Continued radial velocity monitoring of the nearby M4V red dwarf star GJ 876 with Keck/HIRES has revealed the presence of a Uranus-mass fourth planetary companion in the system. The new planet has a mean period of P e = 126.6 days (over the 12.6-year baseline of the radial velocity observations), and a minimum mass of m e sin i e = 12.9 ± 1.7 M ⊕ . The detection of the new planet has been enabled by significant improvements to our radial velocity data set for GJ 876. The data have been augmented by 36 new high-precision measurements taken over the past five years. In addition, the precision of all of the Doppler measurements have been significantly improved by the incorporation of a high signal-to-noise template spectrum for GJ 876 into the analysis pipeline. Implementation of the new template spectrum improves the internal RMS errors for the velocity measurements taken during 1998-2005 from 4.1 m s −1 to 2.5 m s −1 . Self-consistent, N-body fits to the radial velocity data set show that the four-planet system has an invariable plane with an inclination relative to the plane of the sky of i = 59.5 • . The fit is not significantly improved by the introduction of a mutual inclination between the planets "b" and "c," but the new data do confirm a non-zero eccentricity, e d = 0.207 ± 0.055 for the innermost planet, "d." In our best-fit coplanar model, the mass of the new component is m e = 14.6 ± 1.7 M ⊕ . Our best-fitting model places the new planet in a 3-body resonance with the previously known giant planets (which have mean periods of P c = 30.4 and P b = 61.1 days). The critical argument, ϕ Laplace = λ c − 3λ b + 2λ e , for the Laplace resonance librates with an amplitude of ∆ϕ Laplace = 40±13 • about ϕ Laplace = 0 • . Numerical integration indicates that the four-planet system is stable for at least a billion years (at least for the coplanar cases). This resonant configuration of three giant planets orbiting an M-dwarf primary differs from the well-known Laplace configuration -3of the three inner Galilean satellites of Jupiter, which are executing very small librations about ϕ Laplace = 180 • , and which never experience triple conjunctions.The GJ 876 system, by contrast, comes close to a triple conjunction between the outer three planets once per every orbit of the outer planet, "e."

We present 11 years of HIRES precision radial velocities (RV) of the nearby M3V star Gliese 581, combining our data set of 122 precision RVs with an existing published 4.3-year set of 119 HARPS precision RVs. The velocity set now indicates 6 companions in Keplerian motion around this star. Differential photometry indicates a likely stellar rotation period of ∼ 94 days and reveals no significant periodic variability at any of the Keplerian periods, supporting planetary orbital motion as the cause of all the radial velocity variations. The combined data set strongly confirms the 5.37-day, 12.9-day, 3.15-day, and 67-day planets previously announced by Bonfils et al. (2005), Udry et al. (2007), and Mayor et al. (2009). The observations also indicate a 5th planet in the system, GJ 581f, a minimum-mass 7.0 M ⊕ planet orbiting in a 0.758 AU orbit of period 433 days and a 6th planet, GJ 581g, a minimum-mass 3.1 M ⊕ planet orbiting at 0.146 AU with a period of 36.6 days. The estimated equilibrium temperature of GJ 581g is 228 K, placing it squarely in the middle of the habitable zone of the star and offering a very compelling case for a potentially habitable planet arounda very nearby star. That a system harboring a potentially habitable planet has been found this nearby, and this soon in the relatively early history of precision RV surveys, indicates that η ⊕ , the fraction of stars with potentially habitable planets, is likely to be substantial. This detection, coupled with statistics of the incompleteness of present-day precision RV surveys for volume-limited samples of stars in the immediate solar neighborhood suggests that η ⊕ could well be on the order of a few tens of percent. If the local stellar neighborhood is a representative sample of the galaxy as a whole, our Milky Way could be teeming with potentially habitable planets.