Rugheimer, SarahWang, Dingding2025-11-112025-11-112025-05-062025-11-11https://hdl.handle.net/10315/43218Ultra-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.Author owns copyright, except where explicitly noted. Please contact the author directly with licensing requests.AstronomyAstrophysicsElectronic Thesis or Dissertation2025-11-11ExoplanetExoplanet atmospheresUltra-hot JupitersTransmission spectroscopyNear-ultraviolet spectroscopyHubble Space Telescope