Kepler is a space telescope that searches Sun-like stars for planets. Its major goal is to determine ηEarth, the fraction of Sun-like stars that have planets like Earth. When a planet "transits" or moves in front of a star, Kepler can measure the concomitant dimming of the starlight. From analysis of the first four months of those measurements for over 150,000 stars, Kepler's Science Team has determined sizes, surface temperatures, orbit sizes, and periods for over a thousand new planet candidates. In this paper, we characterize the period probability distribution function of the super-Earth and Neptune planet candidates with periods up to 132?days, and find three distinct period regimes. For candidates with periods below 3?days, the density increases sharply with increasing period; for periods between 3 and 30?days, the density rises more gradually with increasing period, and for periods longer than 30?days, the density drops gradually with increasing period. We estimate that 1%-3% of stars like the Sun are expected to have Earth analog planets, based on the Kepler data release of 2011 February. This estimate of ηEarth is based on extrapolation from a fiducial subsample of the Kepler planet candidates that we chose to be nominally "complete" (i.e., no missed detections) to the realm of the Earth-like planets, by means of simple power-law models. The accuracy of the extrapolation will improve as more data from the Kepler mission are folded in. Accurate knowledge of ηEarth is essential for the planning of future missions that will image and take spectra of Earth-like planets. Our result that Earths are relatively scarce means that a substantial effort will be needed to identify suitable target stars prior to these future missions.
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