Optically Thin Core Accretion: How Planets Get Their Gas in Nearly Gas-Free Disks

TL;DR

This paper develops models of planetary atmospheres in optically thin environments, showing that super-Earths and sub-Neptunes can accrete their gas envelopes efficiently without the need for dense, opaque disks, challenging traditional assumptions.

Contribution

It introduces time-dependent models of optically thin planetary atmospheres, demonstrating gas accretion processes in gas-poor disks that align with observed exoplanet characteristics.

Findings

Super-Earths can accrete ~1% gas envelopes in gas-poor environments.

Envelope growth rate is similar to traditional radiative diffusion models.

Atmospheric temperature inversions can occur but do not significantly affect accretion rates.

Abstract

Models of core accretion assume that in the radiative zones of accreting gas envelopes, radiation diffuses. But super-Earths/sub-Neptunes (1-4$R_\oplus$, 2-20$M_\oplus$) point to formation conditions that are optically thin: their modest gas masses are accreted from short-lived and gas-poor nebulae reminiscent of the transparent cavities of transitional disks. Planetary atmospheres born in such environments can be optically thin to both incident starlight and internally generated thermal radiation. We construct time-dependent models of such atmospheres, showing that super-Earths/sub-Neptunes can accrete their $\sim$1%-by-mass gas envelopes, and super-puffs/sub-Saturns their $\sim$20%-by-mass envelopes, over a wide range of nebular depletion histories requiring no fine tuning. Although nascent atmospheres can exhibit stratospheric temperature inversions effected by atomic Fe and various oxides that absorb strongly at visible wavelengths, the rate of gas accretion remains controlled by the radiative-convective boundary (rcb) at much greater pressures. For dusty envelopes, the temperature at the rcb $T_{\rm rcb} \simeq 2500$ K is still set by ${\rm H}_2$ dissociation; for dust-depleted envelopes, $T_{\rm rcb}$ tracks the temperature of the visible or thermal photosphere, whichever is deeper, out to at least $\sim$5 AU. The rate of envelope growth remains largely unchanged between the old radiative diffusion models and the new optically thin models, reinforcing how robustly super-Earths form as part of the endgame chapter in disk evolution.