1D atmospheric modelling of K2-18b

The atmospheric composition of exoplanets with masses between 2 and 10 M⊕ is poorly known. In that regard, the sub-Neptune K2-18b offers a valuable opportunity for the characterisation of such atmospheres under Earth-like stellar irradiation. Previous analyses of its transmission spectrum from the Kepler, Hubble and Spitzer space telescopes data using both retrieval algorithms and forward-modelling suggest the presence H₂O, as well as a low amount of CH₄ in a H₂–He atmosphere.

We present here simulations of the atmosphere of K2-18 b using Exo-REM, our self-consistent 1D atmospheric model — recently adapted for transiting, high-metallicity giant exoplanets — to study the atmosphere of K2-18b. We compared the transmission spectra computed by our model with the above-mentionned data (0.4 to 5 μm) to infer the planet atmospheric composition assuming a H₂–He dominated atmosphere. We investigated the effect of irradiation, eddy diffusion coefficient, internal source, clouds, C/O ratio and metallicity on the atmospheric structure and transit spectrum. 
We will put an emphasis of the relative contributions of the various absorbers to the transmission spectrum. We will show that our simulations favor atmospheric metallicities from 100 to 200 times solar. We will also discuss the possibility of a CH₄-depleted atmosphere and of liquid H₂O cloud formation.