X-ray irradiation of planets traversing the photoevaporation valley
Evidence was recently uncovered for a gap in the radius distribution of small planets. The feature has been referred to as the `photoevaporation valley' as it is thought to result from high energy photons driving substantial atmospheric mass loss. Young planetary systems, where the irradiation level is at its highest, provide the best testing ground for this theory. K2 recently discovered seven new planets in the 800 Myr Praesepe cluster, most of which have radii consistent with being in or just above the photoevaporation valley. We will present an analysis of XMM-Newton observations for four of these planets. We will discuss the implication of the measured irradiation levels on the planet's atmospheres, how they may have evolved in the past, and what the future could hold.