HAT-P-12b JWST NIRISS SOSS transmission spectrum

Atmospheric retrievals were performed for all combinations of JWST data, supplemented by archival HST observations in select cases. The analysis further included evaluations of molecular detection significances and assumptions about the atmospheric structure. The same four molecules previously reported are significantly detected: H_2_O, CO_2_, CO, and H_2_S.

Except for H_2_O, all require NIRSpec coverage for detection, while H_2_S is only detected in multi-instrument retrievals. Abundance constraints obtained using HST WFC3 instead of JWST NIRISS SOSS are largely consistent, particularly when combining instruments, but NIRISS SOSS proved essential to establishing robust evidence for nongray cloud behavior. A moderately steep scattering slope (p<4) is consistently retrieved across different HST STIS reductions.

Single-instrument retrievals, even when yielding significant detections, tend to overestimate molecular abundances. In contrast, retrievals that combine spectra from multiple JWST instruments generally converge toward consistent abundance constraints. The derived C/O ratio remains sensitive to subtle differences between NIRSpec reductions, owing to the instrument's exclusive coverage of the carbon-bearing molecules CO_2_ and CO, so its interpretation requires caution.

The results are broadly consistent with information content studies of the benchmark target WASP-39b, although differences, including the absence of a preference for non-isothermal T-P profiles, highlight variations in information content across exoplanet types.