Cloudy solutions for the clear skies of WASP-80b: 3D cloud feedback on the atmosphere and spectra of a warm Jupiter

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 (GCMs). We used 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 was integrated with the GCM to study the effects on the atmosphere and the spectrum. We find that the emission and transmission spectra of WASP-80b are only compatible with cloudless atmospheres or with clouds composed of sufficiently large particles, namely Na$_2$S ($\geq 10 \mu$m), KCl ($\geq 1 \mu$m), and MgSiO$_3$ ($\geq 5 \mu$m). For these large-particle cloud cases, efficient gravitational settling confines the clouds to deeper atmospheric layers, resulting in weak spectral signatures. Smaller particles are ruled out due to their strong radiative feedback on the atmospheric structure. Overall, our results suggest that WASP-80b's atmosphere is either effectively cloud-free or contains clouds composed of large, settled particles whose opacity has little impact on the observable atmosphere. This underscores the importance of particle size and vertical cloud distribution in interpreting exoplanet spectra. Future observations at shorter wavelengths may help distinguish between large-particle cloud scenarios and a truly cloudless atmosphere.