Phase-averaged detection limit applied to Gl 581. Planets with minimum masses above the limit are excluded with a 99% confidence level for half of our 12 trial phases. The upper curve shows the limit before any planetary signal is removed from the RV time series. The sharp decrease in detection sensitivity around the period of 5.3 days is caused by the RV signal of Gl 581b. The lower curve shows the limit after the best four-planet Keplerian fit has been subtracted. The sharp decrease in sensitivity around the period of two days is due to sampling. The Venus and Mars criteria delineate the habitable zone, which is shown in blue. The vertical yellow dashed line marks the duration of the survey.