High-precision, high-cadence radial velocity monitoring over the past 8 yr 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 msini=5.89+/-0.54 M⊕ and that its orbital period is 1.93776 (+/-7×10-5) 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+/-0.70 M⊕, 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.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.
The Astrophysical Journal
- Pub Date:
- November 2005
- Stars: Planetary Systems;
- Planets and Satellites: General;
- Stars: Individual: Alphanumeric: GJ 876;
- 49 pages, 16 figures, 1 electronic table available upon request