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Talk presented at the Exoplanets III (2020) conference.
Overview: Observations have revealed a lack of planets of sizes ~1.5-2.0 Earth radii, i.e. a radius ‘valley’ in the size distribution of small, short-period exoplanets. Typically, this observation has been attributed to atmospheric mass-loss due to photoevaporation. However, in recent work, Ginzburg et al. (2018) and Gupta & Schlichting (2019, 2020) have demonstrated that atmospheric mass-loss, powered by the cooling luminosity of the planetary core, yields a valley in the exoplanet size distribution, even in the absence of any photoevaporation. In this talk, I describe the key physical processes that drive this core-powered mass-loss mechanism, present detailed comparisons with observations and discuss our recent results that make testable predictions for the core-powered mass-loss mechanism as a function of stellar mass, metallicity and age. I show that, consistent with observations, the core-powered mass-loss mechanism produces a radius valley that moves to larger planet radii as a function of stellar mass. We thus find no evidence for a linear correlation between the planet and stellar mass, however, we can’t rule it out either. Furthermore, under core-powered mass-loss, we find that planets typically lose the bulk of their atmospheres on ~ Gyr timescales to become super-Earths whereas, under photoevaporation, this is expected to happen on a ~ 100 Myr timescale. We thus expect even ~ Gyr old planets to be undergoing atmospheric mass-loss. These predictions are consistent with the latest observational studies.


Understanding the Radius Valley as a by-product of Planet Formation: Observational Signatures of the Core-Powered Mass-Loss Mechanism