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

Authors:

Silburt et al

Abstract:

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.

.5% of G Dwarf Stars, .8 % of K Dwarf Stars Have Ultra Short Period Planets

A Study of the Shortest-Period Planets Found With Kepler

Authors:

Sanchis-Ojeda et al

Abstract:

We present the results of a survey aimed at discovering and studying transiting planets with orbital periods shorter than one day (ultra--short-period, or USP, planets), using data from the {\em Kepler} spacecraft. We computed Fourier transforms of the photometric time series for all 200,000 target stars, and detected transit signals based on the presence of regularly spaced sharp peaks in the Fourier spectrum. We present a list of 106 USP candidates, of which 18 have not previously been described in the literature. In addition, among the objects we studied, there are 26 USP candidates that had been previously reported in the literature which do not pass our various tests. All 106 of our candidates have passed several standard tests to rule out false positives due to eclipsing stellar systems. A low false positive rate is also implied by the relatively high fraction of candidates for which more than one transiting planet signal was detected. By assuming these multi-transit candidates represent coplanar multi-planet systems, we are able to infer that the USP planets are typically accompanied by other planets with periods in the range 1-50 days, in contrast with hot Jupiters which very rarely have companions in that same period range. Another clear pattern is that almost all USP planets are smaller than 2 R⊕, possibly because gas giants in very tight orbits would lose their atmospheres by photoevaporation when subject to extremely strong stellar irradiation. Based on our survey statistics, USP planets exist around approximately (0.51±0.07)% of G-dwarf stars, and (0.83±0.18)% of K-dwarf stars.