Mind the Trap: Non-negligible effect of volatile trapping in ice on C/O ratios in protoplanetary disks and exoplanetary atmospheres

TL;DR

This paper demonstrates that volatile trapping in ice significantly influences C/O ratios in protoplanetary disks and exoplanetary atmospheres, affecting planetary composition and challenging previous models that ignored this effect.

Contribution

It introduces a simple model incorporating volatile trapping effects on C/O ratios, highlighting its importance in planet formation and atmospheric composition analysis.

Findings

Volatile trapping reduces and homogenizes C/O ratios across disks.

Trapping decreases gaseous carbon and oxygen available for planetary atmospheres.

It impacts the elemental budgets of hydrogen and nitrogen in planet-forming environments.

Abstract

The ability of bulk ices (H$_{2}$O, CO$_{2}$) to trap volatiles has been well studied in any experimental sense, but largely ignored in protoplanetary disk and planet formation models as well as the interpretation of their observations. We demonstrate the influence of volatile trapping on C/O ratios in planet-forming environments. We created a simple model of CO, CO$_{2}$, and H$_{2}$O snowlines in protoplanetary disks and calculated the C/O ratio at different radii and temperatures. We included a trapping factor, which partially inhibits the release of volatiles (CO, CO$_{2}$) at their snowline and releases them instead, together with the bulk ice species (H$_{2}$O, CO$_{2}$). Our aim has been to assess its influence on trapping solid-state and gas phase C/O ratios throughout planet-forming environments. Volatile trapping significantly affects C/O ratios in protoplanetary disks. Variations in the ratio are reduced and become more homogeneous throughout the disk when compared to models that do not include volatile trapping. Trapping reduces the proportion of volatiles in the gas and, as such, reduces the available carbon- and oxygen-bearing molecules for gaseous accretion to planetary atmospheres. Volatile trapping is expected to also affect the elemental hydrogen and nitrogen budgets. Volatile trapping is an overlooked, but important effect to consider when assessing the C/O ratios in protoplanetary disks and exoplanet atmospheres. Due to volatile trapping, exoplanets with stellar C/O have the possibility to be formed within the CO and CO$_{2}$ snowline.