Kepler-445,kepler-446 and the Occurrence of Compact Multiples Orbiting Mid-M Dwarf Stars

We confirm and characterize the exoplanetary systems Kepler -445 and Kepler -446: two mid-M dwarf stars, each with multiple, small, short-period transiting planets. Kepler -445 is a metal-rich ([Fe/H] = +0.25 ± 0.10) M4 dwarf with three transiting planets, and Kepler -446 is a metal-poor ([Fe/H] = −0.30 ± 0.10) M4 dwarf also with three transiting planets. Kepler -445c is similar to GJ 1214b: both in planetary radius and the properties of the host star. The Kepler -446 system is similar to the Kepler -42 system: both are metal-poor with large galactic space velocities and three short-period, likely rocky transiting planets that were initially assigned erroneously large planet-to-star radius ratios. We independently determined stellar parameters from spectroscopy and searched for and fitted the transit light curves for the planets, imposing a strict prior on stellar density in order to remove correlations between the fitted impact parameter and planet-to-star radius ratio for short-duration transits. Combining Kepler -445, Kepler -446, and Kepler -42, and isolating all mid-M dwarf stars observed by Kepler with the precision necessary to detect similar systems, we calculate that 21 % of mid-M dwarf stars host compact multiples (multiple planets with periods of less than 10 days) for a wide range of metallicities. We suggest that the inferred planet masses for these systems support highly efficient accretion of protoplanetary disk metals by mid-M dwarf protoplanets.