Wednesday, June 25, 2014

Between 5.3% to 10% of Sun-like Stars Have Terrestrial Planets in the Habitable Zone

A Statistical Reconstruction of the Planet Population Around Kepler Solar-Type Stars


Silburt et al


Using the most recent Kepler catalog, we reconstruct the occurrence rate of small (Neptune-sized or below) planets as a function of orbital period and planet radius, taking careful account of various detection biases. We analyze a sample of 76,000 Sun-like stars and their associated planet candidates with periods between 20 and 200 days, and sizes between 1 and 4R⊕. Such planets have likely experienced little photoevaporation, and may reflect the "primordial" planet population. Assuming that the size distribution of planets are independent of their orbital periods (and vice versa), we conclude that Kepler planets are preferentially peaked at 2−2.8R⊕, with their numbers decreasing gradually toward smaller sizes. These planets are found roughly uniformly in logarithmic period. The average number of planets per star, in the stated period and size ranges, is 0.46±0.03. This number rises by ∼0.2 if one includes planets inward of 20 days. Upon extrapolation we obtain an occurrence rate, for Earth-like planets within the "habitable zone" (as calculated by 1-D climate models), of 6.4+3.4−1.1%. We discuss the astrophysical implications of our results.

In our study, we introduce a number of novel statistical approaches, including the adoption of the "iterative simulation" technique (in addition to the standard MCMC technique), incorporation of uncertainties in planet radii, and an improved consideration of detection bias. Our results largely agree with those from an earlier work by Petigura et al. (2013), based on different statistical treatments and noise models. However, this agreement masks two substantial underlying discrepancies that (to first order) cancel each other out.

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