Brown Dwarfs

Bridging the Gap: Using Brown Dwarfs to Examine Silicate Clouds in Giant Exoplanet Atmospheres

By Keith Cowing
Status Report
astro-ph.EP
March 17, 2026
Filed under , , , , , , , , , ,
Bridging the Gap: Using Brown Dwarfs to Examine Silicate Clouds in Giant Exoplanet Atmospheres
Mg/Si ratios of JWST-observed hot Jupiter host stars versus separation (semi-major axis). Mg/Si ratios compiled from Brewer et al. (2016) with an error of 3.3% as quoted in Brewer & Fischer (2016) as well as Hejazi et al. (2023). Diamond markers indicate systems in which the hot Jupiter companion is reported to have spectral signatures of SiO2(s) clouds. Square markers indicate no current robust detection for a particular silicate species. The purple dashed line indicates the Mg/Si ratio point that distinguishes the onset of either forsterite (>0.9) or quartz (<0.9) condensates (Calamari et al. 2024). The navy dashed line indicates the theoretical gas-phase Mg/Si ratio in a solar-type disk from Thiabaud et al. (2015). -- astro-ph.EP

We present results from examining the silicate cloud modeling of four JWST-observed hot Jupiters in the context of brown dwarf theory to further explore signatures of formation in present-day atmospheres.

We contextualize our understanding of protoplanetary disk refractory chemistry with empirical evidence from chondritic meteorites to show that giant planets forming and accreting in the outer disk adopt their stellar Mg/Si value.

We show that current silicate cloud species determinations of WASP-17 b, WASP-107 b, WASP-39 b and HD 189733 b are in line with predictions laid out in Calamari et al. 2024 based on each system’s host star Mg/Si ratio, further supporting this hypothesis. We discuss physical motivations for potential atmospheric scenarios where apparent silicate cloud species is not in agreement with that predicted by its host star chemistry.

Additionally, we compare current transit spectroscopy for three of these four exoplanets against brown dwarf spectra to examine molecular absorption trends across the substellar mass temperature regime.

Emily Calamari, Jacqueline K. Faherty, Channon Visscher, Marina E. Gemma, Austin Rothermich, Francisco Ardévol Martínez, Sherelyn Alejandro Merchan, Genaro Suárez

Comments: 18 pages, 1 table, 6 figures. Accepted for publication in ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2603.12479 [astro-ph.EP] (or arXiv:2603.12479v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2603.12479
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Related DOI:
https://doi.org/10.3847/2041-8213/ae497d
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Submission history
From: Emily Calamari
[v1] Thu, 12 Mar 2026 21:58:03 UTC (2,653 KB)
https://arxiv.org/abs/2603.12479
Astrobiology, Astrochemistry,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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