Can hydrogen coronae be inferred around a CO2-dominated exoplanetary atmosphere?

To date, almost 20% of the known exoplanets are in the Earth/super-Earth mass regime, and more have to be confirmed, as they represent about 40% of the Kepler candidates. Some of these planets like the Kepler-11 system exhibit very low densities, which can be explained by a high content in water (“waterworlds”) or a large hydrogen/helium atmosphere. Recent theoretical work has been done to explain how a low mass planet could sustain such an atmosphere for several Gyr. On the other hand, some authors have proposed methods based on transit absorption spectroscopy to detect and characterize these possible atmospheres. In this paper, we explore the possibility of inferring the presence of an atomic hydrogen corona by looking at its influence on the thermospheric emissions of a lower CO2 atmosphere. Two emission lines are studied in details, namely the O(1S − 1D) “green-line” at 557 nm and the CO2+(B2Σu+-X2Πg) UV-doublet around 289 nm. We use a 1D transport code coupled to a radiative transfer model to calculate the emissions of the planet and the contrast with its parent star at the two emission lines. We find that in the case of a telluric planet at 1 AU from a G-type star or at 0.03 AU from an active M dwarf, the contrasts between the star and the planet are too low to be observed with current or planned instruments.