Magnesium Silicate Clouds in the Atmosphere of HD 209458b from a Rule-based Tree-structured Data Reduction

HD 209458b is the canonical hot Jupiter: the first to have its atmosphere measured and the first to hint at the role of aerosols in exoplanet atmospheres through the muting of Na absorption signatures in the optical. Here, we present JWST Mid-InfraRed Instrument (MIRI) Low Resolution Spectrograph (LRS) transmission observations of HD 209458b from 5 to 12 μ m, directly measuring the absorption signatures of its clouds for the first time. The observations indicate the presence of magnesium silicates, most likely Mg 2 SiO 4 or a mixture of Mg 2 SiO 4 +MgSiO 3 . We also present a new methodology to reduce observational data, whereby the analysis is formulated as a rule-based model with a tree structure, enabling key decisions to be identified and uncertain decisions to be incorporated into subsequent modeling. With this data reduction, and using a combination of ARCiS free retrievals and PICASO+Virga self-consistent forward models, we are able to show that amorphous Mg 2 SiO 4 clouds explain the LRS data to high significance over either a clear (Δ ln ( Z ) = 16.63) or gray cloud atmosphere (Δ ln ( Z ) = 22.26). By combining the LRS dataset with archival JWST NIRCam and Hubble Space Telescope optical and near-infrared observations, we are able to more robustly constrain the properties of the magnesium silicate condensates, finding particle sizes of approximately 0.1 μ m and atmospheric pressures of the clouds of roughly 1–10 mbar. Our results add to the growing detections of silicate clouds as a dominant atmospheric component of hot Jupiters, with the exact silicate species contextualizing the atmospheric chemistry and potentially the formation conditions of these planets.