  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 ~ P^{\beta - 1},
with \beta = 0.71 +/- 0.08; and an extrapolation to longer periods gives the
frequency of terrestrial planets in the habitable zones of FGK stars as
\eta_\oplus = (34 +/- 14)%. Thus about one-third of FGK stars are predicted to
have at least one terrestrial, habitable-zone planet.
