Data from Kepler ’ s first 136 days of operation are analyzed to determine the distribution of exoplanets with respect to radius , period , and host-star spectral type .
The analysis is extrapolated to estimate the percentage of terrestrial , habitable-zone exoplanets .
The Kepler census is assumed to be complete for bright stars ( magnitude < 14.0 ) having transiting planets > 0.5 Earth radius and periods < 42 days .
It is also assumed that the size distribution of planets is independent of orbital period , and that there are no hidden biases in the data .
Six significant statistical results are found : there is a paucity of small planet detections around faint target stars , probably an instrumental effect ; the frequency of mid-size planet detections is independent of whether the host star is bright or faint ; there are significantly fewer planets detected with periods < 3 days , compared to longer periods , almost certainly an astrophysical effect ; the frequency of all planets in the population with periods < 42 days is 29 % , broken down as terrestrials 9 % , ice giants 18 % , and gas giants 3 % ; the population has a planet frequency with respect to period which follows a power-law relation dN / dP \sim P ^ { \beta - 1 } , with \beta \simeq 0.71 \pm 0.08 ; and an extrapolation to longer periods gives the frequency of terrestrial planets in the habitable zones of FGK stars as \eta _ { \oplus } \simeq ( 34 \pm 14 ) \% .
Thus about one-third of FGK stars are predicted to have at least one terrestrial , habitable-zone planet .