LICK-CARNEGIE EXOPLANET SURVEY: A URANUS-MASS FOURTH PLANET FOR GJ 876 IN AN EXTRASOLAR LAPLACE CONFIGURATION

Continued radial velocity (RV) monitoring of the nearby M4V red dwarf star GJ 876 with Keck/High Resolution Echelle Spectrograph has revealed the presence of a Uranus-mass fourth planetary companion in the system. The new planet has a mean period of P-e = 126.6 days (over the 12.6-year baseline of the RV observations), and a minimum mass of m(e) sin i(e) = 12.9 +/- 1.7 M-circle dot. The detection of the new planet has been enabled by significant improvements to our RV data set for GJ 876. The data have been augmented by 36 new high-precision measurements taken over the past five years. In addition, the precision of all of the Doppler measurements have been significantly improved by the incorporation of a high signal-to-noise template spectrum for GJ 876 into the analysis pipeline. Implementation of the new template spectrum improves the internal rms errors for the velocity measurements taken during 1998-2005 from 4.1 m s(-1) to 2.5 m s(-1). Self-consistent, N-body fits to the RV data set show that the four-planet system has an invariable plane with an inclination relative to the plane of the sky of i = 59 degrees.5. The fit is not significantly improved by the introduction of a mutual inclination between the planets b and c, but the new data do confirm a non-zero eccentricity, e(d) = 0.207 +/- 0.055 for the innermost planet, d. In our best-fit coplanar model, the mass of the new component is m(e) = 14.6 +/- 1.7 M-circle plus. Our best-fitting model places the new planet in a three-body resonance with the previously known giant planets (which have mean periods of P-c = 30.4 and P-b = 61.1 days). The critical argument, phi(Laplace) = lambda(c) - 3 lambda(b) + 2 lambda(e), for the Laplace resonance librates with an amplitude of Delta phi(Laplace) = 40 degrees +/- 13 degrees about phi(Laplace) = 0 degrees. Numerical integration indicates that the four-planet system is stable for at least a billion years (at least for the coplanar cases). This resonant configuration of three giant planets orbiting an M dwarf primary differs from the well-known Laplace configuration of the three inner Galilean satellites of Jupiter, which are executing very small librations about phi(Laplace) = 180 degrees and which never experience triple conjunctions. The GJ 876 system, by contrast, comes close to a triple conjunction between the outer three planets once per every orbit of the outer planet, e.