The UV Transmission Spectrum of Ultra-hot Jupiter WASP-178b: Strong Absorption by Silicate Cloud Precursors

The extreme atmospheres of ultra-hot Jupiters provide a limiting case for the condensation of gaseous species in sub-stellar atmospheres. At some point, a planet will be so close to their host star that the temperatures in the atmosphere become too high for refractory material to condense. We observed a transit of the new ultra-hot Jupiter WASP-178b, which orbits the second hottest planet-hosting star, with the Hubble Space Telescope’s newly-implement WFC3-UVIS/G280 low-resolution grism (0.2-0.8 microns). We detect transit depths in the ultraviolet that are nearly 20 equilibrium scale-heights above the optical continuum, resulting in one of the largest spectral features ever observed in an exoplanet’s atmosphere. Retrieval analyses show SiO, Mg II, and Fe II are responsible for this absorption, which we confirm to be present on both the morning and evening terminator. By re-interpreting NUV transit spectra of HAT-P-41b and WASP-121b, we show that the condensation of important magnesium and iron silicates likely begins in planets with equilibrium temperatures between 1,950 and 2,450 K. In agreement with theoretical expectations, these observations indicate that silicate clouds are likely responsible for muting the spectra of hot Jupiters.