THE GJ 876 PLANETARY SYSTEM: A PROGRESS REPORT1

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 6 m s1) 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, 1 kc 2kb þ$c and 2 kc 2kb þ$b, both librate around 0, with amplitudes j1j max 7N0 1N8 and j2j max 34 12 (kb and kc are the mean longi-tudes, and $b and $c are the longitudes of periastron). The planets are also locked in a secular resonance, which causes them to librate about apsidal alignment with j$1 $2j max 34 12. The joint line of apsides for the system is precessing at a rate $̇41 yr1. 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, is, of the system is is> 2