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The atmospheres of small, close-in exoplanets are vulnerable to rapidmass-loss during protoplanetary disc dispersal via a process referred to as`boil-off', in which confining pressure from the local gas disc reduces,inducing atmospheric loss and contraction. We construct self-consistent modelsof planet evolution during gaseous core accretion and boil-off. As thesurrounding disc gas dissipates, we find that planets lose mass via subsonicbreeze outflows which allow causal contact to exist between disc and planet.Planets initially accrete of order $\sim 10\%$ in atmospheric mass, however,boil-off can remove $\gtrsim 90\%$ of this mass during disc dispersal. We showthat a planet's final atmospheric mass fraction is strongly dictated by theratio of cooling timescale to disc dispersal timescale, as well as the planet'score mass and equilibrium temperature. With contributions from core cooling andradioactivity, we show that core luminosity eventually leads to the transitionfrom boil-off to core-powered mass-loss. We find that smaller mass planetsclosest to their host star may have their atmospheres completely strippedthrough a combination of boil-off and core-powered mass-loss during discdispersal, implying the existence of a population-level radius gap emerging asthe disc disperses. We additionally consider the transition fromboil-off/core-powered mass-loss to X-ray/EUV (XUV) photoevaporation byconsidering the penetration of stellar XUV photons below the planet's sonicsurface. Finally, we show that planets may open gaps in their protoplanetarydiscs during the late stages of boil-off, which may enhance mass-loss rates.

Under the light of a new star: evolution of planetary atmospheres through protoplanetary disc dispersal and boil-off