BOWIE-ALIGN: JWST reveals hints of planetesimal accretion and complex sulphur chemistry in the atmosphere of the misaligned hot Jupiter WASP-15b

We present a transmission spectrum of the misaligned hot Jupiter WASP-15b from 2.8–5.2 microns observed with JWST’s NIRSpec/G395H grating. Our high signal-to-noise data, which has negligible red noise, reveals significant absorption by H₂O (4.2σ) and CO₂ (8.9σ). From independent data reduction and atmospheric retrieval approaches, we infer that WASP-15b’s atmospheric metallicity is supersolar ( 15× solar) and its carbon-to-oxygen ratio is consistent with solar, that together imply planetesimal accretion. Our general circulation model simulations for WASP-15b suggest that the carbon-to-oxygen ratio we measure at the limb is likely representative of the entire photosphere due to the mostly uniform spatial distribution of H₂O, CO₂, and CO. We additionally see evidence for absorption by SO₂ and absorption at 4.9 μm, for which the current leading candidate is OCS (carbonyl sulphide), albeit with several caveats. If confirmed, this would be the first detection of OCS in an exoplanet atmosphere and point towards complex photochemistry of sulphur-bearing species in the upper atmosphere. These are the first observations from the BOWIE-ALIGN survey which is using JWST’s NIRSpec/G395H instrument to compare the atmospheric compositions of aligned/low-obliquity and misaligned/high-obliquity hot Jupiters around F stars above the Kraft break. The goal of our survey is to determine whether the atmospheric composition differs across two populations of planets that have likely undergone different migration histories (disc versus disc-free) as evidenced by their obliquities (aligned versus misaligned).