The Dynamic Proto-atmospheres around Low-Mass Planets with Eccentric Orbits

TL;DR

This study uses radiation-hydrodynamics simulations to explore how low-mass planets on eccentric orbits develop and retain proto-atmospheres, revealing the effects of bow shocks, gas recycling, and orbital dynamics on atmospheric mass and stability.

Contribution

It introduces a 2D radiative model for proto-atmospheres around eccentric low-mass planets, highlighting the impact of orbital eccentricity and gas recycling on atmospheric retention.

Findings

Eccentric planets have atmospheres 3-4 orders of magnitude less massive than circular orbit planets.

Supersonic environments can help planets retain early atmospheres despite bow shock stripping.

Atmospheric properties oscillate with orbital phase, showing lag and interaction with disk gas.

Abstract

Protoplanets are able to accrete primordial atmospheres when embedded in the gaseous protoplanetary disk. The formation and structure of the proto-atmosphere are subject to the planet--disk environment and orbital effects. Especially, when planets are on eccentric orbits, their relative velocities to the gas can exceed the sound speed. The planets generate atmosphere-stripping bow shocks. We investigate the proto-atmospheres on low-mass planets with eccentric orbits with radiation-hydrodynamics simulations. A 2D radiative model of the proto-atmosphere is established with tabulated opacities for the gas and dust. The solutions reveal large-scale gas recycling inside a bow shock structure. The atmospheres on eccentric planets are typically three to four orders of magnitude less massive than those of planets with circular orbits. Overall, however, a supersonic environment is favorable for planets to keep an early stable atmosphere, rather than harmful, due to the steady gas supply through the recycling flow. We also quantitatively explore how such atmospheres are affected by the relative velocity of the planet to the gas, the planet mass, and the background gas density. Our time-dependent simulations track the orbital evolution of the proto-atmosphere with the planet--disk parameters changing throughout the orbit. Atmospheric properties show oscillatory patterns as the planet travels on an eccentric orbit, with a lag in phase. To sum up, low-mass eccentric planets can retain small proto-atmospheres despite the stripping effects of bow shocks. The atmospheres are always connected to and interacting with the disk gas. These findings provide important insights into the impacts of migration and scattering on planetary proto-atmospheres.