Compositional Signatures of Hot Jupiter Formation Inside the Water Snowline

Many well characterized hot Jupiters display a puzzlingly low abundance of water relative to carbon monoxide (CO) in their atmospheres. Previous studies have proposed that their peculiar compositions might be explained by formation close to the CO snowline at tens of au. However, it is unclear if migration from tens of AU to 0.05 AU is feasible or compatible with the dynamical and orbital properties of hot Jupiters. Here, I propose a novel alternative: namely, that the measured atmospheric compositions of these hot Jupiters are consistent with envelope accretion within the water snowline. Giant planets that finish forming within the water snowline accrete material with a Si/O ratio of up to four times the solar value. The excess Si preferentially binds to oxygen in hot Jupiter atmospheres and is lost when these materials condense into Si-rich clouds, thereby depleting the corresponding gas-phase water abundance. We show that accounting for potential contributions from pristine dust rich in refractory carbon that drifts in from the outer disk further improves the quality of the fit to current observations. We identify unique compositional signatures for this scenario that can be used to distinguish it from formation near the carbon monoxide snowline, and show that these signatures should be detectable by JWST (mid-IR) and HST (UV).