
Discovery site		Lick, Keck, Haute-Provence and La Silla Observatories
Discovery status		Published
Database references
Extrasolar Planets Encyclopaedia		data
SIMBAD		data
Exoplanet Archive		data
Open Exoplanet Catalogue		data

Gliese 876 b is an exoplanet orbiting the red dwarf Gliese 876. It completes one orbit in approximately 61 days. Discovered in June 1998, Gliese 876 b was the first planet to be discovered orbiting a red dwarf.

Discovery

Gliese 876 b was discovered independently by two different teams, one led by Geoffrey Marcy (with data from Keck Observatory and Lick Observatory) and the other by Xavier Delfosse (at Geneva Observatory). 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. It was the first discovered of four known planets in the Gliese 876 system.

Orbit and mass

The orbits of the planets of Gliese 876. Gliese 876 b is the third planet from the star.

Gliese 876 b is in a 1:2:4 Laplace resonance with the inner planet Gliese 876 c and the outer planet Gliese 876 e: in the time it takes planet e to complete one orbit, planet b completes two and planet c completes four. This is the second known example of a Laplace resonance, the first being Jupiter's moons Io, Europa and Ganymede. As a result, the orbital elements of the planets change fairly rapidly as they dynamically interact with one another. The planet's orbit has a low eccentricity, similar to the planets in the Solar System. The semimajor axis of the orbit is only 0.208 AU, less than that of Mercury in the Solar System. However Gliese 876 is such a faint star that this puts it in the outer part of the habitable zone.

A limitation of the radial velocity method used to detect Gliese 876 b 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. However, because Gliese 876 is only 15 light years from Earth Benedict et al. (2002) were able to use one of the Fine Guidance Sensors on the Hubble Space Telescope to detect the astrometric wobble created by Gliese 876 b. This constituted the first unambiguous astrometric detection of an extrasolar planet. Their analysis suggested that the orbital inclination is 84°±6° (close to edge-on). In the case of Gliese 876 b, modelling the planet-planet interactions from the Laplace resonance shows that the actual inclination of the orbit is 59°, resulting in a true mass of 2.2756 times the mass of Jupiter.

Physical characteristics

Given the planet's high mass, it is likely that Gliese 876 b 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 876 b is cloudless, though cooler regions of the planet may be able to form water clouds.

This planet, like c and e, has likely migrated inward.

Gliese 876 b currently lies beyond the outer edge of the habitable zone but because Gliese 876 is a slowly evolving main-sequence red dwarf its habitable zone is very slowly moving outwards and will continue to do so for trillions of years. Therefore, Gliese 876 b will, in trillions of years time, lie inside Gliese 876's habitable zone, as defined by the ability of an Earth-mass planet to retain liquid water at its surface, and remain there for at least 4.6 billion years. 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 876 b for the lifetime of the system. On the other hand, it is unclear whether such moons could form in the first place. However, the large mass of the gas giant may make it more likely for larger moons to form.

See also

• Appearance of extrasolar planets