Gliese 876 c

Gliese 876 c is an exoplanet orbiting the red dwarf Gliese 876, taking about 30 days 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 designated Gliese 876 b. On January 9, 2001, it was announced that further analysis of the star's radial velocity had 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. Gliese 876 c is in a 1:2:4 Laplace resonance with the outer planets Gliese 876 b and Gliese 876 e: for every orbit of planet e, planet b completes two orbits and planet c completes four. This leads to strong gravitational interactions between the planets, causing the orbital elements to change rapidly as the orbits precess. This is the second known example of a Laplace resonance, the first being Jupiter's moons Io, Europa and Ganymede. The orbital semimajor axis is only 0.13 AU, around a third of the average distance between Mercury and the Sun, and is more eccentric than the orbit of any of the major planets of the Sun's Solar System. 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. This is because the measured mass value depends on the inclination 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.