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 (\({T}_{eq}=335{4}_{-34}^{+27}\) 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 (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.
Data availability
This work is based on observations made with the Gemini South Telescope. The raw data products are available within the Gemini Observatory Archive [https://archive.gemini.edu/searchform] under program IDs GS-2021B-Q-113 and GS-2023A-Q-231. The data generated and analyzed in this study, including reduced data cubes, model spectra, files used to generate cross correlation maps, and the main retrieval outputs have been deposited in following Zenodo repository via https://doi.org/10.5281/zenodo.18462071. Stellar abundances quoted in this analysis are from refs. 18, 41 and 68. Solar abundances quoted in this analysis are from40. Source data are provided with this paper.
Code availability
The CHIMERA code used in this analysis is based upon that presented in ref. 21, and can be obtained as described in ref. 21. The data products used in the analysis were reduced using the cubify package: https://zenodo.org/records/14194202. The code also made use of the publicly available fastchem tool59: https://github.com/NewStrangeWorlds/FastChem. This analysis also made use of the nested-sampling package pymultinest: https://johannesbuchner.github.io/PyMultiNest/, joblib loop parallelization package: https://joblib.readthedocs.io/en/latest/, and corner.py: https://corner.readthedocs.io/en/latest/.
References
Guillot, T. et al. Giant Planets from the Inside-Out. In Inutsuka, S., Aikawa, Y., Muto, T., Tomida, K. & Tamura, M. (eds.) Protostars and Planets VII, vol. 534 of Astronomical Society of the Pacific Conference Series, 947 (2023).
Venturini, J. & Helled, R. Jupiter’s heavy-element enrichment expected from formation models. Astron. Astrophys. (AA) 634, A31 (2020).
Madhusudhan, N. Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects. ARAA 57, 617–663 (2019).
Öberg, K. I., Murray-Clay, R. & Bergin, E. A. The Effects of Snowlines on C/O in Planetary Atmospheres. ApJL 743, L16 (2011).
Madhusudhan, N. C/O Ratio as a Dimension for Characterizing Exoplanetary Atmospheres. Astrophysical J. 758, 36 (2012).
Cridland, A. J., Pudritz, R. E. & Alessi, M. Composition of early planetary atmospheres - I. Connecting disc astrochemistry to the formation of planetary atmospheres. MNRAS 461, 3274–3295 (2016).
Mordasini, C., van Boekel, R., Mollière, P., Henning, T. & Benneke, B. The Imprint of Exoplanet Formation History on Observable Present-day Spectra of Hot Jupiters. Astrophysical J. 832, 41 (2016).
Kempton, E. M. R. & Knutson, H. A. Transiting Exoplanet Atmospheres in the Era of JWST. Rev. Mineral. Geochem. 90, 411–464 (2024).
Lichtenberg, T. & Krijt, S. System-level Fractionation of Carbon from Disk and Planetesimal Processing. ApJL 913, L20 (2021).
Lothringer, J. D. et al. A New Window into Planet Formation and Migration: Refractory-to-Volatile Elemental Ratios in Ultra-hot Jupiters. Astrophysical J. 914, 12 (2021).
Chachan, Y., Knutson, H. A., Lothringer, J. & Blake, G. A. Breaking Degeneracies in Formation Histories by Measuring Refractory Content in Gas Giants. Astrophysical J. 943, 112 (2023).
Thiabaud, A., Marboeuf, U., Alibert, Y., Leya, I. & Mezger, K. Elemental ratios in stars vs planets. AA 580, A30 (2015).
Pelletier, S. et al. CRIRES+ and ESPRESSO Reveal an Atmosphere Enriched in Volatiles Relative to Refractories on the Ultrahot Jupiter WASP-121b. AJ 169, 10 (2025).
Smith, P. C. B. et al. The Roasting Marshmallows Program with IGRINS on Gemini South. II. WASP-121 b has Superstellar C/O and Refractory-to-volatile Ratios. Astronomical J. 168, 293 (2024).
Dorn, C., Hinkel, N. R. & Venturini, J. Bayesian analysis of interiors of HD 219134b, Kepler-10b, Kepler-93b, CoRoT-7b, 55 Cnc e, and HD 97658b using stellar abundance proxies. Astron. Astrophys. (AA) 597, A38 (2017).
Unterborn, C. T. & Panero, W. R. The Effects of Mg/Si on the Exoplanetary Refractory Oxygen Budget. Astrophysical J. 845, 61 (2017).
Spaargaren, R. J., Ballmer, M. D., Bower, D. J., Dorn, C. & Tackley, P. J. The influence of bulk composition on the long-term interior-atmosphere evolution of terrestrial exoplanets. Astron. Astrophys. (AA) 643, A44 (2020).
Lendl, M. et al. The hot dayside and asymmetric transit of WASP-189 b seen by CHEOPS. Astron. Astrophys. (AA) 643, A94 (2020).
Deline, A. et al. The atmosphere and architecture of WASP-189 b probed by its CHEOPS phase curve. Astron. Astrophys. (AA) 659, A74 (2022).
Park, C. et al. Design and early performance of IGRINS (Immersion Grating Infrared Spectrometer). In Ramsay, S. K., McLean, I. S. & Takami, H. (eds.) Ground-based and Airborne Instrumentation for Astronomy V, vol. 9147 of Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 91471D (2014).
Line, M. R. et al. A solar c/o and sub-solar metallicity in a hot jupiter atmosphere. Nature 598, 580–584 (2021).
Brogi, M. et al. The Roasting Marshmallows Program with IGRINS on Gemini South I: Composition and Climate of the Ultrahot Jupiter WASP-18 b. AJ 165, 91 (2023).
Snellen, I. A. G., de Kok, R. J., de Mooij, E. J. W. & Albrecht, S. The orbital motion, absolute mass and high-altitude winds of exoplanet HD209458b. Nature 465, 1049–1051 (2010).
Birkby, J. L. Exoplanet Atmospheres at High Spectral Resolution. arXiv e-prints arXiv:1806.04617 (2018). 1806.04617.
Smith, P. C. B. et al. A Combined Ground-based and JWST Atmospheric Retrieval Analysis: Both IGRINS and NIRSpec Agree that the Atmosphere of WASP-77A b Is Metal-poor. Astronomical J. 167, 110 (2024).
Ramkumar, S., Gibson, N. P., Nugroho, S. K., Maguire, C. & Fortune, M. High-resolution emission spectroscopy retrievals of MASCARA-1b with CRIRES+: strong detections of CO, H2O, and Fe emission lines and a C/O consistent with solar. MNRAS 525, 2985–3005 (2023).
Cont, D. et al. Silicon in the dayside atmospheres of two ultra-hot Jupiters. AA 657, L2 (2022).
Prinoth, B. et al. Titanium oxide and chemical inhomogeneity in the atmosphere of the exoplanet WASP-189 b. Nat. Astron. 6, 449–457 (2022).
Gandhi, S. et al. Retrieval Survey of Metals in Six Ultrahot Jupiters: Trends in Chemistry, Rain-out, Ionization, and Atmospheric Dynamics. Astronomical J. 165, 242 (2023).
Yan, F. et al. Detection of CO emission lines in the dayside atmospheres of WASP-33b and WASP-189b with GIANO. AA 661, L6 (2022).
Lesjak, F. et al. Retrieving wind properties from the ultra-hot dayside of WASP-189b with CRIRES+. arXiv e-prints arXiv:2411.19662 (2024). 2411.19662.
van Sluijs, L. et al. First Results from WINERED: Detection of Emission Lines from Neutral Iron and a Combined Set of Trace Species on the Dayside of WASP-189 b. Astronomical J. 170, 217 (2025).
Brogi, M. & Line, M. R. Retrieving temperatures and abundances of exoplanet atmospheres with high-resolution cross-correlation spectroscopy. Astronomical J. 157, 114 (2019).
Gibson, N. P. et al. Detection of Fe I in the atmosphere of the ultra-hot Jupiter WASP-121b, and a new likelihood-based approach for Doppler-resolved spectroscopy. MNRAS 493, 2215–2228 (2020).
Foreman-Mackey, D., Hogg, D. W., Lang, D. & Goodman, J. emcee: the mcmc hammer. Publ. Astronomical Soc. Pac. 125, 306 (2013).
Stock, J. W., Kitzmann, D., Patzer, A. B. C. & Sedlmayr, E. FastChem: A computer program for efficient complex chemical equilibrium calculations in the neutral/ionized gas phase with applications to stellar and planetary atmospheres. MNRAS 479, 865–874 (2018).
Kopparapu, R. K., Kasting, J. F. & Zahnle, K. J. A Photochemical Model for the Carbon-rich Planet WASP-12b. ApJ 745, 77 (2012).
Moses, J. I., Madhusudhan, N., Visscher, C. & Freedman, R. S. Chemical Consequences of the C/O Ratio on Hot Jupiters: Examples from WASP-12b, CoRoT-2b, XO-1b, and HD 189733b. ApJ 763, 25 (2013).
Lothringer, J. D., Barman, T. & Koskinen, T. Extremely Irradiated Hot Jupiters: Non-Oxide Inversions, H- Opacity, and Thermal Dissociation of Molecules. ArXiv e-prints: 1805.00038 (2018). 1805.00038.
Asplund, M., Amarsi, A. M. & Grevesse, N. The chemical make-up of the Sun: A 2020 vision. Astron. Astrophys. (AA) 653, A141 (2021).
Saffe, C. et al. Chemical analysis of early-type stars with planets. Astron. Astrophys. (AA) 647, A49 (2021).
Pelletier, S. et al. Vanadium oxide and a sharp onset of cold-trapping on a giant exoplanet. Nature 619, 491–494 (2023).
Dorn, C. et al. Can we constrain the interior structure of rocky exoplanets from mass and radius measurements? Astron. Astrophys. (AA) 577, A83 (2015).
Brewer, J. M. & Fischer, D. A. C/O and Mg/Si Ratios of Stars in the Solar Neighborhood. ApJ 831, 20 (2016).
Zuckerman, B. & Young, E. Characterizing the chemistry of planetary materials around white dwarf stars. In Handbook of Exoplanets, 1–22 (Springer, 2017).
Xu, S. et al. Compositions of Planetary Debris around Dusty White Dwarfs. AJ 158, 242 (2019).
Johnson, T. M. et al. Unusual Abundances from Planetary System Material Polluting the White Dwarf G238-44. ApJ 941, 113 (2022).
Rogers, L. K. et al. Seven white dwarfs with circumstellar gas discs II: tracing the composition of exoplanetary building blocks. MNRAS 532, 3866–3880 (2024).
Bonsor, A. et al. Host-star and exoplanet compositions: a pilot study using a wide binary with a polluted white dwarf. MNRAS 503, 1877–1883 (2021).
Lodders, K. Solar System Abundances and Condensation Temperatures of the Elements. ApJ 591, 1220–1247 (2003).
Adibekyan, V. et al. A compositional link between rocky exoplanets and their host stars. Science 374, 330–332 (2021).
Mace, G. et al. IGRINS at the Discovery Channel Telescope and Gemini South. In Evans, C. J., Simard, L. & Takami, H. (eds.) Ground-based and Airborne Instrumentation for Astronomy VII, vol. 10702 of Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 107020Q (2018).
Lee, J.-J. & Gullikson, K. Plp: V2.1 Alpha 3 (2016).
Kanumalla, K. et al. IGRINS Observations of WASP-127 b: H2O, CO, and Super-solar Atmospheric Metallicity in the Inflated Sub-Saturn. AJ 168, 201 (2024).
de Kok, R. J. et al. Detection of carbon monoxide in the high-resolution day-side spectrum of the exoplanet HD 189733b. Astron. Astrophys. (AA) 554, A82 (2013).
Gibson, N. P., Nugroho, S. K., Lothringer, J., Maguire, C. & Sing, D. K. Relative abundance constraints from high-resolution optical transmission spectroscopy of WASP-121b, and a fast model-filtering technique for accelerating retrievals. MNRAS 512, 4618–4638 (2022).
Fortney, J. J., Lodders, K., Marley, M. S. & Freedman, R. S. A Unified Theory for the Atmospheres of the Hot and Very Hot Jupiters: Two Classes of Irradiated Atmospheres. ApJ 678, 1419–1435 (2008).
Arcangeli, J. et al. Climate of an ultra hot Jupiter. Spectroscopic phase curve of WASP-18b with HST/WFC3. Astron. Astrophys. (AA) 625, A136 (2019).
Stock, J. W., Kitzmann, D. & Patzer, A. B. C. FASTCHEM 2: an improved computer program to determine the gas-phase chemical equilibrium composition for arbitrary element distributions. MNRAS 517, 4070–4080 (2022).
Borsato, N. W. et al. The Mantis Network. III. Expanding the limits of chemical searches within ultra-hot Jupiters: New detections of Ca I, V I, Ti I, Cr I, Ni I, Sr II, Ba II, and Tb II in KELT-9 b. Astron. Astrophys. (AA) 673, A158 (2023).
Prinoth, B. et al. An atlas of resolved spectral features in the transmission spectrum of WASP-189 b with MAROON-X. Astron. Astrophys. (AA) 685, A60 (2024).
Bell, T. J. et al. Nightside clouds and disequilibrium chemistry on the hot Jupiter WASP-43b. Nat. Astron. 8, 879–898 (2024).
Grimm, S. L. et al. HELIOS-K 2.0 Opacity Calculator and Open-source Opacity Database for Exoplanetary Atmospheres. ApJS 253, 30 (2021).
Husser, T. O. et al. A new extensive library of PHOENIX stellar atmospheres and synthetic spectra. AA 553, A6 (2013).
Cont, D. et al. Atmospheric characterization of the ultra-hot Jupiter WASP-33b. Detection of Ti and V emission lines and retrieval of a broadened line profile. Astron. Astrophys. (AA) 668, A53 (2022).
Anderson, D. R. et al. WASP-189b: an ultra-hot Jupiter transiting the bright A star HR 5599 in a polar orbit. arXiv e-prints arXiv:1809.04897 (2018). 1809.04897.
Yan, F. et al. A temperature inversion with atomic iron in the ultra-hot dayside atmosphere of WASP-189b. Astron. Astrophys. 640, L5 (2020).
Lam, M. B., Hoeijmakers, H. J., Prinoth, B. & Thorsbro, B. Secrets in the shadow: High precision stellar abundances of fast-rotating A-type exoplanet host stars through transit spectroscopy. AA 691, A141 (2024).
Bergin, E. A., Blake, G. A., Ciesla, F., Hirschmann, M. M. & Li, J. Tracing the ingredients for a habitable earth from interstellar space through planet formation. Proc. Natl. Acad. Sci. 112, 8965–8970 (2015).
Allègre, C. J., Poirier, J.-P., Humler, E. & Hofmann, A. W. The chemical composition of the Earth. Earth Planet. Sci. Lett. 134, 515–526 (1995).
Acknowledgements
J.A.S., M.R.L., S.K.K., and J.L.B. acknowledge support from NSF grant AST-2307177/8. P.C.B.S. acknowledges support provided by NASA through the NASA FINESST grant 80NSSC22K1598. L.W. and M.W.M. acknowledge support through the 51 Pegasi b Fellowship awarded by the Heising-Simons Foundation. The results reported herein benefited from collaborations and/or information exchange within NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate, grant 80NSSC23K1356, PI Steve Desch. This material is based upon work supported by the National Science Foundation under grant MSIP-1836008. We acknowledge Research Computing at Arizona State University for providing high-performance computing and storage resources that have significantly contributed to the research results reported within this manuscript. This work used the Immersion Grating Infrared Spectrometer (IGRINS) that was developed under a collaboration between the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI) with the financial support of the Mount Cuba Astronomical Foundation, of the US National Science Foundation under grants AST-1229522 and AST-1702267, of the McDonald Observatory of the University of Texas at Austin, of the Korean GMT Project of KASI, and Gemini Observatory. This program is based on observations obtained at the international Gemini Observatory, a program of NSF’s NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation on behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovaç μes e Comunicaç μes (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). J.A.S. and M.R.L. would especially like to thank the anonymous queue observers who successfully completed the observing programs this work is based on. We would like to thank Monika Lendl for useful discussion on stellar abundance analyses.
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J.A.S. performed the analysis, wrote the manuscript, and prepared one of the proposals that resulted in the post-eclipse dataset. P.C.B.S. developed code, contributed text to the manuscript, provided scientific guidance, and reviewed the paper. K.K. performed additional retrieval analyses and integrated the FastChem code into the CHIMERA retrieval pipeline. L.W. provided scientific input, guided J.A.S. throughout the writing process, and advised on manuscript focus. M.R.L. conceived the paper, wrote the original Gemini L.L.P. proposal from which the pre-eclipse dataset originates, edited and revised the manuscript, and supervised J.A.S. throughout the project. S.P. provided code and spectral comparisons that helped identify a bug in the atomic opacities. S.D., P.Y., and J.P. provided comments on multiple drafts and scientific guidance on planetary formation and composition. J.B., M.B., D.J., and G.N.M. contributed to the development and operation of IGRINS and its associated data pipeline. M.W.M. and A.B.S. contributed to data analysis, retrieval tests, and manuscript comments. V.P., L.P., L.v.S., and J.P.W. provided theoretical context on atmospheric chemistry and manuscript feedback. All authors read and approved the final version of the manuscript and contributed to the discussion of results.
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Sanchez, J.A., Smith, P.C.B., Kanumalla, K. et al. A Stellar magnesium to silicon ratio in the atmosphere of an exoplanet. Nat Commun (2026). https://doi.org/10.1038/s41467-026-69610-x
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DOI: https://doi.org/10.1038/s41467-026-69610-x
