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

Suman Saha, James S. Jenkins

TL;DR

The paper tackles how to characterize WASP-178b, an extreme ultra-hot Jupiter, using JWST NIRSpec/G395H transmission spectra. It advances a robust two-pipeline reanalysis with independent data reduction, GP-detrended lightcurve modeling, and comprehensive atmospheric retrievals (line-by-line opacities, Guillot and spline PT profiles, free vs equilibrium chemistry, cloud parameters). Key findings include secure detections of CO and CO₂, a super-solar C/O ratio of $0.954 \pm 0.033$, and a high atmospheric metallicity of $11.44_{-6.94}^{+12.54}$ × solar, with no definitive H₂O or cloud signatures. These results reinforce a growing trend of elevated C/O in UHJs and provide a benchmark for future atmospheric-evolution models and population-level studies of extreme exoplanets, highlighting WASP-178b’s formation and migration history as a testbed for planetary science models.

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 $σ$) and CO$_2$ (7.22 $σ$), along with marginal evidence for C$_2$H$_2$ (1.34 $σ$), 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.

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

TL;DR

The paper tackles how to characterize WASP-178b, an extreme ultra-hot Jupiter, using JWST NIRSpec/G395H transmission spectra. It advances a robust two-pipeline reanalysis with independent data reduction, GP-detrended lightcurve modeling, and comprehensive atmospheric retrievals (line-by-line opacities, Guillot and spline PT profiles, free vs equilibrium chemistry, cloud parameters). Key findings include secure detections of CO and CO₂, a super-solar C/O ratio of , and a high atmospheric metallicity of × solar, with no definitive H₂O or cloud signatures. These results reinforce a growing trend of elevated C/O in UHJs and provide a benchmark for future atmospheric-evolution models and population-level studies of extreme exoplanets, highlighting WASP-178b’s formation and migration history as a testbed for planetary science models.

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 ) and CO (7.22 ), along with marginal evidence for CH (1.34 ), but no clear evidence for HO, suggesting depletion. From these retrieved abundances, we constrain the C/O ratio to a precise super-solar value of 0.9540.033, consistent with an emerging trend in other UHJs. We also infer a very high atmospheric metallicity for a Jupiter-sized gas giant11.44 solarindicating 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.

Paper Structure

This paper contains 9 sections, 1 equation, 13 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. Observational data
  4. Data reduction and analyses
  5. Eureka! reduction
  6. exoTEDRF reduction
  7. Lightcurve modeling
  8. Atmospheric retrieval analyses
  9. Results and Discussion

Figures (13)

  • Figure 1: Planetary radius versus equilibrium temperature for known hot and ultra-hot Jupiters. Extreme ultra-hot Jupiters (EUHJs) are highlighted as colored stars within the blue dashed ellipse, highlighting a distinct population. KELT-9b is marked as a brown triangle. Dashed black lines indicate approximate boundaries separating EUHJs from the broader population.
  • Figure 2: Observed transmission spectra of WASP-178b from NIRSpec/G395H, extracted with two independent reduction pipelines—Eureka! (orange) and exoTEDRF (green). The lower panel shows their difference in units of Eureka! incertainties. The mean absolute difference is $\sim$1$\sigma$, indicating excellent statistical agreement.
  • Figure 3: Observed transmission spectrum of WASP-178b (yellow, Eureka!) with the best-fit model from free chemistry retrieval. Residuals are shown in units of observational uncertainties. Additional retrievals excluding CO, CO$_2$, and C$_2$H$_2$ are overplotted, which were used to assess the statistical significance of their detection.
  • Figure 4: Pressure-temperature (PT) profiles from the equilibrium and free-chemistry retrievals using Guillot (top), dtdp6 (bottom left), and dtdp10 (bottom right) parametrization, together with the retrieved median abundances of CO, CO$_2$, and C$_2$H$_2$.
  • Figure 5: Currently known and well-constrained C/O ratios for several exoplanets plotted against their equilibrium temperatures, including the estimate for WASP-178b from this work (magenta bowtie). Literature measurements based on JWST observations are shown as circles, while others are shown as squares. The distribution of C/O ratios is modeled as a quadratic function of equilibrium temperature, with the dashed blue line indicating the mean trend and the shaded area the 1$\sigma$ confidence interval.
  • ...and 8 more figures