A REVISED ESTIMATE OF THE OCCURRENCE RATE OF TERRESTRIAL PLANETS IN THE HABITABLE ZONES AROUND <i>KEPLER</i> M-DWARFS

Because of their large numbers, low mass stars may be the most abundant planet hosts in our Galaxy. Furthermore, terrestrial planets in the habitable zones (HZs) around M-dwarfs can potentially be characterized in the near future and hence may be the first such planets to be studied. Recently Dressing & Charbonneau(2013) used Kepler data and calculated the frequency of terrestrial planets in the HZ of cool stars to be 0.15^{+0.13}_{-0.06} per star for Earth-size planets (0.5-1.4 R_{Earth}). However, this estimate was derived using the Kasting et al.(1993) HZ limits, which were not valid for stars with effective temperatures lower than 3700 K. Here we update their result using new HZ limits from Kopparapu et al.(2013) for stars with effective temperatures between 2600 K and 7200 K, which includes the cool M stars in the Kepler target list. The new habitable zone boundaries increase the number of planet candidates in the habitable zone. Assuming Earth-size planets as 0.5 - 1.4 R_{Earth}, when we reanalyze their results, we obtain a terrestrial planet frequency of 0.48^{+0.12}_{-0.24} and 0.53^{+0.08}_{-0.17} planets per M-dwarf star for conservative and optimistic limits of the HZ boundaries, respectively. Assuming Earth-size planets as 0.5 - 2 R_{Earth}, the frequency increases to 0.51^{+0.10}_{-0.20} per star for the conservative estimate and to 0.61^{+0.07}_{-0.15} per star for the optimistic estimate. Within uncertainties, our optimistic estimates are in agreement with a similar optimistic estimate from the radial velocity survey of M-dwarfs (0.41^{+0.54}_{-0.13}, Bonfils et al.(2011)). So, the potential for finding Earth-like planets around M stars may be higher than previously reported.