Gliese 876 b is an extrasolar planet orbiting the red dwarf star Gliese 876 every 60.940 days. Discovered in June 1998, Gliese 876 b was the first planet to be discovered orbiting a red dwarf. It is the outermost known planet in its planetary system.
Gliese 876 b was discovered independently by two different teams, one led by Geoffrey Marcy and the other by Xavier Delfosse. 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 876 b is in a 2:1 orbital resonance with the inner planet Gliese 876 c, which leads to strong gravitational interactions between the two planets. As a result, the orbital elements of the planet change fairly rapidly as the orbits precess. 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.
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. 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. 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.
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.
Gliese 876 b 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 876 b for the lifetime of the system. On the other hand, it is unclear whether such moons could form in the first place.
* Gliese 876 d
* Gliese 876 c
* Appearance of extrasolar planets
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