Guardians of the Stars
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.
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 our 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. The planet’s true mass to be 0.72 times that of Jupiter.
Gliese 876 c is a gas giant with no solid surface and a cloudless upper atmosphere. It’s composition is similar to Jupiter with an environment close to chemical equilibrium. Gliese 876 c lies at the inner edge of the system’s habitable zone. Unfortunately tidal interactions between the moons, the planet, and the star have destroyed moons massive enough to be habitable over the lifetime of the system. What few moons exist lie within it’s ring system. This planet, like b and e, has likely migrated inward.