Early Insights for Atmospheric Retrievals of Exoplanets Using JWST Transit Spectroscopy

We have entered the era of the James Webb Space Telescope (JWST). We use the first JWST transmission spectrum of the hot Saturn-mass exoplanet, WASP-39 b, obtained with the NIRSpec instrument in the 3-5 μm range to investigate (a) what atmospheric constraints are possible with JWST-quality data in this spectral range, (b) requirements for atmospheric models used in retrievals, (c) effect of differences between data reduction pipelines on retrieved atmospheric properties, and (d) complementarity between JWST data in the 3-5 μm range and Hubble Space Telescope (HST) observations at shorter wavelengths. JWST spectra in the 3-5 μm range provide a promising avenue for chemical detections while encompassing a window in cloud opacity for several prominent aerosols. We confirm recent inferences of CO₂, SO₂, H₂O, and CO in WASP-39 b; report tentative evidence for H₂S; and retrieve elemental abundances consistent with Saturn's metallicity. We retrieve molecular abundances with ~0.3-0.6 dex precision with this relatively limited spectral range. When considering the 3-5 μm data alone, reported differences in spectra with different reduction pipelines can affect abundance estimates by up to ~1 dex and the detectability of less prominent species. Complementing with data at shorter wavelengths, e.g., with other JWST instruments or HST WFC3 (~0.8-1.7 μm), can significantly improve the accuracy and precision of the abundance estimates. The high data quality enables constraints on aerosol properties, including their composition, modal size, and extent, motivating their consideration in retrievals. Our results highlight the promise of JWST exoplanet spectroscopy, while underscoring the importance of robust data reduction and atmospheric retrieval approaches in the JWST era.