Re-analysis of ten hot-Jupiter atmospheres with disequilibrium chemistry retrieval

¹ Université Paris Cité and Univ Paris Est Creteil, CNRS, LISA, 75013 Paris, France
² Kapteyn Institute, University of Groningen, 9747 AD Groningen, The Netherlands
³ Department of Physics and Astronomy, University College London, London, UK
⁴ Institut d’Astrophysique de Paris (CNRS, Sorbonne Université), 98bis Bd Arago, 75014 Paris, France

^★ Corresponding author: deborah.bardet@lmd.ipsl.fr

Received: 19 December 2024
Accepted: 6 June 2025

Abstract

Context. Constraining the chemical structure of exoplanetary atmospheres is pivotal for interpreting spectroscopic data and understanding planetary evolution. Traditional retrieval methods often assume thermochemical equilibrium or free profiles, which may fail to capture disequilibrium processes such as photodissociation and vertical mixing. This study leverages the TauREx 3.1 retrieval framework coupled with FRECKLL, a disequilibrium chemistry model, to address these challenges.

Aims. The study aims to (1) assess the impact of disequilibrium chemistry on constraining metallicity and C/O ratios; (2) evaluate the role of refractory species (TiO and VO) in spectral retrievals; (3) explore consistency between transit and eclipse observations for temperature and chemical profiles; and (4) determine the effects of retrieval priors and data reduction methods.

Methods. Ten hot-Jupiter atmospheres were re-analysed using Hubble Space Telescope (HST) WFC3 data in eclipse and transit. The TauREx-FRECKLL model incorporated disequilibrium chemistry calculations with a Bayesian framework to infer atmospheric properties. Retrieval scenarios included tests with and without TiO and/or VO and comparisons across different data reduction pipelines.

Results. The disequilibrium approach significantly alters retrieved metallicity and C/O compared to equilibrium models, impacting insights into planet formation. TiO and/or VO additions improve fits for only two planets, with limited effect on parameter convergence. Retrievals reconcile transit and eclipse temperature profiles in deeper atmospheric layers but not in upper layers. These results are highly dependent on spectral resolution and retrieval priors, emphasising the limitations of HST data and the need for broader spectral coverage from instruments such as JWST.

Conclusions. This study demonstrates the feasibility and importance of incorporating disequilibrium chemistry in atmospheric retrievals, highlighting its potential for advancing our understanding of exoplanetary atmospheres with next-generation telescopes.