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Journal Articles Nature Year : 2022

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


Aerosols have been found to be nearly ubiquitous in substellar atmospheres 1,2,3. 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 4. However, some giant planets are thought to be hot enough to avoid condensation altogether 5,6. Here, we present the near-UV transmission spectrum of an ultra-hot Jupiter, WASP-178b (∼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. We observed one transit of the ultra-hot Jupiter WASP-178b/KELT-26b 7,8 (WASP-178b hereafter) with HST/WFC3/UVIS using the G280 grism (0.2-0.8 µm, R∼70) on September 5th, 2020 as part of Program 16086 (PI: Lothringer). WASP-178b is unique among known exoplanets for its especially hot host star: at A1 IV-V and T eff =9360 K, WASP-178 is second-only to KELT-9 as the hottest planet-hosting star. Timeseries spectra were obtained over 7.5 hours centered around the transit event, and were used to extract the transmission spectra which probes the middle and upper atmosphere around the day-night terminator. Further details regarding the data reduction and observational setup are given in the Methods.
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hal-03848956 , version 1 (11-11-2022)


  • HAL Id : hal-03848956 , version 1


Joshua D Lothringer, David K Sing, Zafar Rustamkulov, Hannah R Wakeford, Kevin B Stevenson, et al.. UV Absorption by Silicate Cloud Precursors in Ultra-hot Jupiter WASP-178b. Nature, 2022. ⟨hal-03848956⟩


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