Formation of giant planets with large metal masses and metal fractions via giant impacts in a rapidly dissipating disk

TL;DR

This paper proposes a new formation mechanism for giant planets with large metal masses and fractions, involving giant impacts and photoevaporation in rapidly dissipating disks, explaining planets like HD 149026b and TOI-849b.

Contribution

It introduces a model combining pebble accretion, giant impacts, and photoevaporation to explain the formation of metal-rich giant planets.

Findings

Giant impacts can increase interior metal mass despite small pebble isolation mass.

Runaway gas accretion can be quenched by photoevaporation, leading to high metal-mass fractions.

The model explains the origins of planets with large metal content like HD 149026b.

Abstract

According to planetary interior models, some giant planets contain large metal masses with large metal-mass fractions. HD 149026b and TOI-849b are characteristic examples of these giant planets. It has been suggested that the envelope mass loss during giant impacts plays a key role in the formation of such giant planets. The aim of the present letter is to propose a mechanism that can explain the origin of such giant planets. We investigate the formation of giant planets in a rapidly dissipating disk using N-body simulations that consider pebble accretion. The results show that although the pebble isolation mass is smaller than the metal mass (> 30 Earth masses) in some giant planets, the interior metal mass can be increased by giant impacts between planets with the isolation mass. Regarding the metal fraction, the cores accrete massive envelopes by runaway gas accretion during the disk-dissipation phase of 1-10 Myr in a disk that evolves without photoevaporation. Although a large fraction of the envelope can be lost during giant impacts, the planets can reaccrete the envelope after impacts in a slowly dissipating disk. Here, we demonstrate that, by photoevaporation in a rapidly dissipating disk, the runaway gas accretion is quenched in the middle, resulting in the formation of giant planets with large metal-mass fractions. The origins of HD 149026b and TOI-849b, which are characterized by their large metal-mass fractions, can be naturally explained by a model that considers a disk evolving with photoevaporation.