The GJ 876 Planetary System: A Progress Report
Abstract
We present an updated analysis of the GJ 876 planetary system based on an augmented data set that incorporates 65 new highprecision radial velocities obtained with the Keck Telescope from 2001 to 2004. These new radial velocities permit a more accurate characterization of the planetplanet interactions exhibited by the system. Selfconsistent threebody 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, eccentricitytype meanmotion resonance variables, θ_{1}=λ_{c}2λ_{b}+ϖ_{c} and θ_{2}=λ_{c}2λ_{b}+ϖ_{b}, both librate around 0°, with amplitudes θ_{1}_{max}=7.0d+/1.8d and θ_{2}_{max}=34^{deg}+/12^{deg} (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 ϖ_{1}ϖ_{2}_{max}=34^{deg}+/12^{deg}. The joint line of apsides for the system is precessing at a rate ϖ˙~41^{deg}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. Threebody 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^{deg}. Configurations with modest mutual inclination are, however, also consistent with the current radial velocity data. For noncoplanar 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 asyetundetected 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.
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.
 Publication:

The Astrophysical Journal
 Pub Date:
 April 2005
 DOI:
 10.1086/424686
 arXiv:
 arXiv:astroph/0407441
 Bibcode:
 2005ApJ...622.1182L
 Keywords:

 Stars: Planetary Systems;
 Planets and Satellites: General;
 Stars: Individual: Alphanumeric: GJ 876;
 Astrophysics
 EPrint:
 26 pages, 8 figures, Accepted at Astrophysical Journal