Connecting planet formation and astrochemistry

A main sequence for C/O in hot exoplanetary atmospheres

¹ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
e-mail: cridland@strw.leidenuniv.nl
² Max-Planck-Institut für Extraterrestrishe Physik, Gießenbachstrasse 1, 85748 Garching, Germany
³ Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4E8, Canada
⁴ Origins Institute, McMaster University, Hamilton, Ontario L8S 4E8, Canada

Received: 14 June 2019
Accepted: 29 October 2019

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; planets of higher solid accretion fraction have 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 ≲ C/O ≲ 1) than is found in our models (C/O ~ 10⁻²), and suggest that some form of chemical processing may affect their observed C/O over the few billion years 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.