Gliese 876 c

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Gliese 876 c
Extrasolar planet Lists of extrasolar planets
Parent star
Star Gliese 876
Constellation Aquarius
Right ascension (α) 22h 53m 16.73s
Declination (δ) −14° 15′ 49.3″
Spectral type M3.5V
Orbital elements
Semimajor axis (a) 0.1303 ± 0.0075 AU
Eccentricity (e) 0.2243 ± 0.0013
Orbital period (P) 30.340 ± 0.013 d
Inclination (i)  ?°
Longitude of
(ω) 198.30 ± 0.90°
Time of periastron (τ) 2,452,464.0 ± 0.1 JD
Physical characteristics
Mass (m) >0.619 ± 0.088 MJ
Radius (r)  ? RJ
Density (ρ)  ? kg/ m3
Temperature (T)  ? K
Discovery information
Discovery date 2001
Discoverer(s) Marcy et al.
Detection method Radial velocity
Discovery status Confirmed

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


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 876 c. The orbital 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 higher eccentricity of the orbit of Gliese 876 b.

Orbit and mass

Gliese 876 c is in a 1:2 orbital resonance with the outer planet Gliese 876 b. This leads to strong gravitational interactions between the two planets, causing the orbital elements to change rapidly as the orbits precess. 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.

A limitation of the radial velocity method used to detect Gliese 876 c is that only a lower limit on the planet's mass can be obtained. In the case of Gliese 876 c, 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. 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.


Based on its high mass, Gliese 876 c 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.

Gliese 876 c 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. In addition it is unclear whether such moons could form in the first place.

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