Continued radial velocity monitoring of the nearby M4V red dwarf star GJ 876 with Keck/HIRES 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 radial velocity observations ) , and a minimum mass of m _ { e } \sin { i _ { e } } = 12.9 \pm 1.7 M _ { \oplus } .
The detection of the new planet has been enabled by significant improvements to our radial velocity 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 radial velocity 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 ^ { \circ } .
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 \pm 0.055 for the innermost planet , “ d. ” In our best-fit coplanar model , the mass of the new component is m _ { e } = 14.6 \pm 1.7 M _ { \oplus } .
Our best-fitting model places the new planet in a 3-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 , \varphi _ { Laplace } = \lambda _ { c } -3 \lambda _ { b } +2 \lambda _ { e } , for the Laplace resonance librates with an amplitude of \Delta \varphi _ { Laplace } = 40 \pm 13 ^ { \circ } about \varphi _ { Laplace } = 0 ^ { \circ } .
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 \varphi _ { Laplace } = 180 ^ { \circ } , 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 . ”