AAS 207th Meeting, 8-12 January 2006
Session 191 Extrasolar Planets
Oral, Thursday, 10:00-11:30am, January 12, 2006, Virginia

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[191.03] A ~7.5 Earth-Mass Planet Orbiting the Nearby Star, GJ 876

E. J. Rivera (UCO/Lick), J. J. Lissauer (NASA/Ames), R. P. Butler (DTM/CIW), G. W. Marcy (UC Berkeley), S. S. Vogt (UCO/Lick), D. A. Fischer (SFSU), T. M. Brown (High Altitude Observatory/NCAR), G. Laughlin (UCO/Lick), G. W. Henry (Center of Excellence in Information Systems/TSU)

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\,±,0.54\,M\oplus and that its orbital period is 1.93776\,(±,7\times10-5) days. Assuming coplanar orbits, an inclination of the GJ 876 planetary system to the plane of the sky of ~0\circ gives the best fit. This inclination yields a mass for companion d of m=7.53\,±,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.

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