Hot Super-Earths with Hydrogen Atmospheres: A Model Explaining Their Paradoxical Existence

TL;DR

This paper proposes a model explaining how some hot super-Earths retain hydrogen atmospheres through tidal confinement on the nightside, with implications for their formation and evolution.

Contribution

It introduces a new mechanism involving tidal forces that can confine hydrogen atmospheres on tidally locked super-Earths, explaining their atmospheric survival.

Findings

55 Cancri e likely formed with a 17-18.5 hour day-length and initial mass less than 12 Earth masses.

CoRoT-7b, without an atmosphere, formed with a different day-length and initial mass below 9 Earth masses.

The model constrains formation scenarios and explains atmospheric retention on hot super-Earths.

Abstract

In this paper we propose a new mechanism that could explain the survival of hydrogen atmospheres on some hot super-Earths. We argue that on close-orbiting tidally-locked super-Earths the tidal forces with the orbital and rotational centrifugal forces can partially confine the atmosphere on the nightside. Assuming a super terran body with an atmosphere dominated by volcanic species and a large hydrogen component, the heavier molecules can be shown to be confined within latitudes of $\lesssim 80^{\circ}$ whilst the volatile hydrogen is not. Because of this disparity the hydrogen has to slowly diffuse out into the dayside where XUV irradiation destroys it. For this mechanism to take effect it is necessary for the exoplanet to become tidally locked before losing the totality of its hydrogen envelop. Consequently, for super-Earths with this proposed configuration it is possible to solve the tidal-locking and mass-loss timescales in order to constrain their formation `birth' masses. Our model predicts that 55 Cancri e formed with a day-length between approximately $17-18.5$ hours and an initial mass less than $\rm \sim12 M_{\oplus}$ hence allowing it to become tidally locked before the complete destruction of its atmosphere. For comparison, CoRoT-7b, an exoplanet with very similar properties to 55 Cancri e but lacking an atmosphere, formed with a day-length significantly different from $\sim 20.5$ hours whilst also having an initial mass smaller than $\rm \sim9 M_{\oplus}$