Tracing the formation and migration history: molecular signatures in the atmosphere of misaligned hot Jupiter WASP-94 A b using JWST NIRSpec/G395H

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 μm 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 H2O and CO2 at detection significances of ∼4σ and ∼11σ, respectively. In addition, we detect tentative evidence of CO absorption at ∼3σ, as well as hints of sulphur with the detection of H2S at a ∼2.5σ 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 ± 0.10. The retrieved atmospheric metallicity is similar to the star’s metallicity, as both are ∼2× 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.