  Aerosols have been found to be nearly ubiquitous in substellar atmospheres.
The precise temperature at which these aerosols begin to form in exoplanets has
yet to be observationally constrained. Theoretical models and observations of
muted spectral features suggest that silicate clouds play an important role in
exoplanets between at least 950 and 2,100 K. However, some giant planets are
thought to be hot enough to avoid condensation altogether. Here, we present the
near-UV transmission spectrum of an ultra-hot Jupiter, WASP-178b
($\sim$2,450~K), that exhibits significant NUV absorption. This
short-wavelength absorption is among the largest spectral features ever
observed in an exoplanet in terms of atmospheric scale heights. Bayesian
retrievals indicate the presence of gaseous refractory species containing
silicon and magnesium, which are the precursors to condensate clouds at lower
temperatures. SiO in particular has not been detected in exoplanets before, but
the presence of SiO in WASP-178b is consistent with theoretical expectation as
the dominant Si-bearing species at high temperatures. These observations allow
us to re-interpret previous observations of HAT-P-41b and WASP-121b that did
not consider SiO to suggest that silicate cloud formation begins on exoplanets
with equilibrium temperatures between 1,950 and 2,450~K.

Earth and Planetary Astrophysics