Clouds remain a key challenge in observations of exoplanets by altering the local atmospheric composition through condensation (e.g. C/O), as well as obscuring deeper atmospheric layers. WASP-96b is a hot Saturn exoplanet, recently observed with JWST as part of Early Release Observations. With an equilibrium temperature of Teq = 1300 K, WASP-96b is well within the regime of thermodynamically expected cloud formation. However, previous observations with Hubble/WFC3, Spitzer/IRAC, and VLT/FORS2 have suggested a cold but cloud-free atmosphere.
The apparent contradiction between theoretical expectations and transit spectra indicative of a cloud-free atmosphere prompts the question: Why do observations of WASP-96b appear cloud-free?
To study this, we use 1D profiles extracted from the 3D general circulation model expeRT/MITgcm as input for a kinetic, non-equilibrium model to study the formation of mineral cloud particles of mixed composition. In addition, the ARCiS retrieval framework was applied to the pre-JWST WASP-96b transit spectrum.
We find that the atmosphere of WASP-96b is unlikely to be cloud free. Retrievals of HST, Spitzer, and VLT spectra also show that multiple cloudy solutions reproduce the data. In addition, the microphysical model consistently produces mixed silicate and metal oxide clouds. However, reduced vertical mixing or increased cloud particle porosity are needed to allow for clearer molecular features consistent with observations.
So the conundrum remains, for modelling: What motivates the reduction in mixing, or particle porosity? And for observations: Are cloud-free retrieval assumptions valid given the temperatures retrieved?