Very Hot Super-Earths with an Atmosphere: A Model Explaining Their Paradoxical Existence
The aim of this research is to constrain the interior structures and evolutions of hot super-Earths, particularly that of 55 Cancri e. Herewith, we propose an alternative model for the paradoxical nature of small, hot super-Earths with atmospheres. Our model does not require these bodies to contain large quantities of ices in order to account for their low densities, which has been a subject of dispute considering their high surface temperatures and the potentially strong internal heat processes such as tidal flexing or radiogenic heating. The first aspect of our research involved calculating the total H2 reservoir in 55 Cancri e which is ~ 2×1023 kg (0.04 M⨁). We then encountered a theoretical enigma since the UV and X-Ray induced mass loss should have been strong enough to evaporate the atmosphere billions of years ago, which is inconsistent with astronomical data showing a currently plentiful atmosphere. This issue can be completely avoided by showing that for a tidally locked setup, the mass loss rates on the night-side are negligible thus allowing the planet to maintain a H2-rich atmosphere above half its surface. In the case of 55 Cancri e, it became tidally locked approximately 50 ± 250 Myrs after it formed implying that from that moment onwards the radius and mass of the body changed negligibly. Prior to this time mass loss rates were very strong and approximately homogeneous which when modelled, showed that 55 Cancri e was born as a Neptunian-or-Jovian-type exoplanet. Finally, we propose that the bimodal distribution in exoplanet radii may be the result of two different evolutionary paths; one where a super-Earth loses all of its atmosphere before it becomes tidally locked (resulting in the peak at ~ 1.3 R⨁), and the other when super-Earths become tidally locked before losing their atmosphere allowing them to maintain it (resulting in the other peak at ~ 2.4 R⨁).