Göttingen 2025 – scientific programme

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EP: Fachverband Extraterrestrische Physik

EP 9: Exoplanets and Astrobiology

EP 9.3: Talk

Wednesday, April 2, 2025, 17:00–17:15, ZHG005

WASP-121 b’s transmission spectrum observed with JWST/NIRSpec G395H reveals thermal dissociation and SiO in the atmosphere — •Cyril Gapp for the WASP-121 b JWST/NIRSpec transit collaboration — Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

WASP-121 b has been established as a benchmark Ultra-Hot Jupiter, serving as a laboratory for the atmospheric chemistry and dynamics of strongly irradiated extrasolar gas giants. Here, we present and analyze WASP-121 b’s transmission spectrum observed with NIRSpec G395H onboard the James Webb Space Telescope (JWST) and find evidence for the thermal dissociation of H₂O and H₂ on the planet’s permanent day side. Additionally, we detect SiO at a statistical significance of 5.2 σ. Constraining the abundance of SiO and abundance ratios between Silicon and volatile atoms in WASP-121 b’s atmosphere could help discriminate between possible migration histories of the planet. The three-dimensional nature of thermal dissociation on WASP-121 b’s day side and recombination on its night side, however, poses a challenge to constrain molecular abundances and elemental abundance ratios from the transmission spectrum. To account for this, we implemented an atmospheric model in the NEMESIS framework that splits the planet’s atmosphere into day side and night side. A retrieval applying our atmospheric model to WASP-121 b’s transmission spectrum favors a higher H₂O abundance on the night side than on the day side, demonstrating the impact of hemispheric heterogeneity when attempting to constrain WASP-121 b’s bulk H₂O inventory.

Keywords: Hot Jupiter; Transmission spectroscopy; JWST; Infrared spectroscopy; Exoplanet atmospheric composition