The Road to the Radius Gap

Exoplanet surveys have revealed over the past decade that, to our surprise, the most prevalent class of planet is small and close-in to its host star. This population is bifurcated into two distinct types by the so-called ‘radius gap’. In this talk I will discuss the atmospheric processes which may effect such planets and finally lead to the radius gap being carved out. Specifically, I will present new results of sophisticated simulations in which planets undergo dramatic atmospheric mass-loss during protoplanetary disc dispersal, sometimes referred to as the ‘boil-off’ phase. Once this process is over, planets are subject to other mass-loss processes such as XUV photoevaporation and core-powered mass-loss, which are typically accredited with the formation of the radius gap. I will show that the boil-off phase is required in order to bring the predictions of gas accretion during planet formation and atmospheric mass-loss into alignment. I will also discuss how we may observationally separate these late-time processes to determine which (if any) is the dominant creator of the radius gap.