UV absorption by silicate cloud precursors in ultra-hot Jupiter WASP-178b

Abstract

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.

Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph. II. Emergence and Sedimentation of Silicate Clouds in L Dwarfs, and Analysis of the Full M5--T9 Field Dwarf Spectroscopic Sample

“…The polynomial relationship between effective temperature and spectral type in Filippazzo et al (2015, Table 10) has an r.m.s. scatter of 113 K. A ≈2000 K onset temperature for silicate absorption is consistent with the recent finding that silicate condensation must begin between equilibrium temperatures 1950 K < 𝑇 eq < 2450 K in exoplanetary atmospheres (Lothringer et al 2022). Specifically, Lothringer et al (2022) detect the gas-phase SiO constituent of silicate molecules (MgSiO 3 , Mg 2 SiO 4 ) in the UV transmission spectra of 𝑇 eq = 2350 K and 𝑇 eq = 2450 K transiting planets, but do not see it in the UV transmission spectrum of a cooler 𝑇 eq = 1950 K transiting planet.…”

Patchy nightside clouds on ultra-hot Jupiters: General Circulation Model simulations with radiatively active cloud tracers

“…Our finding that the western limb is cloudy to low pressures and the eastern limb is cloud free further implies strong limb-to-limb contrasts in the cloud coverage of ultra-hot Jupiters, as expected from previous microphysical modeling (Powell et al 2019, Helling et al 2021. However, as found in the case of WASP-178b (Lothringer et al 2022), we predict that SiO vapor can persist in the gas phase and produce strong transmission spectral features in the NUV. We included a case with a significantly enhanced visible opacity in our suite of models in order to mimic the potential radiative impact of absorption of incident starlight by atomic metals in the atmospheres of ultrahot Jupiters (Lothringer et al 2018).…”

“…We expect that the transition point between cloudy and cloud-free limbs as probed in transmission will occur within the ultra-hot Jupiter regime. From an emerging sample of NUV spectra of ultra-hot Jupiters, there is tentative evidence that such a transition occurs between equilibrium temperatures of 1950 − 2450 K (Lothringer et al 2022). For WASP-121b (T eq ≈ 2350 K), Mikal- Evans et al (2022) found sufficiently cold nightside temperatures from HST/WFC3 phase curve observations for cloud condensation, while previous transmission spectra of WASP-121b point toward a cloud-free limb -implying that atmospheric motions keep condensate clouds aloft in order to prevent rain-out to deeper atmospheric levels.…”

Atmospheric characterization of the ultra-hot Jupiter WASP-33b

“…For instance, neutral or ionic species of H, Li, O, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Sr, and Y have been identified in the atmospheres of several UHJs, including KELT-9b (Yan & Henning 2018;Hoeijmakers et al 2018Hoeijmakers et al , 2019Wyttenbach et al 2020;Borsa et al 2021a), WASP-33b (Yan et al 2019(Yan et al , 2021Nugroho et al 2020a;Borsa et al 2021b;Cont et al 2021Cont et al , 2022Herman et al 2022), WASP-189b (Yan et al 2020;Stangret et al 2022;Prinoth et al 2022), WASP-76b (Seidel et al 2019;Ehrenreich et al 2020;Tabernero et al 2021;Deibert et al 2021;Casasayas-Barris et al 2021;Kesseli et al 2022), and KELT-20b/MASCARA-2b (Casasayas-Barris et al 2018, 2019Stangret et al 2020;Nugroho et al 2020b;Hoeijmakers et al 2020a;Rainer et al 2021;Yan et al 2022b;Borsa et al 2022). Also, the spectral signatures of an increasing number of molecular species such as CO, HCN, H 2 O, OH, SiO, or TiO have been identified in UHJ atmospheres (e.g., Nugroho et al 2021;Cont et al 2021;Landman et al 2021;Fu et al 2022;Prinoth et al 2022;Sánchez-López et al 2022;Yan et al 2022a;van Sluijs et al 2022;Lothringer et al 2022). Detecting the spectral lines of different species not only...…”