Linking Models to Observations: Unlocking the 3D Climate Structure of WASP-107b

Current space missions including CHEOPS and JWST, as well as upcoming missions such as PLATO, will help diagnose cloud properties and global climate regimes on gas giant exoplanets with unprecedented detail and in 3D. One key target of interest is WASP-107b, a warm (~750 K) cloudy transiting planet with a Neptune-like mass but a Jupiter-like radius, suggesting an unusually large, inflated atmosphere. A wide range of observations (i.e., with HST and JWST, including limb transmission spectra) are available for this planet covering the optical to infrared wavelengths (~0.8 – 12 μm). With these observations, spectroscopic features due to H₂O, CH₄, CO, CO₂, SO₂, and NH₃, as well as a 200 K temperature difference between the morning and evening terminators, have been detected. Understanding the chemistry and horizontal temperature variations on WASP-107b requires constraints on the kinetic gas-phase chemistry (e.g., CH₄) and photochemistry (e.g., SO₂) as well as the planet’s interior temperature. Thus, a full 3D cloudy atmosphere model is needed with coherent observational constraints.

In this work, an iterative coupling between the ExoRad 3D global circulation model (GCM), which produces 3D temperature and gas abundance profiles assuming chemical equilibrium, with a kinetic cloud formation model (DRIFT) is used. The latter takes into account nucleation, surface growth, gravitational settling, mixing, element conservation, and equilibrium gas-phase chemistry in the whole computational volume.

Our iterative 3D GCM-cloud framework required approximately 5 iterations to provide the best fit to the observations. The results suggest that the iterative modeling approach reproduces the observed evening to morning limb temperature differences of 200 K, highlighting how clouds shape the 3D thermodynamics of the planet and are thus vital to properly interpret the chemical abundances of planetary atmospheres. Further, iron-free clouds with a reduced cloud mass load in the upper atmosphere that contains small cloud condensation nuclei are inferred. Depleted levels of CH₄ along with increased abundances of SO₂ and NH₃ compared to equilibrium chemistry provide evidence of disequilibrium chemical processes. Finally, detailed analysis of the 4.3 μm CO₂ feature allowed us to place constraints on the atmospheric metallicity of WASP-107b, where current estimates range from 10 – 43x solar metallicity.