Abstract
We use the optical and near-infrared photometry from the Kepler Input Catalog to provide improved estimates of the stellar characteristics of the smallest stars in the Kepler target list. We find 3897 dwarfs with temperatures below 4000K, including 64 planet candidate host stars orbited by 95 transiting planet candidates. We refit the transit events in the Kepler light curves for these planet candidates and combine the revised planet/star radius ratios with our improved stellar radii to revise the radii of the planet candidates orbiting the cool target stars. We then compare the number of observed planet candidates to the number of stars around which such planets could have been detected in order to estimate the planet occurrence rate around cool stars. We find that the occurrence rate of 0.5-4 Earth radius planets with orbital periods shorter than 50 days is 0.90 (+0.04/-0.03) planets per star. The occurrence rate of Earth-size (0.5-1.4 Earth radius) planets is constant across the temperature range of our sample at 0.51 (+0.06/-0.05) Earth-size planets per star, but the occurrence of 1.4-4 Earth radius planets decreases significantly at cooler temperatures. Our sample includes 2 Earth-size planet candidates in the habitable zone, allowing us to estimate that the mean number of Earth-size planets in the habitable zone is 0.15 (+0.13/-0.06) planets per cool star. Our 95% confidence lower limit on the occurrence rate of Earth-size planets in the habitable zones of cool stars is 0.04 planets per star. With 95% confidence, the nearest transiting Earth-size planet in the habitable zone of a cool star is within 21 pc. Moreover, the nearest non-transiting planet in the habitable zone is within 5 pc with 95% confidence.
Highlights
The Kepler mission has revolutionized exoplanet statistics by increasing the number of known extrasolar planets and planet candidates by a factor of five and discovering systems with longer orbital periods and smaller planet radii than prior exoplanet surveys (Batalha et al 2011; Borucki et al 2012; Fressin et al 2012; Gautier et al 2012)
For a typical cool star, we find that the revised radius is only 69% of the original radius listed in the Kepler Input Catalog (KIC) and that the revised temperature is 130K cooler than the original temperature estimate
In order to investigate the dependence of the planet occurrence rate on host star temperature, we repeat the analysis described in Section 5 for each group separately
Summary
The Kepler mission has revolutionized exoplanet statistics by increasing the number of known extrasolar planets and planet candidates by a factor of five and discovering systems with longer orbital periods and smaller planet radii than prior exoplanet surveys (Batalha et al 2011; Borucki et al 2012; Fressin et al 2012; Gautier et al 2012). The majority of Kepler ’s target stars are solar-like F GK dwarfs and most of the work on the planet occurrence rate from Kepler has been focused on planets orbiting that sample of stars (e.g., Borucki et al 2011; Catanzarite & Shao 2011; Youdin 2011; Howard et al 2012; Traub 2012) Those studies revealed that the planet occurrence rate increases toward smaller planet radii and longer orbital periods. In addition to nearly doubling the number of planet candidates, Batalha et al (2012) improved the stellar parameters for many target stars by comparing the estimated temperatures, radii, and surface gravities in the Kepler Input Catalog (KIC; Batalha et al 2010; Brown et al 2011) to the values expected from Yonsei-Yale evolutionary models (Demarque et al 2004). The Yonsei-Yale models overestimate the observed radii and luminosity of cool stars at a given effective temperature (Boyajian et al 2012)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
