The Two Modes of Gas Giant Planet Formation

TL;DR

This paper proposes two distinct modes of gas giant planet formation—disk instability at large radii and core accretion at smaller radii—and discusses their conditions, implications, and observational signatures.

Contribution

It introduces the concept of two formation modes for gas giants, supported by simulations, and predicts a bimodal distribution of planet semi-major axes based on formation mechanisms.

Findings

Disk instability likely forms gas giants beyond 100 AU.

Core accretion dominates within 100 AU.

Bimodal distribution of gas giant semi-major axes is expected.

Abstract

I argue for two modes of gas giant planet formation and discuss the conditions under which each mode operates. Gas giant planets at disk radii $r>100$ AU are likely to form in situ by disk instability, while core accretion plus gas capture remains the dominant formation mechanism for $r<100$ AU. During the mass accretion phase, mass loading can push disks toward fragmentation conditions at large $r$. Massive, extended disks can fragment into clumps of a few to tens of Jupiter masses. This is confirmed by radiation hydrodynamics simulations. The two modes of gas giant formation should lead to a bimodal distribution of gas giant semi-major axes. Because core accretion is expected to be less efficient in low-metallicity systems, the ratio of gas giants at large $r$ to planets at small $r$ should increase with decreasing metallicity.