Connecting planet formation and astrochemistry: A main sequence for C/O in hot-exoplanetary atmospheres

TL;DR

This study models the chemical evolution of protoplanetary disks to understand how planet formation influences the atmospheric C/O ratio in exoplanets, revealing a main sequence linking solid accretion to atmospheric composition.

Contribution

It introduces a population-based model connecting planet formation processes with atmospheric C/O ratios, incorporating astrochemical evolution and refractory carbon erosion effects.

Findings

Neptune-mass planets are more oxygen-rich than Hot-Jupiters.

Hot-Jupiters' C/O ratios are mainly determined by gas accretion.

The model reproduces observed mass-metallicity and C/O-metallicity trends.

Abstract

To understand the role that planet formation history has on the observable atmospheric carbon-to-oxygen ratio (C/O) we have produced a population of astrochemically evolving protoplanetary disks. Based on the parameters used in a pre-computed population of growing planets their combination allows us to trace the molecular abundances of the gas that is being collected into planetary atmospheres. We include atmospheric pollution of incoming (icy) planetesimals as well as the effect of refractory carbon erosion noted to exist in our own solar system. We find that the carbon and oxygen content of Neptune-mass planets are determined primarily through solid accretion and result in more oxygen-rich (by roughly two orders of magnitude) atmospheres than Hot-Jupiters, whose C/O are primarily determined by gas accretion. Generally we find a `main-sequence' between the fraction of planetary mass accreted through solid accretion and the resulting atmospheric C/O - with planets of higher solid accretion fraction having lower C/O. Hot-Jupiters whose atmospheres have been chemically characterized agree well with our population of planets, and our results suggest that Hot Jupiter formation typically begins near the water ice line. Lower mass hot-Neptunes are observed to be much more carbon-rich (with 0.33 $\lesssim$ C/O $\lesssim$ 1) than is found in our models (C/O $\sim 10^{-2}$), and suggest that some form of chemical processing may affect their observed C/O over the few Gyrs between formation and observation. Our population reproduces the general mass-metallicity trend of the solar system and qualitatively reproduces the C/O-metallicity anti-correlation that has been inferred for the population of characterized exoplanetary atmospheres.