Effect of improved atmospheric opacities in modelling sub-Neptunes A case study for GJ 1214 b

Aims. We investigate the impact of updated atmospheric mean opacity input values on modelled transit radius and the distribution of interior layer mass fractions.Methods. We developed and applied a coupled interior-atmosphere model. Our straightforward semi-grey calculation of atmospheric temperature enables us to perform thousands of model realisations in a Monte Carlo approach to address potential degeneracies in interior and atmospheric mass fraction. Our main constraints are planetary mass and radius from which our model infers distributions of the internal structure of exoplanetary classes ranging from Super-Earth to Mini-Neptune. We varied the relative masses of gas, envelope, mantle, and core layers subject to constraints on the bulk density from observations, and investigated the effect of updating atmospheric mean opacities.Results. First, we validate our model output with observed temperature profiles for modern Neptune. We can reproduce the basic features in the middle atmosphere but not the temperature inversion in the upper layers, which is likely because our model lacks aerosol heating. Calculated interiors are generally consistent with modern Neptune. Second, we compare with the well-studied object GJ 1214 b and obtain results that are broadly consistent with previous findings; they suggest correlations between modelled gas, water, and core mass fractions, although these are generally weak. Updating the opacities leads to a change on the order of a few percent in the modelled transit radius. This is comparable in magnitude to the planned accuracy of the PLATO data for planetary radius, suggesting that the opacity update in the model is important to implement.