Thermodynamics of Giant Planet Formation: Shocking Hot Surfaces on Circumplanetary Disks

TL;DR

This paper uses high-resolution 3D simulations to show that shock fronts on circumplanetary disks significantly influence the thermodynamic state and observational appearance of forming giant planets by concealing their intrinsic luminosity.

Contribution

It introduces detailed 3D radiative hydrodynamic simulations revealing how shock surfaces on circumplanetary disks regulate planet entropy and luminosity during formation.

Findings

Shock fronts are extended, optically thick, and can hide planetary luminosity.

Gas entropy decreases significantly after passing through the shock.

Shock surfaces may affect the observational signatures of forming gas giants.

Abstract

The luminosity of young giant planets can inform about their formation and accretion history. The directly imaged planets detected so far are consistent with the "hot-start" scenario of high entropy and luminosity. If nebular gas passes through a shock front before being accreted into a protoplanet, the entropy can be substantially altered. To investigate this, we present high resolution, 3D radiative hydrodynamic simulations of accreting giant planets. The accreted gas is found to fall with supersonic speed in the gap from the circumstellar disk's upper layers onto the surface of the circumplanetary disk and polar region of the protoplanet. There it shocks, creating an extended hot supercritical shock surface. This shock front is optically thick, therefore, it can conceal the planet's intrinsic luminosity beneath. The gas in the vertical influx has high entropy which when passing through the shock front decreases significantly while the gas becomes part of the disk and protoplanet. This shows that circumplanetary disks play a key role in regulating a planet's thermodynamic state. Our simulations furthermore indicate that around the shock surface extended regions of atomic - sometimes ionized - hydrogen develop. Therefore circumplanetary disk shock surfaces could influence significantly the observational appearance of forming gas-giants.