The 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 ⊕ . 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.°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 ⊕ . 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, φ Laplace = λ c − 3λ b + 2λ e , for the Laplace resonance librates with an amplitude of Δφ Laplace = 40° ± 13° about φ Laplace = 0°. 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 φ Laplace = 180° 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.”