Atmospheric Recycling Prevents Premature Collapse of Protoplanetary Atmospheres of Close-in Super-Earths and Mini-Neptunes

Abstract

In recent years, space missions such as Kepler and the Transiting Exoplanet Survey Satellite (TESS) have discovered numerous close-in planets that are more massive than Earth but less massive than gas giants: super-Earths and mini-Neptunes. Their often significant atmospheres consisting of mainly hydrogen and helium indicate that these planets formed early in gas-rich disks while avoiding the runaway gas accretion that would otherwise have turned them into hot Jupiters. This challenges current planet formation models. As a possible mechanism to prevent Kelvin-Helmholtz contraction of the atmosphere, hydrodynamical atmosphere-disk recycling has been suggested. However, because atmospheric recycling can only be captured by three-dimensional simulations which are inherently more computationally expensive, this effect has often been ignored in previous studies. This thesis investigates the efficacy of the recycling hypothesis in preventing the collapse of the atmosphere. Additionally, the effects of the core mass, optical depth, orbital separation, and the effect of circumstellar gas on sub-Keplerian orbits (headwind) on the recycling process are investigated. Using three-dimensional radiation-hydrodynamics simulations the formation of the protoplanetary atmosphere is modeled. As part of this thesis, a new particle integrator was implemented to utilize tracer particles in addition to tracer fluids to measure the recycling timescale. This new method allows studying the recycling process in an unprecedented amount of detail. For the explored parameter space, all simulations except for those with the largest orbital separation of a_p = 1 au, eventually converge to a thermodynamic equilibrium where gas accretion stops. For the first time, it is shown that atmospheric recycling is capable of fully compensating radiative cooling. In all simulations the atmosphere-to-core mass ratio stays well below 10%, preventing the atmosphere from becoming self-gravitating and entering runaway gas accretion. For close-in planets, recycling naturally halts the cooling of planetary proto-atmospheres, preventing them from contracting toward the runaway regime and collapsing into gas giants. Thereby explaining the high frequency of observed super-Earths and mini-Neptunes that avoided atmospheric collapse.