2025MNRAS.540.2535A

The discovery of hot Jupiters that orbit very close to their host stars has long challenged traditional models of planetary formation and migration. Characterizing their atmospheric composition - mainly in the form of the carbon-to-oxygen (C/O) ratio and metallicity - can provide insights into their formation locations and evolution pathways. With JWST, we can characterize the atmospheres of these types of planets more precisely than previously possible, primarily because it allows us to determine both their atmospheric oxygen and carbon composition. Here, we present a JWST NIRSpec (Near-Infrared Spectrograph)/G395H transmission spectrum from 2.8 to 5.1 $\mu {\rm m}$ of WASP-94 A b, an inflated hot Jupiter with a retrograde misaligned orbit around its F-type host star. We find a relatively cloud-free atmosphere, with absorption features of ${\rm H}_{2}{\rm O}$ and ${\rm CO}_{2}$ at detection significances of ${\sim} 4\sigma$ and ${\sim} 11\sigma$, respectively. In addition, we detect tentative evidence of ${\rm CO}$ absorption at ${\sim} 3\sigma$, as well as hints of sulphur with the detection of ${\rm H}_{2}{\rm S}$ at a ${\sim} 2.5\sigma$ confidence level. Our favoured equilibrium chemistry model determines a C/O ratio of $0.49^{+0.08}_{-0.13}$ for WASP-94 A b's atmosphere, which is substellar compared to the star's C/O ratio of $0.68 \pm 0.10$. The retrieved atmospheric metallicity is similar to the star's metallicity, as both are ${\sim} 2\times$ solar. We find that this substellar C/O ratio and stellar metallicity can be best explained by pebble accretion or planetesimal accretion in combination with large-distance migration of the planet.