Paper: astro-ph/0510508
Date: Tue, 18 Oct 2005 01:16:36 GMT (578kb)

Title: A ~ 7.5 Earth-Mass Planet Orbiting the Nearby Star, GJ 876

Authors: E. J. Rivera, J. J. Lissauer, R. P. Butler, G. W. Marcy, S. S. Vogt,
D. A. Fischer, T. M. Brown, G. Laughlin, and G. W. Henry

Comments: 49 pages, 16 figures, 1 electronic table
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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.

\\ ( http://arXiv.org/abs/astro-ph/0510508 , 574kb)