A photoevaporative investigation of planets either side of the radius-period valley

A dearth of planets around 2 Earth radii uncovered in the exoplanet population, dubbed the “radius gap” or “radius-period valley”, has been the subject of much study over the past few years. There are currently two leading mechanisms proposed to explain the observed valley: X-ray and EUV-driven photoevaporation, and core-powered mass loss. We present XMM-Newton observations and evolution simulations of two systems containing planets residing either side of the valley, TOI-431 and HD 136352, in order to explore their compatibility with the photoevaporation mechanism. With our measured X-ray fluxes, we estimate the current ongoing high-energy-driven mass loss from the planets. In the evolution simulations, we investigate the past history of the planets and their atmospheres resulting from XUV-driven escape, constraining possible early life properties.