C/O and O/H Ratios Suggest Some Hot Jupiters Originate Beyond the Snow Line

TL;DR

This study compares planetary and stellar compositions of hot Jupiters to infer their formation locations, suggesting some originate beyond the snow line based on elemental ratios, though current data is inconclusive.

Contribution

It provides the first comparative analysis of C/O and O/H ratios in hot Jupiters and their host stars, proposing a link between elemental ratios and formation beyond the snow line.

Findings

Most hot Jupiters have super-stellar C/O ratios.

HD 209458b shows significant C/O enrichment and O/H depletion.

Current uncertainties prevent definitive conclusions.

Abstract

The elemental compositions of planet hosting stars serve as proxies for the primordial compositions of the protoplanetary disks within which the planets form. The temperature profile of the disk governs the condensation fronts of various compounds, and although these chemically distinct regions migrate and mix during the disk lifetime, they can still leave an imprint on the compositions of the forming planets. Observable atmospheric compositions of hot Jupiters when compared against their host stars could potentially constrain their formation and migration processes. We compared the measured planetary and stellar abundances of carbon and oxygen for ten systems with hot Jupiters. If the planets formed by core accretion with significant planetesimal accretion and migrated through the disk, the hot Jupiter atmospheres should be substantially super-stellar in O/H and sub-stellar in C/O. On the contrary, however, we find that currently reported abundances of hot Jupiters have generally super-stellar C/O ratios, though present uncertainties on the reported O/H and C/O ratios are too large to reach a firm conclusion. In one case however, HD 209458b, the elevated C/O and depleted O/H of the planet compared to the host star is significant enough to suggest an origin far beyond the ice line, with predominantly gas accretion, and subsequent disk-free migration. Improved measurements from the James Webb Space Telescope will enable more precise measurements for more hot Jupiters and we predict, based on the current marginal trend, that a sizable fraction of hot Jupiters will show enrichment of C/O and lower O/H than their hosts, similar to HD 209458b.