ar X iv : a st ro - p h / 04 07 44 1 v 1 2 1 Ju l 2 00 4 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. Selfconsistent three-body orbital fits (which incorporate both the estimated instrumental uncertainties and Gaussian stellar jitter with σ = 6ms) 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 = λc − 2λb + ̟c and θ2 = λc − 2λb +̟b both librate around 0 , with amplitudes |θ1|max = 7.0 ± 1.8 , and |θ2|max = 34 ± 12 . (λb and λc are the mean longitudes, 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 |̟1 −̟2|max = 34 ± 12 . The joint line of apsides for the system is precessing at a rate ̟̇ ∼ −41 yr. 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) co-planar inclination, is, of the system is is > 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 −4yr, 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 Based on observations obtained at Lick Observatory, which is operated by the University of California, and on observations obtained at the W.M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology. 2 Department of Astronomy, University of California, Berkeley, CA USA 94720 3 Department of Physics and Astronomy, San Francisco State University, San Francisco, CA USA 94132 4 Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington DC, USA 20015-1305 5 UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA, USA 95064