The evolutionary track of H/He envelope in the observed population of sub-Neptunes and Super-Earths planets

The observational detection of a localized reduction in the small planet occurrence rate, sometimes termed a “gap”, is an exciting discovery because of the implications for planet evolutionary history. This gap appears to define a transition region in which sub-Neptune planets are believed to have lost their H/He envelope, potentially by photoevaporation or core powered mass loss, and have thus been transformed into bare cores terrestrial planets. Here we investigate the transition between sub-Neptunes and super-Earths by using envelope evolution models of the H/He envelope together with the mass-radius diagram and a photoevaporation model. We find that photoevaporation cannot explain the loss of the H/He envelope if the super-Earths and sub-Neptunes have the same core mass distribution. We explore the possibility that these planets families have different core mass and find that the primordial envelope fractions of the strongly irradiated super-Earths and the sub-Neptunes at an age of 100Myr is $\sim$12$\%$ of their original mass. This agreement suggests that these two groups are part of the same primordial, parent population. Our analysis also shows that sub-Neptunes with R$<$3.5 R$_{\oplus}$ typically lose $\sim$ 70$\%$ of their primordial envelope.