The Radius Distribution of M dwarf-hosted Planets and its Evolution

M dwarf stars are not only the most promising hosts for detection and characterization of small and potentially habitable planets, they provide leverage relative to solar-type stars to test models of planet formation and evolution. Using Gaia astrometry, adaptive optics imaging, and calibrated gyrochronologic relations to estimate stellar properties, filter binaries, and assign ages, we refined the radii of 179 transiting planets orbiting 119 single late K- and early M-type stars detected by the Kepler mission, and assigned stellar rotation-based ages ) to 115 of these. We constructed the radius distribution of <4R_⊕ planets and assessed its evolution with time. As for solar-type stars, the inferred distribution contains distinct populations of "super-Earths" (at  1.3R_⊕) and "sub-Neptunes" (at  2.2Rearth) separated by a gap or "valley" at ≈1.7R_⊕ that has a period dependence that is significantly weaker (power law index of -0.026^+0.026_-0.017) than for solar-type stars. Sub-Neptunes are largely absent at short periods (<2 days) and high irradiance, a feature analogous to the "Neptune desert" observed around solar-type stars. The relative number of sub-Neptunes to super-Earths declines between the younger and older halves of the sample (median age 3.8 Gyr), although the formal significance is low (p = 0.06) because of the small sample size. The decline in sub-Neptunes appears to be more pronounced at long orbital periods vs. short periods; this is not due to detection bias and could indicate that these objects are inflated by a mechanism that operates at elevated irradiance, e.g. a runaway water greenhouse augmented by H/He.