Towards Chemical Constraints on Hot Jupiter Migration

TL;DR

This paper proposes that atmospheric chemical compositions of hot Jupiters can distinguish their migration origins, favoring disk-free mechanisms for planets with sub-solar C and O abundances.

Contribution

It introduces a novel approach of using atmospheric chemistry to constrain hot Jupiter migration pathways, highlighting the significance of chemical depletions.

Findings

Sub-solar C and O abundances suggest disk-free migration mechanisms.

Super-solar C/O ratios are consistent with various formation and migration pathways.

Low oxygen in atmospheres may indicate dynamical encounters during migration.

Abstract

The origin of hot Jupiters -- gas giant exoplanets orbiting very close to their host stars -- is a long-standing puzzle. Planet formation theories suggest that such planets are unlikely to have formed in-situ but instead may have formed at large orbital separations beyond the snow line and migrated inward to their present orbits. Two competing hypotheses suggest that the planets migrated either through interaction with the protoplanetary disk during their formation, or by disk-free mechanisms such as gravitational interactions with a third body. Observations of eccentricities and spin-orbit misalignments of hot Jupiter systems have been unable to differentiate between the two hypotheses. In the present work, we suggest that chemical depletions in hot Jupiter atmospheres might be able to constrain their migration mechanisms. We find that sub-solar carbon and oxygen abundances in Jovian-mass hot Jupiters around Sun-like stars are hard to explain by disk migration. Instead, such abundances are more readily explained by giant planets forming at large orbital separations, either by core accretion or gravitational instability, and migrating to close-in orbits via disk-free mechanisms involving dynamical encounters. Such planets also contain solar or super-solar C/O ratios. On the contrary, hot Jupiters with super-solar O and C abundances can be explained by a variety of formation-migration pathways which, however, lead to solar or sub-solar C/O ratios. Current estimates of low oxygen abundances in hot Jupiter atmospheres may be indicative of disk-free migration mechanisms. We discuss open questions in this area which future studies will need to investigate.