The GJ 876 Planetary System: A Progress Report^*

We present an updated analysis of the GJ 876 planetary system based on an augmented data set that incorporates 65 new high-precision radial velocities obtained with the Keck Telescope from 2001 to 2004. These new radial velocities permit a more accurate characterization of the planet-planet interactions exhibited by the system. Self-consistent three-body orbital fits (which incorporate both the estimated instrumental uncertainties and Gaussian stellar jitter with σ = 6 m s^-1) continue to show that GJ 876 b (the outer planet of the system), and GJ 876 c (the inner planet of the system) are participating in a stable and symmetric 2 : 1 resonance condition in which the lowest order, eccentricity-type mean-motion resonance variables, θ₁ = λ_c - 2λ_b + ϖ_c and θ₂ = λ_c - 2λ_b + ϖ_b, both librate around 0°, with amplitudes |θ₁|_max = 70 ± 18 and |θ₂|_max = 34^° ± 12^° (λ_b and λ_c are the mean longitudes, and ϖ_b and ϖ_c are the longitudes of periastron). The planets are also locked in a secular resonance, which causes them to librate about apsidal alignment with |ϖ₁ - ϖ₂|_max = 34^° ± 12^°. The joint line of apsides for the system is precessing at a rate ~ -41^° yr^-1. The small libration widths of all three resonances likely point to a dissipative history of differential migration for the two planets in the system. Three-body fits to the radial velocity data set, combined with a Monte Carlo analysis of synthetic data sets, indicate that the (assumed) coplanar inclination, i_s, of the system is i_s > 20^°. Configurations with modest mutual inclination are, however, also consistent with the current radial velocity data. For non-coplanar configurations, the line of nodes of the inner planet precesses at rates of order -4° yr^-1, and in these cases, the inner planet can be observed to transit the parent star when either the ascending or descending node precesses through the line of sight. Therefore, GJ 876 c may possibly be observed to transit in the relatively near future even if it is not transiting at the present time. We comment briefly on the orbital stability of as-yet-undetected terrestrial planets in habitable orbits and assess the suitability of the system as a potential target for upcoming space missions such as the Terrestrial Planet Finder.