Analysis of disequilibrium chemistry in five exoplanets’ atmosphere

Studying chemical composition is fundamental to model the formation history of planets and planetary systems. With the first JWST data and the upcoming Ariel satellite, we expect a leap forward in the exoplanet’s atmosphere field. Most current atmospheric retrieval methods assume thermochemical equilibrium or freely parameterized profiles. These approaches can miss important disequilibrium processes like photochemistry and vertical mixing, which significantly affect the observed spectra. This study explores how including disequilibrium chemistry affects the retrieval of key atmospheric parameters, such as metallicity and the carbon-to-oxygen (C/O) ratio. We focus on whether these effects are detectable using data from the Hubble Space Telescope and the James Webb Space Telescope. Our five targets are HAT-P-12b, HD 209458b, WASP-6b, WASP-17b, and WASP-39b, which have temperatures ranging from 1000K to 1700K and radii ranging from 0.9 to 1.9 Jupiter radius.

We used the TauREx 3.1 atmospheric retrieval framework (Al-Refaie et al. 2019), coupled with FRECKLL (Al-Refaie et al. 2024), a disequilibrium chemistry plugin that includes vertical mixing and photochemistry. FRECKLL computes steady-state chemical abundances under disequilibrium conditions and gives them to TauREx for radiative transfer and retrieval analysis. A Bayesian nested sampling algorithm is used for parameter estimation. The principle is to start from a transmission spectroscopy dataset and to look for the model that characterizes the atmosphere in the most probable way, in other words, the spectrum that best fits the data points. For very hot planets, thermochemical equilibrium may be close to reality, but for less hot planets, vertical mixing and photodissociation bring these planets out of equilibrium. This work is a follow-up study of Panek et al. (2023) and is complementary of Bardet et al. (2025) (in review), a study that was mainly looking at the same effect on emission spectra.

We will present one of the very rare retrieval analysis taking into accounts disequilibrium chemistry and we will evaluate how this new sophisticated method improve the analysis of observations. The detectability of these effects strongly depends on spectral resolution and wavelength coverage. We expect to see the biggest improvement with JWST data, thanks to its better spectral coverage. This study demonstrates the feasibility and importance of incorporating disequilibrium chemistry into retrieval models. Doing so can significantly improve our interpretation of exoplanetary atmospheres and help refine models of planet formation and evolution. These findings also highlight the need to revisit previous retrieval studies that used only equilibrium models, especially for cooler planets where disequilibrium chemistry is expected to play a major role.