Gas composition of main volatile elements in protoplanetary discs and its implication for planet formation

Abstract

Direct observations of gaseous exoplanets reveals that their gas envelope is commonly enriched in C/O ratio compared to that of the host star. This has been explained by considering that the gas phase of the disc could be inhomogeneous, exceeding the stellar C/O ratio in regions where these planets formed; but few studies have considered the drift of the gas and the migration of planets. We aim to derive the gas composition in planets to evaluate if the formation of giant planets with an enriched C/O ratio is possible. The study focusses on the effects of different processes on the C/O ratio like the disc evolution, the drift of gas, and the migration of the planet. We used our previous models for computation of the chemical composition together with the planet formation model of Alibert et al. (2013), to which we added the composition and drift of the gas phase of the disc composed of major volatile species, H2 and He. The study focusses on the region where ice lines are present and influence the C/O ratio of the planets. Modelling shows that the condensation of volatile species allows for C/O enrichment in specific parts of the protoplanetary disc, of up to 4 times the solar value. This leads to the formation of planets that can be enriched in C/O in their envelope up to 3 times the solar value. The migration of planets, the evolution of the gas phase, and the irradiation of the disc enables the evolution of the initial C/O ratio with a decrease in the outer part of the disc and an increase in the inner part. The total C/O ratio of the planets is governed by the contribution of ices accreted, suggesting that high C/O ratios measured in planetary atmospheres are indicative of a lack of exchange of material between the core of a planet and its envelope or an observational bias, and suggesting that the observed C/O ratio is not representative of the total C/O ratio of the planet.