A Stellar magnesium to silicon ratio in the atmosphere of an exoplanet
- Jorge A. Sanchez
- Peter C. B. Smith
- Krishna Kanumalla
- Luis Welbanks
- Michael R. Line
- Stefan Pelletier
- Steven Desch
- Patrick Young
- Jennifer Patience
- Jacob Bean
- Matteo Brogi
- Dan Jaffe
- Gregory N. Mace
- Megan Weiner Mansfield
- Vatsal Panwar
- Vivien Parmentier
- Lorenzo Pino
- Arjun Baliga Savel
- Lennart van Sluijs
- Joost P. Wardenier
2026-02-18
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 ( $${T}_{eq}=335{4}_{-34}^{+27}$$ T e q = 335 4 − 34 + 27 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 2 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.