  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.
