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Description
We present 11 years of HIRES precision radial velocities (RVs) 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 (Mayor et al., 2009, Cat. J/A+A/507/487). The velocity set now indicates six 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 RV variations. The estimated equilibrium temperature of the sixth planet, GJ 581g, is 228K, placing it squarely in the middle of the habitable zone of the star and offering a very compelling case for a potentially habitable planet around a very nearby star. 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 {eta}_{oplus}_, the fraction of stars with potentially habitable planets, could well be on the order of a few tens of percent.
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