A Metal-Rich Atmosphere with a Super-Solar C/O Ratio for the Extreme Ultra-Hot Jupiter WASP-178b

Abstract

The population of ultra-hot Jupiters (UHJs) provide unique opportunities to probe the extreme formation and evolutionary pathways in exoplanets. Owing to their very high temperatures and inflated atmospheres, UHJs are among the most favorable targets for both transmission and emission spectroscopy, enabling detailed characterization of their atmospheric properties. Here, we present a reanalysis of the JWST NIRSpec/G395H transmission spectra of the extreme ultra-hot Jupiter (EUHJ) WASP-178b, aimed at precisely characterizing its atmospheric composition. Our approach combines data reduction using two independent pipelines, lightcurve modeling with robust detrending techniques, and rigorous atmospheric retrievals. We report statistically significant detections of CO (7.24 $\sigma$) and CO$_2$ (7.22 $\sigma$), along with marginal evidence for C$_2$H$_2$ (1.34 $\sigma$), but no clear evidence for H$_2$O, suggesting depletion. From these retrieved abundances, we constrain the C/O ratio to a precise super-solar value of 0.954$\pm$0.033, consistent with an emerging trend in other UHJs. We also infer a very high atmospheric metallicity for a Jupiter-sized gas giant$\unicode{x2014}$11.44$_{-6.94}^{+12.54}$ $\times$solar$\unicode{x2014}$indicating unique atmospheric evolutions. These findings provide a critical benchmark for an extreme exoplanet atmosphere, offering a testbed for developing next-generation atmospheric evolution models and enabling comparative population-level studies across the UHJ population.