Reliability of 1D radiative-convective photochemical-equilibrium retrievals on transit spectra of WASP-107b

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
² Centre for Exoplanet Science, University of Edinburgh, Edinburgh EH9 3FD, UK
³ School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3FF, UK
⁴ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁵ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁶ University of Liège, STAR Institute, Allée du Six Août 19C, 4000 Liège, Belgium

^★ Corresponding author.

Received: 31 March 2025
Accepted: 13 July 2025

Abstract

Context. WASP-107b has been observed in unprecedented detail with the James Webb Space Telescope. These observations suggest that it has a metal-rich and carbon-deprived atmosphere with an extremely hot interior based on detections of SO₂, H₂O, CO₂, CO, NH₃, and CH₄.

Aims. In this paper, we aim to determine the reliability of a 1D radiative-convective photochemical-equilibrium (1D-RCPE) retrieval method in inferring atmospheric properties of WASP-107b. We aim to explore its sensitivity to modelling assumptions and different cloud parametrizations, and investigate the data information content. Additionally, we aim to characterize chemical trends and map dominant pathways to develop a comprehensive understanding of the 1D-RCPE model grid before running the retrievals.

Methods. We built a grid of radiative-convective balanced pressure-temperature profiles and 1D photochemical equilibrated models, which cover a range of metallicities (Z), carbon-to-oxygen ratios (C/O), intrinsic temperatures (T_int), and eddy diffusion coefficients (K_zz). We adopted a nested sampling algorithm within a Bayesian framework to estimate model parameters from previously analysed transit observations of WASP-107b discontinuously covering 1.1–12.2 μm.

Results. Our model grid reproduces established chemical trends such as the dependence of SO₂ production on metallicity and demonstrates that mixing-induced quenching at high T_int reduces the bulk CH₄ and NH₃ content. We obtain good fits with our 1D-RCPE retrievals that are mostly based on a few molecular features of H₂O, CO₂, SO₂, and CH₄, but find no substantial contribution of NH₃. We find that the degeneracy between metallicity, cloud pressure, and a model offset is broken by the presence of strong SO₂ features, confirming that SO₂ is a robust metallicity indicator. We systematically retrieve sub-solar C/O based on the relative amplitude of a strong CO₂ feature with respect to the broad band of H₂O, which is sensitive to a wavelength-dependent scattering slope. We find that high-altitude clouds obscure the CH₄−rich layers, preventing the retrievals from constraining T_int, but that higher values of K_zz can transport material above the cloud deck, allowing a fit of the CH₄ feature. However, T_int and K_zz can vary substantially between retrievals depending on the adopted cloud parametrization.

Conclusions. We conclude that the 1D-RCPE retrieval method can provide useful insights if the underlying grid of forward models is well understood. We find that WASP-107b’s atmosphere is enriched in metals (3–5 Z_⊙) and carbon-deprived (C/O ≲ 0.20). However, we lack robust constraints on the intrinsic temperature and vertical mixing strength.