Reduced gas accretion on super-Earths and ice giants

TL;DR

This study shows that planetary envelopes around super-Earths and ice giants are limited in growth due to non-hydrostatic gas flows, explaining their low-mass atmospheres despite theoretical expectations of runaway accretion.

Contribution

The paper introduces 3D radiative hydrodynamical simulations demonstrating how non-hydrostatic flows stall envelope growth around cores up to 15 Earth masses.

Findings

Gas flows enter through poles and exit in the disc midplane.

Envelope accretion stalls for cores around five Earth masses due to gas flow dynamics.

Runaway gas accretion occurs only for cores larger than 15 Earth masses with low disc opacity.

Abstract

A large fraction of giant planets have gaseous envelopes that are limited to about 10 % of their total mass budget. Such planets are present in the Solar System (Uranus, Neptune) and are frequently observed in short periods around other stars (the so-called Super-Earths). In contrast to these observations, theoretical calculations based on the evolution of hydrostatic envelopes argue that such low mass envelopes cannot be maintained around cores exceeding five Earth masses. Instead, under nominal disc conditions, these planets would acquire massive envelopes through runaway gas accretion within the lifetime of the protoplanetary disc. In this work, we show that planetary envelopes are not in hydrostatic balance, which slows down envelope growth. A series of 3-dimensional, global, radiative hydrodynamical simulations reveal a steady state gas flow, which enters through the poles and exits in the disc midplane. Gas is pushed through the outer envelope in about 10 orbital timescales. In regions of the disc that are not significantly dust-depleted, envelope accretion onto cores of about five Earth masses can get stalled as the gas flow enters the deep interior. Accreted solids sublimate deep in the convective interior, but small opacity-providing grains are trapped in the flow and do not settle, which further prevents rapid envelope accretion. The transition to runaway gas accretion can however be reached when cores grow larger than typical Super-Earths, beyond 15 Earth masses, and preferably when disc opacities are below kappa=1 cm^2/g. These findings offer an explanation for the typical low-mass envelopes around the cores of Super-Earths.