Heavy-metal Jupiters by major mergers: metallicity vs. mass for giant planets

TL;DR

This paper proposes that heavy-metal Jupiters with high metallicity can form through planetary mergers near 10 au, explaining observed mass-metallicity relations and metallicity scatter.

Contribution

It introduces a merger-based formation model for heavy-metal Jupiters, linking core mass, metallicity, and planet mass, supported by theoretical and observational consistency.

Findings

Heavy-metal Jupiters can form via planetary mergers near 10 au.

Core mass scales with total planet mass as M_core ∝ M^{1/5}.

Merger outcomes can explain observed metallicity scatter.

Abstract

Some Jupiter-mass exoplanets contain $\sim$$100\, M_\oplus$ of metals, well above the $\sim$$10\, M_\oplus$ typically needed in a solid core to trigger giant planet formation by runaway gas accretion. We demonstrate that such `heavy-metal Jupiters' can result from planetary mergers near $\sim$10 au. Multiple cores accreting gas at runaway rates gravitationally perturb one another onto crossing orbits such that the average merger rate equals the gas accretion rate. Concurrent mergers and gas accretion implies the core mass scales with the total planet mass as $M_{\rm core} \propto M^{1/5}$ - heavier planets harbour heavier cores, in agreement with the observed mass-metallicity relation. While the average gas giant merges about once to double its core, others may merge multiple times, as merger trees grow chaotically. We show that the dispersion of outcomes inherent in mergers can reproduce the large scatter in observed planet metallicities, assuming $3-30\, M_\oplus$ pre-runaway cores. Mergers potentially correlate metallicity, eccentricity, and spin.