Magnesium Silicate Clouds in the Atmosphere of HD 209458b from a Rule-Based Tree-Structured Data Reduction
Abstract
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 MIRI/LRS transmission observations of HD 209458b from 5-12 microns, directly measuring the absorption signatures of its clouds for the first time. The observations indicate the presence of magnesium silicates, most likely Mg2SiO4 or a mixture of Mg2SiO4 and MgSiO3. 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 Mg2SiO4 clouds explain the LRS data to high significance over either a clear (Delta ln(Z)=16.63) or gray cloud atmosphere (Delta ln(Z)=22.26). By combining the LRS dataset with archival JWST NIRCam and HST 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 microns and atmospheric pressures of the clouds of roughly 1-10 millibar. 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 formation conditions of these planets.
Versions
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BibTeX
@misc{chubb2026magnesium,
title = {{Magnesium Silicate Clouds in the Atmosphere of HD 209458b from a Rule-Based Tree-Structured Data Reduction}},
author = {Katy L. Chubb and David Grant and Hannah R. Wakeford and Sarah E. Moran and Natasha E. Batalha and Arika Egan and Charlotte Fairman and Diana Powell and Kevin B. Stevenson and Lili Alderson and Peter Gao and Tiffany Kataria and Nikole K. Lewis and Ryan J. MacDonald and Mark Marley Elijah Mullens and David K. Sing and Jeff A. Valenti},
howpublished = {arXiv},
year = {2026},
month = jun,
doi = {10.48550/arxiv.2606.00177},
eprint = {2606.00177},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.00177},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Magnesium Silicate Clouds in the Atmosphere of HD 209458b from a Rule-Based Tree-Structured Data Reduction
AU - Katy L. Chubb
AU - David Grant
AU - Hannah R. Wakeford
AU - Sarah E. Moran
AU - Natasha E. Batalha
AU - Arika Egan
AU - Charlotte Fairman
AU - Diana Powell
AU - Kevin B. Stevenson
AU - Lili Alderson
AU - Peter Gao
AU - Tiffany Kataria
AU - Nikole K. Lewis
AU - Ryan J. MacDonald
AU - Mark Marley Elijah Mullens
AU - David K. Sing
AU - Jeff A. Valenti
PY - 2026
DA - 2026/06/01
PB - arXiv
DO - 10.48550/arxiv.2606.00177
UR - https://arxiv.org/abs/2606.00177
AB - 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 MIRI/LRS transmission observations of HD 209458b from 5-12 microns, directly measuring the absorption signatures of its clouds for the first time. The observations indicate the presence of magnesium silicates, most likely Mg2SiO4 or a mixture of Mg2SiO4 and MgSiO3. 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 Mg2SiO4 clouds explain the LRS data to high significance over either a clear (Delta ln(Z)=16.63) or gray cloud atmosphere (Delta ln(Z)=22.26). By combining the LRS dataset with archival JWST NIRCam and HST 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 microns and atmospheric pressures of the clouds of roughly 1-10 millibar. 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 formation conditions of these planets.
ER -