Information content of JWST transmission spectroscopy of the exoplanet HAT-P-12b from the optical to the mid-infrared

Abstract

The James Webb Space Telescope (JWST) provides low- to medium-resolution spectra with unprecedented precision and broad near- to mid-infrared wavelength coverage, enabling detailed characterization of exoplanet atmospheres. We present a new JWST NIRISS SOSS transit observation of the warm sub-Saturn HAT-P-12b. Combined with NIRSpec G395M and MIRI LRS data, this enables an assessment of the information content across JWST instruments over the full accessible wavelength range. The NIRISS data were reduced and the impact of reduction choices on the transmission spectrum evaluated. Atmospheric retrievals were performed for all JWST combinations, with selected cases including archival HST data. Four molecules are significantly detected: H2O, CO2, CO, and H2S. Except for H2O, detections require NIRSpec coverage, while H2S is only detected in multi-instrument retrievals. NIRISS SOSS is essential to establish robust evidence for non-gray cloud behavior. A moderate scattering slope (p < 4) is consistently retrieved. Single-instrument retrievals tend to overestimate abundances, whereas combined JWST datasets yield more consistent constraints. The C/O ratio remains sensitive to differences between NIRSpec reductions. Results broadly agree with studies of WASP-39b, but highlight variations in information content across exoplanet types.