Small Planet Sizes Evolve Over Billions of Years

The radius valley, a bifurcation in the size distribution of close-in Kepler planets, is hypothesized to be a signature of planetary atmospheric loss. Such an evolutionary phenomenon should depend on the age of the star-planet system. In this work, we study the temporal evolution of the radius valley using two independent determinations of host star ages among the California--Kepler Survey (CKS) sample. We find evidence for a wide and nearly empty void of planets in the period-radius diagram at the youngest system ages ($\lesssim$2--3 Gyr) represented in the CKS sample. At older system ages the radius valley appears progressively more filled in. We show that the orbital period dependence of the radius valley among the younger CKS planets is consistent with that found among those planets with asteroseismically determined host star radii. While the slopes are consistent, the radius valley determined among the younger planetary sample is shifted to smaller radii, compatible with an atmospheric loss timescale on the order of gigayears for the largest and most massive planetary cores. Our results suggest that the radius valley may be a transient feature in the radius distribution of exoplanets, being weaker among older planet populations.