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

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, together 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 ≲80° while the volatile hydrogen is not. Because of this disparity, the hydrogen has to slowly diffuse out into the dayside where X-ray and ultraviolet 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 envelope. 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 hr and an initial mass less than ∼12 M ⊕ , 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 ∼20.5 hr while also having an initial mass smaller than ∼9 M ⊕ .