Into the radius valley.

This article focuses on the "radius valley," an observed gap in the size distribution of small exoplanets between approximately 1.5 and 2 Earth radii, separating super-Earths (rocky planets without atmospheres) from sub-Neptunes (planets with substantial hydrogen-helium envelopes). It reviews detection methods, primarily the transit method used by missions like Kepler and TESS, and discusses theories explaining the radius valley's origin, including photoevaporation, core-powered mass loss, and late gas-poor formation. Observational studies, especially those using short-cadence light curves, have refined the understanding of the valley's depth and dependence on stellar and planetary parameters, generally supporting thermally-driven atmospheric loss models. The article also highlights ongoing challenges in precisely determining planetary masses and compositions, and anticipates future missions such as PLATO and ARIEL to advance knowledge of planet formation and evolution.