Located at a distance of 4.7 parsecs (15.4 light years) in the constellation Aquarius, Gliese 876 – usually abbreviated GJ 876 – is an ordinary red dwarf of spectral type M4. Roughly one-third the size and mass of our Sun, this star is notable as the host to two gas giants and one terrestrial planet.

The age of GJ 876 remains poorly constrained. Marcy et al. (1998) provide an unhelpful estimate of 1-10 billion years, while Rivera et al. (2005) report an extremely slow stellar rotation of about 97 days, implying an age at the far end of the range. The star's metallicity has recently been measured at -0.12 (Bean et al. 2006). This low value is atypical of exoplanetary host stars, which tend to be metal-rich.

It is all the more noteworthy that the aggregate mass of the system’s three planets (2.55 MJUP) exceeds that of the eight planets of the Solar System (less than 1.5 MJUP), even if we include Pluto and all 150+ moons. No compelling arguments have yet been ventured to explain how this low-mass, metal-poor red star should be able to host a planetary system more massive than that of HD 69830 or our own Sun.

The three detected planets of GJ 876 orbit close to the central star in a region that extends from about 0.02 AU to 0.21 AU – a span of less than 18 million miles, substantially narrower than the average separation of Mercury from the Sun. All three planets must have originated at greater distances from the host star and migrated inward to their present locations (see Evolution of Planetary Systems).

The orbital and evolutionary dynamics of the system have clearly been dominated by the third planet, a gas giant with a minimum mass of 1.93 MJUP, a semimajor axis of 0.21 AU, and an orbital eccentricity near zero. This planet maintains a 2:1 mean motion resonance with the second planet (Barnes & Greenberg 2006b), a smaller gas giant with a minimum mass of 0.62 MJUP and a semimajor axis of 0.13 AU. The period of the third planet is 60.9 days; that of the second planet is 30.34 days.

Over long time scales these two planets exhibit the resonance phenomenon of “apsidal corotation,” a dynamic relationship that could not exist if the two planets simply formed in situ without any shared orbital evolution (Beauge et al. 2006). In fact, precise dynamic analyses of the GJ 876 system have furnished important proof of the general hypothesis of giant planet migration.

The orbits of both giant planets closely coincide with the habitable zone of GJ 876 (Rivera & Haghighipour 2006), indicating that both may be cool enough to support water clouds in their dense atmospheres. It is likely that their originally rapid rotational periods have been modified by the effects of the 2:1 orbital resonance as well as by perturbations from the host star. Their resonant interaction may have protected them from tidal locking, and thus avoided the extreme dayside/nightside temperature variations observed on some Hot Jupiters (Harrington et al. 2006).

The gravitational influence of the nearby red dwarf might prevent these two giant planets from maintaining systems of satellites or rings. However, one study suggests that the third planet, GJ 876 b, might sustain moons even as massive as Earth (Barnes & O'Brien 2002). Such heavy companions would be conceivable only if they were captured by the host planet, since theories of satellite formation place far stricter constraints on the maximum size of co-formed moons (Canup & Ward 2006). Objects in the mass range of the Galilean satellites of Jupiter (perhaps up to twice the mass of Ganymede) may be more likely -- if indeed any moons at all are possible.

The innermost planet, GJ 876 d, has a semimajor axis of only 0.02 AU and a period of less than two days. Its mass may be as low as 6 times that of Earth, and probably not more than 7.5 times, making it one of the smallest planets so far detected by the radial velocity method. This planet’s combination of low mass and extremely short orbital period confirm its status as a rocky Super Earth, without any substantial percentage of volatiles in its composition (Lecavelier des Etangs 2006). Its proximity to the host star guarantees that the planet's rotation has been tidally locked. Its starward hemisphere is grilled by GJ 876, while its opposite half faces infinite night.

The innermost planet probably formed as a consequence of the inward migration of the two outer planets, which entered into the 2:1 mean motion resonance as they spiraled through the primordial gas disk of GJ 876. Gravitational perturbations by these two planets forced rocky planetesimals in the inner disk to clump inside the shrinking radius of the second planet’s orbit, where the planetesimals underwent ejections, collisions, and accretion (Fogg & Nelson 2005, Raymond et al. 2006b, Mandell et al. 2007). The ultimate result was the hot Super Earth that we now detect.

Last update February 2008