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

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
e-mail: dominic.samra@oeaw.ac.at
² Institute for Theoretical Physics and Computational Physics, Graz University of Technology, Petersgasse 16 8010 Graz, Austria
³ Center for Exoplanet Science, University of St Andrews, St Andrews KY16 9AJ, UK
⁴ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁵ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ Center for ExoLife Sciences, Niels Bohr Institute, Øster Voldgade 5, 1350 Copenhagen, Denmark

Received: 9 September 2022
Accepted: 26 October 2022

Abstract

Context. WASP-96b is a hot Saturn exoplanet, with an equilibrium temperature of ≈1300 K. This is well within the regime of thermo-dynamically expected extensive cloud formation. Prior observations with Hubble/WFC3, Spitzer/IRAC, and VLT/FORS2 have been combined into a single spectrum 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 exoplanet WASP-96b is explored.

Methods. One-dimensional profiles were 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 was applied to the pre-JWST WASP-96b transit spectrum to investigate the apparent contradiction between cloudy models and assumed cloud-free transit spectrum.

Results. Clouds are predicted to be ubiquitous throughout the atmosphere of WASP-96b. Silicate materials contribute between 40% and 90% cloud particle volume, which means that metal oxides also contribute with up to 40% cloud particle volume in the low-pressure regimes that affect spectra. We explore how these cloudy models match currently available transit spectra. Reduced vertical mixing acts to settle clouds to deeper in the atmosphere, and an increased cloud particle porosity reduces the opacity of clouds in the near-IR and optical region. These two 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. Retrievals of HST, Spitzer, and VLT spectra also show that multiple cloudy solutions reproduce the data. JWST observations will be affected by clouds, where the cloud top pressure varies by an order of magnitude within even the NIRISS wavelength range. The long-wavelength end of NIRSpec and the short-wavelength end of MIRI may probe atmospheric asymmetries between the limbs of the terminator on WASP-96b.