ar X iv : a st ro - p h / 05 10 50 8 v 1 1 8 O ct 2 00 5 A ∼ 7 . 5 Earth - Mass Planet Orbiting the Nearby Star , GJ 876

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 ± 0.54M⊕ and that its orbital period is 1.93776 (± 7 × 10) days. Assuming coplanar orbits, an inclination of the GJ 876 planetary system to the plane of the sky of ∼ 50 gives the best fit. This inclination yields a mass for companion d of m = 7.53 ± 0.70M⊕, making it by far the lowest mass companion yet found around a main sequence star other than our Sun. Precise photometric observations at Fairborn Based on observations obtained at the W.M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology. UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064 NASA/Ames Research Center, Space Science and Astrobiology Division, MS 245-3, Moffett Field, CA 94035 Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington DC, 20015-1305 Department of Astronomy, University of California, Berkeley, CA 94720 Department of Physics and Astronomy, San Francisco State University, San Francisco, CA 94132 High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307 Center of Excellence in Information Systems, Tennessee State University, 3500 John A. Merritt Blvd., Box 9501, Nashville, TN 37209 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235