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

TL;DR

This study presents the first near-UV transmission spectrum of an ultra-hot Jupiter, revealing silicon monoxide and refractory species, suggesting silicate cloud formation begins at temperatures around 1950-2450 K.

Contribution

It provides observational evidence of SiO in an ultra-hot Jupiter, indicating the onset of silicate cloud formation at higher temperatures than previously thought.

Findings

Detection of significant near-UV absorption in WASP-178b

Identification of gaseous silicon monoxide (SiO) in the atmosphere

Reinterpretation of cloud formation temperatures in hot exoplanets

Abstract

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.