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

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