Pyrat Bay 2.0: an Upgraded Framework for Exoplanet Atmosphere Modeling in the JWST Era

This article presents a major update to the open-source Pyrat Bay modeling framework (version 2.0), tailored for the characterization of exoplanet atmospheres with the observational capabilities of current facilities, such as the James Webb Space Telescope (JWST), and future missions. The upgraded framework introduces a standalone chemistry package, chemcat, enabling the modeling of exoplanet atmospheres via thermochemical-equilibrium calculations with custom compositions, self-consistent radiative-equilibrium thermal profiles, and equilibrium-chemistry retrievals. Other key implementations include nested sampling via MultiNest, isotopic-ratio fitting, transit light source modeling, and free-chemistry parameterization allowing vertical variations in volume mixing ratios. We validated the thermochemical- and radiative-equilibrium modules via comparisons with the GGchem, FastChem, and HELIOS codes. We present an application of the new modeling framework by conducting an atmospheric characterization study using simulated transmission spectra of high signal-to-noise observations for a gas-giant exoplanet. We found that vertical abundance variations in planetary atmospheres can be accurately recovered using data of JWST quality. In such cases, retrievals that neglect these variations may result in biased abundance constraints, e.g., when employing the commonly used free-chemistry approach assuming constant abundance profiles. The results highlight the potential of JWST to study the multi-dimensional nature of exoplanetary atmospheres and their underlying physical processes.