Clouds form on the hot Saturn JWST ERO target WASP-96b

Abstract

Context. WASP-96b is a hot Saturn exoplanet, with an equilibrium temperature of ≈ 1300 K, well within the regime of thermodynamically expected extensive cloud formation. Prior observations with Hubble/WFC3, Spitzer/IRAC, and VLT/FORS2 have been combined into a single spectra for which retrievals suggest a cold but cloud-free atmosphere. Recently, the planet was observed with the James Webb Space Telescope (JWST) as part of the Early Release Observations (ERO). Aims. The formation of clouds in the atmosphere of the exoplanet WASP-96b is explored. Methods. 1D profiles are extracted from the 3D GCM expeRT/MITgcm results and used as input for a kinetic, non-equilibrium model to study the formation of mineral cloud particles of mixed composition. The ARCiS retrieval framework is applied to the pre-JWST WASP-96b transit spectra to investigate the apparent contradiction between cloudy models and assumed cloud-free transit spectra. Results. Clouds are predicted to be ubiquitous throughout the atmosphere of WASP-96b. Silicate materials contribute between 40% and 90%, hence, also metal oxides do contribute with up to 40% in the low-pressure regimes that effect the spectra. We explore how to match these cloudy models with currently available atmospheric transit spectra. A reduced vertical mixing acts to settle clouds to deeper in the atmosphere, and an increased cloud particles porosity reduces the opacity of clouds in the near-IR and optical region. Both these effects allow for clearer molecular features to be observed, while still allowing clouds to be in the atmosphere. Conclusions. The atmosphere of WASP-96b is unlikely to be cloud free. Also retrievals of HST, Spitzer and VLT spectra show that multiple cloudy solutions reproduce the data. JWST observations will be affected by clouds, where within even the NIRISS wavelength range the cloud top pressure varies by an order of magnitude. The long wavelength end of NIRSpec and short end of MIRI may probe atmospheric asymmetries between the limbs of the terminator on WASP-96b. Key words. planets and satellites: individual: WASP-96b -planets and satellites: atmospheres -planets and satellites: gaseous planets -planets and satellites: fundamental parameters was assumed because of broad Na I line wings by (Nikolov et al. 2018). Nikolov et al. (2022 more recently used space-based instruments to observe further primary transits of WASP-96b, including the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (Spitzer), and the Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope (HST). These were combined with ground-based observations from the VLT to do a more complete analysis of the planetary atmosphere. These are the set of observations that we use for some comparisons to our atmospheric models in this work. Nikolov et al. (2022) argue that their retrieval approach of these combined data suggests cloud/haze free terminators at the pressures probed by the observations. The retrieval procedure treats cloud particles rain out and the element abundances for Na...