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.
Highlights
It is well established that the study of an exoplanetary atmospheric carbon-to-oxygen ratio (C/O) represents an important step in understanding the physical processes that govern planet formation (Öberg et al 2011; Helling et al 2014; Madhusudhan et al 2014; Cridland et al 2016, 2019a)
As a follow-up to an earlier study of ours on hot Jupiters, we present a population of warm Jupiters extracted from the same planetesimal formation population synthesis model as used in that previous work
The population we study here is extracted from the same population synthesis model as our hot Jupiter model in Paper I
Summary
It is well established that the study of an exoplanetary atmospheric carbon-to-oxygen ratio (C/O) represents an important step in understanding the physical processes that govern planet formation (Öberg et al 2011; Helling et al 2014; Madhusudhan et al 2014; Cridland et al 2016, 2019a). If Jupiter and Saturn formed through planetesimal accretion near the water ice line, they would have to undergo a Grand Tack (Walsh et al 2011) to migrate out to their current orbital radius (from 1–3 AU to 5.5 and 9.5 AU, respectively) This process, is highly sensitive to the mass ratio of the two planets and requires a particular orbital radius arrangement to function (Raymond & Morbidelli 2014; Chametla et al 2020). The two sub-populations could have accreted the bulk of their gas in similar locations in the disk, and while hot Jupiters migrated very close to their host stars, warm Jupiters did not In this case, we might expect very little chemical difference between the two types of planets.
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