How clear are the skies of WASP-80b?: 3D Cloud feedback on the atmosphere and spectra of the warm Jupiter

TL;DR

WASP-80b, a warm Jupiter around an M-dwarf, is analyzed with JWST emission and transmission spectra to probe clouds and chemistry using a 3D General Circulation Model (ADAM) with radiatively active tracer clouds. The study demonstrates that cloudless GCMs can closely reproduce the observed spectra, and that atmospheres containing large cloud particles (Na$_2$S $>10~\mu$m, KCl $>1~\mu$m, MgSiO$_3$ $>5~\mu$m) also fit the data, while smaller particles induce strong radiative feedback that is inconsistent with the observations. Among expected condensates, Na$_2$S, KCl, and MgSiO$_3$ clouds with specific size thresholds can be compatible with an apparently cloudless spectrum, whereas smaller particles are ruled out. Overall, the work highlights the critical roles of particle size, composition, and 3D atmospheric transport in shaping JWST spectra of warm Jupiters and shows that short-wavelength data are essential to break degeneracies between cloudless and cloudy scenarios.

Abstract

Close-in warm Jupiters orbiting M-dwarf stars are expected to exhibit diverse atmospheric chemistry, with clouds playing a key role in shaping their albedo, heat distribution, and spectral properties. We study WASP-80b, a warm Jupiter orbiting an M-dwarf star, using the latest JWST panchromatic emission and transmission spectra to comprehensively characterise its atmosphere, including cloud coverage, chemical composition, and particle sizes, and compare the observations with predictions from the General Circulation Models (GCM). We use a GCM, ADAM (ADvanced Atmospheric MITgcm, formerly known as SPARC/MITgcm), combined with the latest JWST data to study the atmosphere of WASP-80b. A cloud module with radiatively active, tracer-based clouds is integrated with the GCM to study the effects on the atmosphere and the spectrum. Our results indicate that both emission and transmission spectra of WASP-80b are best reproduced by cloudless GCMs or by atmospheres containing large cloud particles ($\geq 10~μ$m for Na$_2$S, $\geq 1~μ$m for KCl, and $\geq 5~μ$m for MgSiO$_3$), with smaller particles ruled out due to their strong radiative feedback. These findings emphasize the importance of particle size and composition in interpreting exoplanet atmospheric spectra and showcase the power of global modelling in constraining cloud properties. Among the expected clouds to form in WASP-80b, we show that only Na$_2$S clouds forming particles larger than 10 $μm$, KCl clouds larger than 1 $μm$, or MgSiO$_3$ clouds with particles larger than $5 μm$ can be compatible with the apparently cloudless emission and transmission spectra. Observations at shorter wavelengths in both emission and transmission could further distinguish between these cloudy scenarios and a truly cloudless atmosphere.

Figures (16)

• Figure 1: Vertically varying chemical abundance profiles in the atmosphere of WASP-80b, based on the best-fit retrieval from wiser2025. The profiles are applied in each GCM column assuming horizontal homogeneity. The retrieved Volume Mixing Ratio (VMR) profiles were interpolated onto the GCM pressure grid and extrapolated beyond the retrieval range using the nearest available values.
• Figure 2: Left Column: Pressure-temperature profiles from the Cloudless GCM models (top: cloudless GCM with low T$_{\rm int}$; bottom: cloudless GCM with high T$_{\rm int}$; grey: profiles at all latitudes and longitudes; light blue: 1D profile from wiser2025). The condensation curves of several important species are plotted as dashed lines. Right Column: ADAM Spectrum plotted with JWST observations in grey (top: emission spectra for cloudless GCM with low and high T$_{\rm int}$ showing significant overlap, indicating minimal sensitivity to internal heating in the terminator region, plotted along with 1D spectrum from wiser2025; bottom: transmission spectra for cloudless GCM with low and high T$_{\rm int}$ showing significant overlap, indicating minimal sensitivity to internal heating in the terminator region)
• Figure 3: Effective temperature calculated from global outgoing flux as a function of simulation time for different initial temperature-pressure profiles ($T_{\rm eq}$ = 625, 800, 825, and 850 K). The convergence of all cases toward the same equilibrium flux demonstrates that the GCM solution is independent of the initial conditions.
• Figure 4: Top: Zonal-mean zonal wind speed (Latitudinal distribution of Zonal component of wind (U) averaged over longitudes) for all models. Within each subplot, the x-axis shows the latitude and the y-axis the pressure. The first row shows cloudless models, low (100 K) and high (381 K) T$_{\rm int}$. Rows 2, 3, and 4 correspond to Na$_2$S, KCl, and MgSiO$_3$ clouds (with high T$_{\rm int}$), respectively, with each column representing a different particle size indicated at the top. Bottom: Same as Top, but for longitudinal distribution of the Vertical component of wind (W) averaged over latitudes. The vertical component shows the strength of upwelling on the dayside and downwelling on the nightside for different models.
• Figure 5: Top: Latitudinal distribution of clouds (shows equatorial and polar regions), averaged over longitudes for all models. Within each subplot, the x-axis shows the latitude and the y-axis the pressure. Rows 1, 2, and 3 correspond to Na$_2$S, KCl, and MgSiO$_3$ (with high T$_{\rm int}$) clouds, respectively, with each column representing a different particle size indicated at the top. Bottom: Same as Top, but for longitudinal distribution of clouds (shows dayside and nightside of the planet), averaged over latitudes for all models.