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 (Teq = 3354^{+27}_{-34} 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_2O), 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.

💡 Research Summary

This paper presents the first high‑resolution thermal‑emission spectroscopic measurement of refractory element ratios in the atmosphere of an ultra‑hot Jupiter, WASP‑189b (equilibrium temperature ≈ 3350 K). Using the Immersion Grating Infrared Spectrometer (IGRINS) on Gemini South (R ≈ 45,000, λ = 1.4–2.5 µm), the authors obtained two observing runs (May 2022 and April 2023) that together yielded 239 individual spectra covering pre‑ and post‑secondary‑eclipse phases. After standard data reduction and detrending to remove telluric and stellar lines, they applied a cross‑correlation technique, shifting a forward model spectrum over a grid of line‑of‑sight velocities (Kp, ΔVsys) to locate the planet’s Doppler‑shifted signal. Distinct trails in velocity‑phase space were recovered for both volatile (H₂O, CO, OH) and refractory (Fe I, Mg I, Si I, Ti I, Ca I, V I) species. Individual detections reached high significance: Fe I (8.5 σ), CO (6.2 σ), Si I (5.9 σ), OH (5.8 σ), Mg I (4.7 σ) and H₂O (4.4 σ).

To translate these detections into elemental abundances, the authors built a thermochemical‑equilibrium atmospheric forward model that predicts molecular and atomic mixing ratios given a set of elemental ratios and a parameterized temperature–pressure (T–P) profile. They coupled this model to an affine‑invariant ensemble MCMC sampler (emcee) and performed a Bayesian retrieval on the full data set, simultaneously fitting for the elemental abundance ratios (