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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 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 the plane of the sky of ~ 50 degrees gives the best fit. This inclination yields a mass for companion d of m = 7. 53 +- 0. 70 Earth masses, 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.
Rivera et al. (Tue,) studied this question.
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