Early Accretion of Large Amounts of Solids for Directly-Imaged Exoplanets

Abstract

As the number of planetary mass objects (PMOs, $⪅$13 M${}_{\mathrm{Jupiter}}$) at wider separation ($⪆$10 AU) grows, there is emerging evidence that they form differently from their higher-mass brown-dwarf (BD) counterparts. Specifically, PMOs' atmospheres are often enriched by metals and show a large dispersion of metallicity, which is usually interpreted as a sign of solid accretion. {As a first step toward a population-level study of the amount and timing of solid accretion, }we analyze a sample of seven directly-imaged exoplanets with measured stellar and planetary chemical abundances (51 Eri b, $\beta$ Pic b, HIP 65426 b, HR 8799 c and e, AF Lep b, and YSES 1 c). Our analysis uses existing data of stellar and planetary atmospheric metallicities, and adopts a Bayesian framework that marginalizes the probabilities of disk conditions, formation locations, {planetary interior structures}, and accretion physics. We show that these PMOs accrete large amounts of solids {regardless of whether they form via core accretion or disk instability}. On average $⪆$50 M${}_{\oplus }$ solids are accreted to enrich planet atmospheres. {Individual planet accretes between 23.3 and 223.2 M${}_{\oplus }$ of solid mass, more than 75\% of which is assumed to stay in the atmosphere and increase the observed metallicity.} The result implies that the solid accretion process and therefore the planet formation process {likely take place} at an early stage {($⪅$2 Myr)} when large amounts of solids are available in young {massive} protoplanetary disks.

accepted to AAS Journals. Significant revision from the previous version