Metal loading of giant gas planets

TL;DR

This paper investigates how pebble accretion influences the collapse of pre-collapse giant gas planets, revealing that increased metallicity can accelerate planetary formation contrary to previous expectations.

Contribution

It introduces a new analytical theory explaining how pebble accretion accelerates collapse in low-mass giant planets, highlighting the importance of dust physics in planet formation models.

Findings

Higher metallicity accelerates planet collapse at certain temperatures.

Addition of 5-10% metals by weight triggers collapse in specific temperature range.

Dust physics critically affects the evolution of gas disc fragments.

Abstract

One of many challenges in forming giant gas planets via Gravitational disc Instability model (GI) is an inefficient radiative cooling of the pre-collapse fragments. Since fragment contraction times are as long at $10^5 -10^7$ years, the fragments may be tidally destroyed sooner than they contract into gas giant planets. Here we explore the role of "pebble accretion" onto the pre-collapse giant planets and find an unexpected result. Despite larger dust opacity at higher metallicities, addition of metals actually accelerates -- rather than slows down -- collapse of high opacity, relatively low mass giant gas planets ($M_p$ below a few Jupiter masses). A simple analytical theory that explains this result exactly in idealised simplified cases is presented. The theory shows that planets with the central temperature in the range between $\sim$ 1000 to 2000K are especially sensitive to pebble accretion: addition of just $\sim 5$ to 10 % of metals by weight is sufficient to cause their collapse. These results show that dust grain physics and dynamics is essential for an accurate modelling of self-gravitating disc fragments and their near environments in the outer massive and cold protoplanetary discs.