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. This is 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 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, nonequilibrium 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 lowpressure 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.

Aggregation and Charging of Mineral Cloud Particles under High-energy Irradiation

“…With telescopes such as the James Webb Space Telescope (JWST), our understanding of cloud formation is being put to the test as it becomes essential in our analysis of transit spectra. Already, the early-release observations from JWST (Pontoppidan et al 2022) have presented planets such as WASP-96 b with strong H 2 O features that are also predicted to have mineral cloud particles including silicate species (40%; Samra et al 2023), indicating the relevance of understanding the behavior of mineral cloud particles in humid environments. The presence of mineral cloud particles is not unusual and has been observed for, e.g., WASP-107 b with the Hubble Space Telescope (Kreidberg et al 2018) and later with JWST (Dyrek et al 2024).…”

Aggregation and charging of mineral cloud particles underhigh-energy irradiation

“…With telescopes such as the James Webb Space Telescope (JWST), our understanding of cloud formation is being put to the test as it becomes essential in our analysis of transit spectra. Already, the early-release observations from JWST (Pontoppidan et al 2022) have presented planets such as WASP-96 b with strong H 2 O features that are also predicted to have mineral cloud particles including silicate species (40%; Samra et al 2023), indicating the relevance of understanding the behavior of mineral cloud particles in humid environments. The presence of mineral cloud particles is not unusual and has been observed for, e.g., WASP-107 b with the Hubble Space Telescope (Kreidberg et al 2018) and later with JWST (Dyrek et al 2024).…”

The Impact of Cometary “Impacts” on the Chemistry, Climate, and Spectra of Hot Jupiter Atmospheres

“…Vertically, our models extend between 10 −5 and 10 2 bar, allowing us to model both the upper atmosphere, which is probed by transmission spectra, as well as the deep, optically thick atmosphere in which the cometary breakup and resulting inertia-driven mass deposition occurs. These regions are linked via vertical mixing, which is parameterized using a constant eddy diffusion coefficient K zz = 10 9 cm 2 s −1 , a value that was chosen to be broadly compatible with prior studies using ATMO (Drummond et al 2016;Venot et al 2020) and of hot Jupiter atmospheres (Moses et al 2011;Miguel & Kaltenegger 2014;Molaverdikhani et al 2020;Miles et al 2023;Samra et al 2023). We note that small changes in the adopted value of the mixing strength (K zz = 10 8 → 10 10 ) did not significantly alter our results, other than the time taken for the model to settle.…”