Growing Planet Envelopes in Spite of Recycling Flows

TL;DR

This study uses 3D radiation-hydrodynamics simulations to explore how material exchange between protoplanet envelopes and the disk affects planet formation, revealing energy contributions and growth dynamics relevant to giant planet development.

Contribution

It introduces a 3D hydrodynamic model of protoplanet envelopes and integrates these effects into a 1D framework, highlighting the role of recycling in planet growth.

Findings

Outer envelope reaches steady state through material exchange.

Recycling increases luminosity beyond Kelvin-Helmholtz predictions.

Recycling minimally affects giant planet formation timescales.

Abstract

The hydrodynamic exchange of a protoplanet's envelope material with the background protoplanetary disk has been proposed as one mechanism to account for the diversity of observed planet envelopes which range in mass fractions of ~1% for super-Earths to ~90% for giants. Here we present and analyze 3D radiation-hydrodynamics models of protoplanet envelopes to understand how the exchange of mass and energy between the protoplanet and background disk influences the formation process. Our protoplanet envelope simulations show an exchange of material bringing the outer <0.4Rbondi envelope to steady state. This exchange provides a continuous source of energy, which acts to increase the observed luminosity beyond that inferred from the binding energy liberated from Kelvin-Helmholtz contraction alone -- a finding important for potential protoplanet observations. The inner <0.4Rbondi, on the other hand, appears insulated -- growing in accordance with 1D quasi-static theory. We incorporate these 3D hydrodynamic effects into an extensible 1D framework with a physically motivated three-layer recycling parameterization. Specializing to the case of Jupiter, recycling produces minimal changes to the growth rate with the planet still entering runaway accretion and becoming a gas giant in ~1 Myr. Even in the inner disk (0.1 AU), our 1D models suggest that recycling is not so robust and ubiquitous as to stop all cores from becoming giants. At the same time however, this recycling can delay a runaway phase by an order-of-magnitude depending on the inner disk conditions and core mass.