The role of cloud particle properties in the WASP-39b transmission spectrum based on JWST/NIRSpec observations

Context . Aerosols are capable of having a huge influence on reflected, emitted, and transmitted planetary spectra, especially at wavelengths similar to their average sizes, but also extending to much longer and shorter wavelengths. The Near InfraRed Spectrograph (NIRSpec) using the PRISM mode on board the James Webb Space Telescope (JWST) is providing valuable data of transit spectra over a wide spectral range that is able to cover the whole contribution of aerosols, potentially disentangling them from other constituents, and thus allowing us to constrain their properties. Aims . Our aim was to investigate whether NIRSpec/PRISM JWST transmission spectroscopy observations, in addition to being useful for detecting and determining the abundance of gases more accurately than any previous instruments, are also capable of studying the physical properties of the aerosols in exoplanetary atmospheres. Methods . We performed nested sampling Bayesian retrievals with the MultiNest library. We used the Planetary Spectrum Generator (PSG) and the Modelled Optical Properties of enSeMbles of Aerosol Particles (MOPSMAP) database as tools for the forward simulations and previously published NIRSpec/PRISM JWST observations of WASP-39b as input data. Results . Retrievals indicate that models including an aerosol extinction weakly increasing or sharply decreasing with wavelength are decisively better than those with a flat transmission and that this increased degree of complexity is supported by the kind of data that JWST/NIRSpec can provide. Given other physical constraints from previous works, the scenario of weakly increasing particle extinction is favoured. We find that this also has an effect on the retrieved gas abundances. Conclusions . JWST observations give us the potential to study some physical characteristics of exoplanetary clouds, in particular their overall dependence of transmissivity on wavelength. It is important to implement more detailed aerosol models as their extinction may affect significantly retrieved molecular abundances.