A ∼7.5M⊕Planet Orbiting the Nearby Star, GJ 876

High precision, high cadence radial velocity monitoring over the past 8 yearsat the W. M. Keck Observatory reveals evidence for a third planet orbiting thenearby (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 significantpower at a periodicity of 1.9379 days. Self-consistently fitting the radialvelocity data with a model that includes an additional body with this periodsignificantly 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+- 0.54 Earth masses and that its orbital period is 1.93776 (+- 7x10^-5) days.Assuming coplanar orbits, an inclination of the GJ 876 planetary system to theplane of the sky of ~ 50 degrees gives the best fit. This inclination yields amass for companion d of m = 7.53 +- 0.70 Earth masses, making it by far thelowest mass companion yet found around a main sequence star other than our Sun.Precise photometric observations at Fairborn Observatory confirm low-levelbrightness variability in GJ 876 and provide the first explicit determinationof the star's 96.7-day rotation period. Even higher precision short-termphotometric measurements obtained at Las Campanas imply that planet d does nottransit GJ 876.Comment: 49 pages, 16 figures, 1 electronic table available upon reques