Contributed Talk - Splinter Exoplanets

Friday, 17 September 2021, 16:00   (virtual Exo)

Atmospheric recycling of super-Earths and mini-Neptunes: The effects of core mass, headwind and opacity

Tobias Moldenhauer, Rolf Kuiper, Willy Kley, Chris Ormel
University of Tübingen, University of Heidelberg, Tsinghua University

Recent observations found close-in planets with significant atmospheres of hydrogen and helium, the so called super-Earths and mini-Neptunes, in great abundance. Their atmospheric composition suggests that they formed early during the gas-rich phase of the circumstellar disk and were able to avoid becoming hot Jupiters. As a possible explanation recent studies explored the recycling hypothesis and showed that atmospheric recycling is able to fully compensate radiative cooling and thereby halt Kelvin-Helmholtz contraction to prevent runaway gas accretion. To understand the parameters that determine whether a proto-planet eventually undergoes runaway gas accretion, we extend our earlier studies by exploring the effects of the core mass, the the effect of circumstellar gas on sub-Keplerian orbits (headwind) and the the optical depth of the surrounding gas on the recycling timescale. Additionally, we analyze their effects on the size and mass of the forming atmosphere. We use three-dimensional (3D) radiation-hydrodynamic simulations to model a local shearing box centered at the planet. Our planet is located at a separation of 0.1 au from its solar-type host star and we scan the core mass range from 1 to 10 M_Earth. In order to measure and track the recycling of the atmosphere, we employ tracer particles as well as tracer fluids after a thermodynamic equilibrium is reached. For the explored parameter space, all simulations eventually reach an equilibrium where heating due to hydrodynamic recycling fully compensates radiative cooling. In this equilibrium the atmosphere-to-core mass ratio stays well below 10 %, preventing the atmosphere from becoming self-gravitating and entering runaway gas accretion. Higher core masses cause the atmosphere to become turbulent which further enhances recycling. Compared to the core mass the effect of the headwind on the recycling timescale is negligible. The opacity has no significant effect on the recycling timescale which demonstrates that the Kelvin-Helmholtz contraction timescale and the atmospheric recycling timescale are independent of each other. Even for our highest core mass of 10 M_Earth atmospheric recycling is efficient enough to fully compensate radiative cooling and prevent the atmosphere from becoming self-gravitating. Hence, in-situ formation of hot Jupiters is very unlikely and migration of gas giants is a key process required to explain their existence. Atmospheric recycling is the prime explanation for the high abundance of close-in super-Earths and mini-Neptunes.