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.

Figures (15)

• Figure 1: White lightcurves created from the first two spectral orders of the NIRISS SOSS data and combined best-fit model. The top panels show the observed data and fitted model lightcurves, while the bottom panels show the standardized residuals. The standardized residuals are the residuals divided by the uncertainty $\sigma_\Phi$ that combines the uncertainties in flux reported by the pipeline $\sigma_t$ with the additional jitter term $\sigma_\omega$ fitted by the juliet framework ($\sigma_\Phi = \sqrt{\sigma_t^2 + \sigma_\omega^2}$). The outer bottom panels compare the distribution of the standardized residuals to the shape expected for Gaussian noise. Shaded regions indicate the time intervals used for baseline normalization (gray) and a suspected spot crossing event (yellow).
• Figure 2: Visualization of the HAT-P-12b transmission spectrum and the underlying opacity contributions. Top: Combined spectra from the three JWST instruments and HST STIS Alexoudi_2020_RoleImpactParameter with $1\sigma$ uncertainties, together with the best-fit model with its $3\sigma$ credible interval. Model spectra including only the opacity of individual contributors (chemical species or non-gray cloud parametrization) are also shown. Bottom: Absorption cross sections $\sigma$ of all tested chemical species; significantly detected species are emphasized with thicker lines.
• Figure 3: Venn diagrams of JWST instrument combinations showing detection significances of the four significantly detected molecules (top) and the preference for two modeling assumptions (bottom). Additional circles were used to show the significances obtained when replacing the JWST NIRISS SOSS with the HST WFC3 instrument data. The significances are reported as log-evidence differences $\Delta \ln{Z}$ and visualized using a custom non-linear color scale. The colorbar also provides a conversion to $n_\sigma$ detection significances following the calibration scale from Benneke_2013_HowDistinguishCloudy.
• Figure 4: Marginal posterior distributions of the chemical abundances for all included molecules, retrieved using different JWST instrument combinations. The shown distributions were derived from the samples via kernel density estimation (KDE; Gaussian kernel with bandwidth $h=0.1$). For each distribution, markers indicate either the $1\sigma$ credible interval, in cases of well-constrained posteriors, or the 95th percentile upper limit (arrows) when only an upper bound can be established.
• Figure 5: Left: Excerpt from corner plot focusing on parameters of the cloud parametrization. The joint posterior distributions (off-diagonal panels) are only shown for the retrieval using the data from all JWST instruments and are visualized using contour lines smoothed with a Gaussian kernel ($\sigma=1$). The marginal posterior distributions (diagonal panels) are additionally shown for several exemplary JWST instrument combinations, as well as the results from the retrievals under the inclusion of the two different HST STIS reductions. The same KDE as in Figure \ref{['fig:marg_abunds_jwst']} was applied. Upper Right: Comparison of $T$--$P$ profiles derived for more complex $N$-point and simpler isothermal parametrization for the case of the three JWST instrument combination. Lines indicate the median value, while shaded regions are used to highlight the [0.5, 1,2,3] $\sigma$ credible intervals.