  The quantity $\eta_\oplus$, 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 Earth-like extrasolar planets.
Here we present a measurement of $\eta_\oplus$ 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 $\eta_\oplus = 3.9_{-1.6}^{+2.2}\%$ (90% CL). We conclude
that each star hosts $3.83_{-0.62}^{+0.76}$ planets with $P \lesssim 3
\mathrm{yr}$ and $R \gtrsim 0.2 R_\oplus$. Our empirical model for
false-positive 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.
