Detecting Silicon Monoxide in the Atmosphere of Ultra-Hot Jupiter WASP-178b

Abstract

Ultra-hot Jupiters (UHJs) host unique atmospheric chemistry where refractory species remain gaseous. WASP-178b (Teq ∼ 2470 ± 60 K) is a UHJ that exhibits strong near-ultraviolet absorption (0.2–0.28 μm) previously attributed to silicon monoxide (SiO), though direct detection remained elusive. We present the first spectroscopic detection of gaseous SiO in an exoplanet atmosphere through re-analysis of archival HST/STIS E230M observations. Using an in-band/out-of-band technique with theoretical SiO templates from the ExoMol SiOUVenIR line list, we detect excess absorption of 0.647 ± 0.148% at > 4.3σ significance (p = 7.6 × 10−6). This confirms SiO remains in the gas phase at these extreme temperatures, constraining the silicate cloud condensation threshold. Our detection methodology provides a framework for systematic SiO searches across the UHJ population, enabling comparative studies of silicon chemistry and thermal structure in these extreme atmospheres.