C/O Ratios and the formation of wide separation exoplanets

TL;DR

This paper investigates the relationship between disk gas and planetary atmospheric compositions, focusing on C/O ratios, and explores formation mechanisms for wide separation exoplanets through observational data and simple modeling.

Contribution

It synthesizes observational estimates of disk C/O ratios and compares them to exoplanet atmospheres, proposing models for planet formation and composition acquisition.

Findings

Disk gas C/O ratios are generally > 1 with sub-solar carbon content.

Wide separation gas giants have atmospheric C/O ratios near stellar values.

Variations in planetary metallicity suggest modest and variable disk carbon depletion.

Abstract

The gas and solid-state C/O ratios provide context to potentially link the atmospheric composition of planets to that of the natal disk. We provide a synthesis of extant estimates of the gaseous C/O and C/H ratios in planet-forming disks obtained primarily through analysis of Atacama Large Millimeter Array observations. These estimates are compared to atmospheric abundances of wide separation (> 10 au) gas giants. The resolved disk gas C/O ratios, from seven systems, generally exhibit C/O > 1 with sub-solar, or depleted, carbon content. In contrast, wide separation gas giants have atmospheric C/O ratios that cluster near or slightly above the presumed stellar value with a range of elemental C/H. From the existing disk composition, we infer that the solid-state mm/cm-sized pebbles have a total C/O ratio (solid cores and ices) that is solar (stellar) in content. We explore simple models that reconstruct the exoplanet atmospheric composition from the disk, while accounting for silicate cloud formation in the planet atmosphere. If wide separate planets formed via the core-accretion mechanism, they must acquire their metals from pebble or planetesimal accretion. Further, the dispersion in giant planet C/H content is best matched by a disk composition with modest and variable factors of carbon depletion. An origin of the wide separation gas giants via gravitational instability cannot be ruled out as stellar C/O ratios should natively form in this scenario. However, the variation in planet metallicity with a stellar C/O ratio potentially presents challenges to these models.