High precision , high cadence radial velocity monitoring over the past 8 years at the W. M. Keck Observatory reveals evidence for a third planet orbiting the nearby ( 4.69 pc ) dM4 star GJ 876 .
The residuals of three-body Newtonian fits , which include GJ 876 and Jupiter mass companions b and c , show significant power at a periodicity of 1.9379 days .
Self-consistently fitting the radial velocity data with a model that includes an additional body with this period significantly improves the quality of the fit .
These four-body ( three-planet ) Newtonian fits find that the minimum mass of companion “ d ” is m \sin { i } = 5.89 \pm 0.54 M _ { \oplus } and that its orbital period is 1.93776 ( \pm 7 \times 10 ^ { -5 } ) days .
Assuming coplanar orbits , an inclination of the GJ 876 planetary system to the plane of the sky of \sim 50 ^ { \circ } gives the best fit .
This inclination yields a mass for companion d of m = 7.53 \pm 0.70 M _ { \oplus } , making it by far the lowest mass companion yet found around a main sequence star other than our Sun .
Precise photometric observations at Fairborn Observatory confirm low-level brightness variability in GJ 876 and provide the first explicit determination of the star ’ s 96.7-day rotation period .
Even higher precision short-term photometric measurements obtained at Las Campanas imply that planet d does not transit GJ 876 .