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

Abstract

Context. Direct observations of gaseous exoplanets reveal that their gas envelope has a higher C/O ratio than that of the host star (e.g., . 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 planet migration. Aims. We aim to derive the gas composition in planets through planet formation 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, such as the disc evolution, the drift of gas, and planet migration. Methods. We used our previous models for computing the chemical composition, together with a planet formation model, to which we added the composition and drift of the gas phase of the disc, which is composed of the main volatile species H 2 O, CO, CO 2 , NH 3 , N 2 , CH 3 OH, CH 4 , and H 2 S, H 2 and He. The study focusses on the region where ice lines are present and influence the C/O ratio of the planets. Results. Modelling shows that the condensation of volatile species as a function of radial distance allows for C/O enrichment in specific parts of the protoplanetary disc of up to four times the solar value. This leads to the formation of planets that can be enriched in C/O in their envelope up to three times the solar value. Planet migration, gas phase evolution and disc irradiation enables the evolution of the initial C/O ratio that decreases in the outer part of the disc and increases in the inner part of the disc. 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. It also suggests that the observed C/O ratio is not representative of the total C/O ratio of the planet.

Setting the volatile composition of (exo)planet-building material

“…The protoplanetary disk is taken from the models of Alibert et al (2013), Marboeuf et al (2014), and Thiabaud et al (2015) which provide the midplane temperature T (R), pressure p(R), and surface density profiles Σ(R) with radius, R. The disk has a parameterised surface density profile 1 ,…”

The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime

“…During the planet-formation process, the molecules accreted (whether gas or ice) are ultimately reprocessed in the planet atmosphere. Recent population synthesis models suggest that the main contribution to the heavy element content in the atmospheres of forming planets are ices accreted during the formation of the planetary embryo and icy planetesimals which are captured during the gas accretion stage and vaporised in the atmosphere (see, e.g., Thiabaud et al 2014).…”

Observing transiting planets with JWST

“…The observational evidence of C/O > 1 planets is debated (Madhusudhan et al 2011b;Crossfield et al 2012;Swain et al 2013;Stevenson et al 2014;Line et al 2014;Kreidberg et al 2015;Benneke 2015) and there have been numerous studies trying to theoretically assess whether the formation of C/O > 1 or C/O → 1 planets is possible (Öberg et al 2011;Ali-Dib et al 2014;Thiabaud et al 2014Thiabaud et al , 2015Helling et al 2014;Marboeuf et al 2014b,a;Madhusudhan et al 2014aMadhusudhan et al , 2016Öberg & Bergin 2016), while one of the most recent works on this topic indicates that hot Jupiters (which usually have masses 3M ) and planets of lower mass may never have C/O > 1 .…”