Connecting planet formation and astrochemistry

C/Os and N/Os of warm giant planets and Jupiter analogues

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: 26 June 2020
Accepted: 5 September 2020

Abstract

The chemical composition of planetary atmospheres has long been thought to store information regarding where and when a planet accretes its material. Predicting this chemical composition theoretically is a crucial step in linking observational studies to the underlying physics that govern planet formation. As a follow-up to an earlier study of ours on hot Jupiters, we present a population of warm Jupiters (semi-major axis between 0.5 and 4 AU) extracted from the same planetesimal formation population synthesis model as used in that previous work. We compute the astrochemical evolution of the proto-planetary disks included in this population to predict the carbon-to-oxygen (C/O) and nitrogen-to-oxygen (N/O) ratio evolution of the disk gas, ice, and refractory sources, the accretion of which greatly impacts the resulting C/Os and N/Os in the atmosphere of giant planets. We confirm that the main sequence (between accreted solid mass and the atmospheric C/O) we found previously is largely reproduced by the presented population of synthetic warm Jupiters. As a result, the majority of the population falls along the empirically derived mass-metallicity relation when the natal disk has solar or lower metallicity. Planets forming from disks with high metallicity ([Fe/H] > 0.1) results in more scatter in chemical properties, which could explain some of the scatter found in the mass-metallicity relation. Combining predicted C/Os and N/Os shows that Jupiter does not fall among our population of synthetic planets, suggesting that it likely did not form in the inner 5 AU of the Solar System before proceeding into a Grand Tack. This result is consistent with a recent analysis of the chemical composition of Jupiter’s atmosphere, which suggests that it accreted most of its heavy element abundance farther than tens of AU away from the Sun. Finally, we explore the impact of different carbon refractory erosion models, including the location of the carbon erosion front. Shifting the erosion front has a major impact on the resulting C/Os of Jupiter- and Neptune-like planets, but warm Saturns see a smaller shift in C/Os since their carbon and oxygen abundances are equally impacted by gas and refractory accretion.