WASP-39b: exo-Saturn with patchy cloud composition, moderate metallicity, and underdepleted S/O

TL;DR

WASP-39b is analyzed with a self-consistent, non-equilibrium cloud model coupled to a 3D GCM to understand how mixed-condensate clouds form and patch across its atmosphere. The study finds global cloud formation with vertically varying, patchy composition, driven by local thermodynamics and metallicity, rather than a globally uniform, grey deck. Sulfur chemistry remains relatively un-depleted by condensation, suggesting S-bearing gas species as potential tracers of primordial abundances. The results demonstrate that simplified cloud parameterizations can bias retrievals and that incorporating microphysical, mixed-material clouds improves interpretation of JWST observations. Overall, a moderately supersolar metallicity with a complex, patchy cloud structure reconciles JWST data without invoking very high metallicities and emphasizes the need for physically grounded cloud models in exoplanet atmospheres.

Abstract

WASP-39b is one of the first extrasolar giant gas planets that has been observed within the JWST ERS program. Fundamental properties that may enable the link to exoplanet formation differ amongst retrieval methods, for example metallicity and mineral ratios. In this work, the formation of clouds in the atmosphere of WASP-39b is explored to investigate how inhomogeneous cloud properties (particle sizes, material composition, opacity) may be for this intermediately warm gaseous exoplanet. WASP-39b's atmosphere has a comparable day-night temperature median with sufficiently low temperatures that clouds may form globally. The presence of clouds on WASP-39b can explain observations without resorting to a high (> 100x solar) metallicity atmosphere for a reduced vertical mixing efficiency. The assessment of mineral ratios shows an under-depletion of S/O due to condensation compared to C/O, Mg/O, Si/O, Fe/O ratios. Vertical patchiness due to heterogeneous cloud composition challenges simple cloud models. An equal mixture of silicates and metal oxides is expected to characterise the cloud top. Further, optical properties of Fe and Mg silicates in the mid-infrared differ significantly which will impact the interpretation of JWST observations. We conclude that WASP-39b's atmosphere contains clouds and the underdepletion of S/O by atmospheric condensation processes suggest the use of sulphur gas species as a possible link to primordial element abundances. Over-simplified cloud models do not capture the complex nature of mixed-condensate clouds in exoplanet atmospheres. The clouds in the observable upper atmosphere of WASP-39b are a mixture of different silicates and metal oxides. The use of constant particles sizes and/or one-material cloud particles alone to interpret spectra may not be sufficient to capture the full complexity available through JWST observations.

Figures (18)

• Figure 1: The JWST ESR targets WASP-39b (light-blue triangle) and WASP-96b (purple triangle) comfortably share the T$_{\rm eq}$, log(g) and T$_{\rm eff}$ parameter ranges with the exoplanet subclass of hot Jupiters like HD 189773b. Comparing known exoplanets in the (R$_{\rm P}$, M$_{\rm P}$)-plane shows both sharing the parameter space with the warm Saturn HATS-6b. The grey symbols indicate the presently known JWST exoplanet targets.
• Figure 2: WASP-39b 2D slices showing atmosphere and cloud structure terminator maps. Top Left: Local atmospheric gas temperature and gas pressure (T$_{\rm gas}$, p$_{\rm gas}$). Top Right: Total nucleation rate, $J_*=\sum_i J_{\rm i}$ [cm$^{-3}$ s$^{-1}$] (i=TiO$_2$, SiO, NaCl, KCl). Bottom left: Dust-to-gas mass ratio $\rho_{\rm d}/ \rho$. Bottom right: Surface averaged mean cloud particle radius $\langle a \rangle_{A}$ [$\mu$m].
• Figure 3: WASP-39b 2D slices showing atmosphere and cloud structure equatorial maps. Top Left: Local atmospheric gas temperature and gas pressure (T$_{\rm gas}$, p$_{\rm gas}$). Top Right: Total nucleation rate, $J_*=\sum_i J_{\rm i}$ [cm$^{-3}$ s$^{-1}$] (i=TiO$_2$, SiO, NaCl, KCl). Bottom left: Dust-to-gas mass ratio $\rho_{\rm d}/ \rho$. Bottom right: Surface averaged mean cloud particle radius $\langle a \rangle_{A}$ [$\mu$m].
• Figure 4: (T$_{\rm gas}$, p$_{\rm gas}$) - profiles extracted from the 3D GCM in the non-grey version presented in 2022arXiv220209183S. Left: The 120 1D profiles extracted from a WASP-39b 3D GCM. The inset highlights the region of the nightside where Rossby vortices form at $\theta\sim\pm68^{\circ}$ (see also Fig. \ref{['fig:gcm_maps']}) Right: WASP-39b and WASP-96b day- and nightside median (T$_{\rm gas}$, p$_{\rm gas}$) profiles with maximum and minimum temperature envelopes. The dayside and the nightside of WASP-96b are on average slightly hotter than WASP-39b.
• Figure 5: Microphysical cloud properties of WASP-39 b. Left column: Individual 1D profiles which describe the local properties of the cloud. Right column: Median dayside and nightside profiles with maximum and minimum planet wide value envelopes. Top row: Total nucleation rate $J_{\rm *,~tot}$. Middle row: Surface averaged mean cloud particle radius $\langle a \rangle_{A}$. Bottom row: Dust-to-gas mass ratio $\rho_{\rm d}/ \rho$.