Under the magnifying glass: A combined 3D model applied to cloudy warm Saturn type exoplanets around M-dwarfs

TL;DR

This paper develops a comprehensive 3D atmospheric and cloud formation model for warm Saturn exoplanets orbiting M-dwarfs, revealing complex cloud effects on temperature, wind, and spectra, aiding future JWST observations.

Contribution

It introduces a novel iterative 3D coupled atmospheric and microphysical cloud model that captures full physical complexity for exoplanets around M-dwarfs.

Findings

Cloud opacity causes temperature inversion at the sub-stellar point.

Clouds cool the atmosphere between 0.01 and 10 bar.

Spectral features include muted gas absorption and silicate cloud signatures.

Abstract

Warm Saturn type exoplanets orbiting M-dwarfs are particularly suitable for in-depth cloud characterisation through transmission spectroscopy due to their favourable stellar to planetary radius contrast. However, modelling cloud formation consistently within the 3D atmosphere remains computationally challenging. The aim is to explore the combined atmospheric and micro-physical cloud structure, and the kinetic gas-phase chemistry for the warm Saturn HATS0-6b orbiting an M-dwarf. A combined 3D cloudy atmosphere model is constructed by iteratively executing the 3D General Circulation Model (GCM) expeRT/MITgcm and a kinetic cloud formation model, each in its full complexity. Resulting cloud particle number densities, sizes, and compositions are used to derive the local cloud opacity which is then utilised in the next GCM iteration. The disequilibrium H/C/O/N gas-phase chemistry is calculated for each iteration to assess the resulting transmission spectrum in post-processing. The cloud opacity feedback causes a temperature inversion at the sub-stellar point and at the evening terminator at gas pressures higher than 0.01 bar. Furthermore, clouds cool the atmosphere between 0.01 bar and 10 bar, and narrow the equatorial wind jet. The transmission spectrum shows muted gas-phase absorption and a cloud particle silicate feature at approximately 10 micron. The combined atmosphere-cloud model retains the full physical complexity of each component and therefore enables a detailed physical interpretation with JWST NIRSpec and MIRI LRS observational accuracy. The model shows that warm Saturn type exoplanets around M-dwarfs are ideal candidates to search for limb asymmetries in clouds and chemistry, identify cloud particle composition by observing their spectral features, and identify the cloud-induced strong thermal inversion that arises on these planets specifically.