In-situ Accretion of Hydrogen-Rich Atmospheres on Short-Period Super-Earths: Implications for the Kepler-11 Planets

TL;DR

This study investigates how in-situ accretion of hydrogen-helium atmospheres on rocky super-Earths depends on planetary mass and disk conditions, with implications for planets like those in Kepler-11.

Contribution

It presents a new model of atmospheric evolution considering disk dispersal effects and applies it to Kepler-11, highlighting conditions for thick atmosphere formation.

Findings

Massive rocky bodies undergo runaway gas accretion.

Light rocky bodies experience significant atmospheric erosion.

Thick atmospheres are possible only under specific disk conditions.

Abstract

Motivated by recent discoveries of low-density super-Earths with short orbital periods, we have investigated in-situ accretion of H-He atmospheres on rocky bodies embedded in dissipating warm disks, by simulating quasi-static evolution of atmospheres that connect to the ambient disk. We have found that the atmospheric evolution has two distinctly different outcomes, depending on the rocky body's mass: While the atmospheres on massive rocky bodies undergo runaway disk-gas accretion, those on light rocky bodies undergo significant erosion during disk dispersal. In the atmospheric erosion, the heat content of the rocky body that was previously neglected plays an important role. We have also realized that the atmospheric mass is rather sensitive to disk temperature in the mass range of interest in this study. Our theory is applied to recently-detected super-Earths orbiting Kepler-11 to examine the possibility that the planets are rock-dominated ones with relatively thick H-He atmospheres. The application suggests that the in-situ formation of the relatively thick H-He atmospheres inferred by structure modeling is possible only under restricted conditions; namely, relatively slow disk dissipation and/or cool environments. This study demonstrates that low-density super-Earths provide important clues to understanding of planetary accretion and disk evolution.