The occurrence of earth-like planets around other stars

The quantity η⊕, the number density of planets per star per logarithmic planetary radius per logarithmic orbital period at one Earth radius and one year period, describes the occurrence of Earthlike extrasolar planets. Here we present a measurement of η⊕ from a parameterised forward model of the (correlated) period-radius distribution and the observational selection function in the most recent (Q17) data release from the Kepler satellite. We find η⊕ = 3.9 +2.2 −1.6% (90% CL). We conclude that each star hosts 3.83+0.76 −0.62 planets with P . 3yr and R & 0.2R⊕. Our empirical model for falsepositive contamination is consistent with the dominant source being background eclipsing binary stars. The distribution of planets we infer is consistent with a highly-stochastic planet formation process producing many correlated, fractional changes in planet sizes and orbits.