A Stellar magnesium to silicon ratio in the atmosphere of an exoplanet
The elemental compositions of exoplanets encode information about their formation environments and internal structures. While volatile ratios such as carbon-to-oxygen (C/O) are used to trace formation location, the rock-forming elements-magnesium (Mg), silicon (Si), and iron (Fe)-govern interior mineralogy and are commonly assumed to reflect the host star's abundances. Yet this assumption remains largely untested. Ultra-hot Jupiters, gas-giant exoplanets with dayside temperatures above 3000 K, provide rare access to refractory elements that remain gaseous. Here we present high-resolution thermal emission spectroscopy of the exoplanet WASP-189b ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mi>e</mml:mi> <mml:mi>q</mml:mi></mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>335</mml:mn> <mml:msubsup><mml:mrow><mml:mn>4</mml:mn></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>34</mml:mn></mml:mrow> <mml:mrow><mml:mo>+</mml:mo> <mml:mn>27</mml:mn></mml:mrow> </mml:msubsup> </mml:math> K) obtained with the Immersion Grating Infrared Spectrometer (IGRINS) on Gemini South. We detect neutral iron (Fe I), magnesium (Mg I), silicon (Si I), water (H<sub>2</sub>O), carbon monoxide (CO), and hydroxyl (OH) at signal-to-noise ratios exceeding 4, and retrieve their elemental abundances. We show that the Mg/Si, Fe/Mg, and Si/Fe ratios are consistent with stellar values, while the refractory-to-volatile ratio is enhanced by roughly a factor of 2. These findings demonstrate that giant-planet atmospheres can preserve stellar-like rock-forming ratios, providing an empirical validation of the stellar-proxy assumption that underpins planetary composition and formation models across exoplanet systems.
Authors
- Peter Smith (ORCID: https://orcid.org/0000-0002-9946-5259)
- Gregory Mace
- P.H. Young
- Stefan Pelletier (ORCID: https://orcid.org/0000-0002-8573-805X)
- Vatsal Panwar (ORCID: https://orcid.org/0000-0002-2513-4465)
- Vivien Parmentier (ORCID: https://orcid.org/0000-0001-9521-6258)
- Megan Mansfield (ORCID: https://orcid.org/0000-0003-4241-7413)
- Lennart van Sluijs (ORCID: https://orcid.org/0000-0002-9610-3367)
- Joost P. Wardenier (ORCID: https://orcid.org/0000-0003-3191-2486)
- Jorge A Sánchez (ORCID: https://orcid.org/0000-0002-6519-4147)
- Luis Welbanks
- A. B. Savel
- Dan Jaffe
- Matteo Brogi
- Jacob L. Bean
- Michael R. Line
- Krishna Kanumalla
- Jennifer Patience
- Steven Desch
Institutions
- University of Geneva (CH)
- University of Bern (CH)
- Centre National de la Recherche Scientifique (FR)
- Université Côte d'Azur (FR)
- University of Michigan (US)
- Arcetri Astrophysical Observatory (IT)
- University of Warwick (GB)
- University of Chicago (US)
- Osservatorio Astrofisico di Torino (IT)
- Observatoire de la Côte d’Azur (FR)
- University of Turin (IT)
- Arizona State University (US)
- University of Maryland, College Park (US)
- Université de Montréal (CA)
- University of Birmingham (GB)
- The University of Texas at Austin (US)
Publication Details
- Journal
- Nature Communications
- Published
- 2026-02-18
- DOI
- https://doi.org/10.1038/s41467-026-69610-x
- Primary Topic
- Stellar, planetary, and galactic studies
- Type
- article
- Field-Weighted Citation Impact
- 0.00
Funders
- National Science Foundation
- National Aeronautics and Space Administration
- Arizona State University
- University of Texas at Austin
- Ministry of Science, ICT and Future Planning
- Nuclear Safety and Security Commission
- Korea Astronomy and Space Science Institute
- Agencia Nacional de Investigación y Desarrollo
- Science Mission Directorate