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Aerosols have been found to be nearly ubiquitous in substellar atmospheres.The precise temperature at which these aerosols begin to form in exoplanets hasyet to be observationally constrained. Theoretical models and observations ofmuted spectral features suggest that silicate clouds play an important role inexoplanets between at least 950 and 2,100 K. However, some giant planets arethought to be hot enough to avoid condensation altogether. Here, we present thenear-UV transmission spectrum of an ultra-hot Jupiter, WASP-178b($\sim$2,450~K), that exhibits significant NUV absorption. Thisshort-wavelength absorption is among the largest spectral features everobserved in an exoplanet in terms of atmospheric scale heights. Bayesianretrievals indicate the presence of gaseous refractory species containingsilicon and magnesium, which are the precursors to condensate clouds at lowertemperatures. SiO in particular has not been detected in exoplanets before, butthe presence of SiO in WASP-178b is consistent with theoretical expectation asthe dominant Si-bearing species at high temperatures. These observations allowus to re-interpret previous observations of HAT-P-41b and WASP-121b that didnot consider SiO to suggest that silicate cloud formation begins on exoplanetswith equilibrium temperatures between 1,950 and 2,450~K.

UV Absorption by Silicate Cloud Precursors in Ultra-hot Jupiter WASP-178b