How Common are Earth and SuperEarth Exoplanets in M Dwarf Habitable Zones?

The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity

Authors:

Dressing et al

Abstract:

We present an improved estimate of the occurrence rate of small planets around small stars by searching the full four-year Kepler data set for transiting planets using our own planet detection pipeline and conducting transit injection and recovery simulations to empirically measure the search completeness of our pipeline. We identified 157 planet candidates, including 2 objects that were not previously identified as Kepler Objects of Interest (KOIs). We inspected all publicly available follow-up images, observing notes, and centroid analyses, and corrected for the likelihood of false positives. We evaluate the sensitivity of our detection pipeline on a star-by-star basis by injecting 2000 transit signals in the light curve of each target star. For periods shorter than 50 days, we found an occurrence rate of 0.57 (+0.06/-0.05) Earth-size planets (1-1.5 Earth radii) and 0.51 (+0.07/-0.06) super-Earths (1.5-2 Earth radii) per M dwarf. Within a conservatively defined habitable zone based on the moist greenhouse inner limit and maximum greenhouse outer limit, we estimate an occurrence rate of 0.18 (+0.18/-0.07) Earth-size planets and 0.11 (+0.10/-0.05) super-Earths per M dwarf habitable zone. Accounting for the cooling effect of clouds by doubling the insolation limit at the inner edge of the habitable zone results in a higher occurrence rate of 0.27 (+0.16/-0.09) Earth-size planets and 0.25 (+0.11/- 0.07) super-Earths per M dwarf habitable zone.