Being one of the first exoplanets observed by the James Webb Space Telescope, WASP-39 b has become an iconic target and many transit spectra recorded with different instruments (NIRISS, NIRCAM, NIRSpec G395H, NIRSpec PRISM and MIRI) are currently available, allowing in-depth studies of its atmosphere. We present here a novel approach to interpret WASP-39 b's transit spectroscopic data, consisting of a multi-step process where ab initio equilibrium chemistry models and blind retrievals are used iteratively to find physically robust, optimal solutions. Following this approach, we have identified a new scenario to explain WASP-39 b's atmospheric composition, in which silicon-based chemistry plays a major role. In this scenario, SiO may explain the spectral absorption at 4.1 μm, currently interpreted as being due to SO2. SiO and the other gas species identified by the retrieval models, i.e. H2O, CO2, Na and K, are consistent with an atmosphere in chemical equilibrium with a temperature-pressure profile constrained by H2O and CO2 absorption bands. In addition, silicate clouds and hazes can produce the spectral features observed by MIRI in the spectral window 5-12 μm. While we advocate the need for more data, possibly at higher spectral resolution, to confirm our results for WASP-39 b's atmospheric composition, we highlight a refined atmospheric retrieval strategy with pre-selection and post-reconstruction to guide the next generation of transit spectroscopy.
@article{arxiv.2504.07823,
title = {A new look into the atmospheric composition of WASP-39 b},
author = {Sushuang Ma and Arianna Saba and Ahmed Faris Al-Refaie and Giovanna Tinetti and Sergei N. Yurchenko and Jonathan Tennyson and Cesare Cecchi Pestellini},
journal= {arXiv preprint arXiv:2504.07823},
year = {2026}
}