# A Stellar magnesium to silicon ratio in the atmosphere of an exoplanet

## TL;DR

This study finds that the magnesium to silicon ratio in the atmosphere of the exoplanet WASP-189b matches that of its host star, supporting the idea that planetary compositions reflect their formation environment.

## Contribution

The study empirically validates the assumption that exoplanet refractory element ratios mirror their host star's.

## Key findings

- Mg/Si, Fe/Mg, and Si/Fe ratios in WASP-189b's atmosphere are consistent with stellar values.
- The refractory-to-volatile ratio in the exoplanet's atmosphere is enhanced by a factor of 2.
- Detection of Fe i, Mg i, Si i, H2O, CO, and OH in the atmosphere confirms the presence of refractory and volatile species.

## Abstract

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 (\documentclass[12pt]{minimal}
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\begin{document}$${T}_{eq}=335{4}_{-34}^{+27}$$\end{document}Teq=3354−34+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 (H2O), 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.

Spectroscopic observations of the ultra-hot Jupiter WASP-189b reveal both volatile (H2O, CO, OH) and refractory (Fe, Mg, Si) gas in its atmosphere. Here, the authors show that the abundance ratio of refractory species reflects that of the host star.