Letter to the Editor

A cloudy fit to the atmosphere of WASP-107 b

¹ Department of Astronomy, Tsinghua University, Haidian DS, 100084, Beijing, China
² SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA, Leiden, The Netherlands
³ Space Telescope Science Institute, Baltimore, MD, USA
⁴ Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV, Groningen, The Netherlands
⁵ Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA, Leiden, The Netherlands

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 December 2025
Accepted: 8 March 2026

Abstract

Context. WASP-107 b has been observed comprehensively by JWST in the near- and mid-IR bands, meaning we can probe its composition and internal dynamics. Recent analyses reveal a 8 − 10 μm silicate feature, but it remains uncertain how silicate clouds form on this planet.

Aims. We aim to fit the complete JWST spectrum of WASP-107 b, from 0.9 μm to 12 μm with a physically motivated cloud model and self-consistent temperature profile.

Methods. We coupled two-stream radiative transfer to a cloud formation model until convergence between cloud and temperature profiles was reached. We searched a model grid that included metallicity, turbulent diffusivity, internal heat flux, and nucleation parameters to find the best-fit model.

Results. The silicate cloud feature at 10 μm and the near-IR molecular band strength can be simultaneously and naturally explained without assuming a parametrized temperature profile. A moderate vertical diffusivity of K_zz = 10⁹ cm² s⁻¹ is needed to bring the cloud particles into the upper atmosphere of WASP-107 b. This K_zz is favored by the joint fitting of the near-IR water feature and mid-IR silicate feature – both of which are sensitive to clouds. Based on the strength of the H2O and CO2 bands, our model suggests a metallicity of 17 times solar.

Conclusions. Even in warm planets such as WASP-107 b, silicate clouds can form in the relatively cool upper atmosphere because turbulence uplifts vapor and cloud particles. Despite having considerably fewer degrees of freedom, the self-consistent modeling approach successfully fits WASP-107 b’s multiwavelength data, instilling confidence in the derived physical parameters.