Gliese 876 c is anexoplanetorbiting thered dwarfGliese 876, taking about 30days to complete an orbit. The planet was discovered in April 2001 and is the second planet in order of increasing distance from its star.
At the time of discovery, Gliese 876 was already known to host an extrasolar planet designatedGliese 876 b. On January 9, 2001, it was announced that further analysis of the star'sradial velocity had revealed the existence of a second planet in the system, which was designated Gliese 876 c.[2][1] Theorbital period of Gliese 876 c 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 highereccentricity of the orbit of Gliese 876 b.
The planet orbits a (M-type)star namedGliese 876. The star has a mass of 0.33M☉ and a radius of around 0.36R☉. It has a surface temperature of 3350K and is 2.55 billion years old. In comparison, the Sun is about 4.6 billion years old[4] and has a surface temperature of 5778 K.[5]
The orbits of the planets of Gliese 876. Gliese 876 c is the second planet from the starGliese 876.
Gliese 876 c is in a 1:2:4Laplace resonance with the outer planets Gliese 876 b andGliese 876 e: for every orbit of planet e, planet b completes two orbits and planet c completes four.[6] This leads to stronggravitational interactions between the planets,[7] causing theorbital elements to change rapidly as the orbitsprecess.[6][8] This is the second known example of a Laplace resonance, the first beingJupiter'smoonsIo,Europa andGanymede.
The orbitalsemimajor axis is only 0.13AU, around a third of the average distance betweenMercury and theSun, and is moreeccentric than the orbit of any of the majorplanets of the Sun'sSolar System.[6] Despite this, it is located in the inner regions of the system'shabitable zone, since Gliese 876 is such an intrinsically faint star.[9]
A limitation of the radial velocity method used to detect Gliese 876 c is that only a lower limit on the planet'smass can be obtained. This is because the measured mass value depends on theinclination of the orbit, which is not determined by the radial velocity measurements. 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 inclination of the orbit can be determined, revealing the planet's true mass to be 0.72 times that ofJupiter.[6]
Based on its high mass, Gliese 876 c is likely to be agas giant with nosolid surface. Since it was detected indirectly through its gravitational effects on the star, properties such as itsradius, composition, andtemperature are unknown. Assuming a composition similar to Jupiter and an environment close tochemical equilibrium, the planet is predicted to have acloudless upperatmosphere.[10]
Gliese 876 c lies at the inner edge of the system's habitable zone. While the prospects forlife on gas giants are unknown, it might be possible for a largemoon of the planet to provide ahabitable environment. Unfortunatelytidal interactions between a hypothetical moon, the planet, and the star could destroy moons massive enough to be habitable over the lifetime of the system.[11] In addition it is unclear whether such moons could form in the first place.[12]
This planet, like b and e, has likely migrated inward.[13]
