A REVISED ESTIMATE OF THE OCCURRENCE RATE OF TERRESTRIAL PLANETS IN THE HABITABLE ZONES AROUND KEPLER 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 used Kepler data and calculated the frequency of terrestrial planets in the HZ of cool stars to be 0.15(-0.06)(+0.13) per star for Earth-size planets (0.5-1.4 R-circle plus). However, this estimate was derived using the Kasting et al. 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. for stars with effective temperatures between 2600 K and 7200 K, which includes the cool M stars in the Kepler target list. The new HZ boundaries increase the number of planet candidates in the HZ. Assuming Earth- size planets as 0.5- 1.4 R-circle plus, when we reanalyze their results, we obtain a terrestrial planet frequency of 0.48(- 0.24)(+0.12) and 0.53(-0.17)(+0.08) planets per M-dwarf star for conservative and optimistic limits of the HZ boundaries, respectively. Assuming Earth- size planets as 0.5-2 R-circle plus, the frequency increases to 0.51(-0.20)(+0.10) per star for the conservative estimate and to 0.61(-0.15)(+0.07) 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(-0.13)(+0.54)). So, the potential for finding Earth- like planets around M stars may be higher than previously reported.