Panchromatic JWST Spectrum Constrains KELT-7b Metallicity at 92 Times Solar

The team observed two transits with NIRISS SOSS and one with NIRSpec G140H, extracting spectra at the group level to fit ramps post-extraction and recover accurate depths between 1 and 1.5 microns. They paired these with re-reduced HST WFC3 G280 data and performed free retrievals, equilibrium chemistry retrievals, and self-consistent grids. The study design comprises three transit observations of a single target; the principal limitation is residual wavelength-dependent systematics that could still bias absolute abundances.

Retrievals consistently require M/H = 92 +24/-23 times solar and C/O ≤0.9, with strong detections of H2O, CO2, and TiO but none for H- or clouds. This points to late-stage metal accretion during formation rather than the lower metallicities predicted by standard core-accretion models for hot Jupiters. GCMs reproduce the lack of limb asymmetry via efficient superrotating jets.

Prior HST-only analyses inferred H- opacity that disappears once the charge-migration bias is removed, illustrating how incomplete wavelength coverage produces spurious species. The panchromatic baseline now supplies the first direct test of formation pathways for ultra-hot Jupiters around young, metal-rich stars.

Future MIRI observations targeting 5-10 microns will test whether the high metallicity produces detectable SO2 or SiO, tightening C/O constraints and enabling demographic comparisons across the ultra-hot Jupiter population.