Discovery of a Transiting Adolescent Sub-Neptune Exoplanet in the Cas-Tau Association With K2

The role of stellar age in the measured properties and occurrence rates of exoplanets is not well understood. This is in part due to a paucity of young planets and the uncertainties in age-dating for most exoplanet host stars. Exoplanets belonging to coeval stellar populations, young or old, are particularly useful as benchmarks for studies aiming to constrain the evolutionary timescales relevant for planets. Such timescales may concern orbital migration, gravitational contraction, or atmospheric photo-evaporation, among other mechanisms. Here we report the serendipitous discovery of a transiting sub-Neptune from K2 photometry of the low-mass star EPIC 247267267. From multiple age indicators we estimate the age of the star to be in the range of 50-500 Myr at 68% confidence. However, based on its kinematics, we propose the star belongs to the Cas-Tau association, which we suggest has an age of 46 ± 8 Myr. Our newly derived age estimate for the association is based on a color-magnitude diagram analysis of high-probability members near the main sequence turnoff. The size of EPIC 247267267 b (R_P = 3.0 ± 0.5 R_⊕) combined with its youth make it an intriguing case study for photo-evaporation models, which predict enhanced atmospheric mass loss during early evolutionary stages.