Super stellar abundances of alkali metals suggest significant migration for Hot Jupiters

TL;DR

This paper proposes that the over-abundance of alkali metals in hot Jupiter atmospheres results from inward migration after formation outside the water snow line, a process supported by their model's predictions for future observations.

Contribution

It introduces a formation and migration model explaining alkali metal enrichment in hot Jupiters' atmospheres, contrasting with in situ formation scenarios.

Findings

Migration leads to alkali metal enrichment in atmospheres.

Water abundances remain similar to host stars.

Model predictions can be tested with JWST and Ariel data.

Abstract

We investigate the origin of the measured over-abundance of alkali metals in the atmospheres of hot gas giants, relative to both their host stars and their atmospheric water abundances. We show that formation exterior to the water snow line followed by inward disc-driven migration results in excess accretion of oxygen-poor, refractory-rich material from within the snow-line. This naturally leads to enrichment of alkali metals in the planetary atmosphere relative to the bulk composition of its host star but relative abundances of water that are similar to the stellar host. These relative abundances cannot be explained by in situ formation which places the refractory elements in the planetary deep interior rather than the atmosphere. We therefore suggest that the measured compositions of the atmospheres of hot Jupiters are consistent with significant migration for at least a subset of hot gas giants. Our model makes robust predictions about atmospheric composition that can be confirmed with future data from JWST and Ariel.