Using the Immersion Grating Infrared Spectrometer (IGRINS) on the Gemini South telescope at the International Gemini Observatory, astronomers have directly measured the atmospheric composition of WASP-189b and found it echoes the elemental makeup of its host star, offering the clearest evidence yet that planets inherit their chemical identities from the disks that formed them.
An artist’s impression of an ultrahot Jupiter. Image credit: Sci.News.
WASP-189 is a 730-million-year-old A-type star located 322 light-years away in the constellation of Libra.
Also known as HD 133112, the star is larger and more than 2,000 degrees Celsius hotter than the Sun.
First discovered in 2018, WASP-189b is a transiting gas giant about 1.6 times the radius of Jupiter.
The planet sits around 20 times closer to the star than Earth does to the Sun, and completes a full orbit in just 2.7 days.
“Ultra-hot Jupiters have temperatures high enough to vaporize rock-forming elements like magnesium, silicon, and iron, offering a rare opportunity to see these elements using spectroscopy — the technique of breaking up light into its component wavelengths to identify the presence of chemicals,” said Arizona State University graduate student Jorge Antonio Sanchez and his colleagues.
Using the IGRINS instrument, the astronomers obtained high-resolution thermal emission spectra of WASP-189b.
They detected neutral iron, magnesium, silicon, water, carbon monoxide, and hydroxyl in the exoplanet’s atmosphere.
“The IGRINS data reveal that WASP-189b shares the same magnesium-to-silicon ratio as its host star,” they said.
“This finding provides the first observational evidence of a widely adopted assumption about planet formation, and opens a new route to understand how exoplanets form and evolve.”
Hot giant planets like WASP-189b are thought to have an outer layer of gas that has a chemical composition influenced by the disk of material in which they formed, known as protoplanetary disks.
And researchers assume that the ratio of rock-forming elements in a protoplanetary disk matches that of the host star, since the two were born from the same primordial cloud of material.
This inferred chemical link between a star and the planets that form around it is commonly used to model the composition of rocky exoplanets.
This link was previously based on measurements within our Solar System, and it had not been directly observed on planets elsewhere, until now.
“WASP-189b gives us a much-needed observational anchor in our understanding of terrestrial planet formation since it offers a measurable quantity that validates the presumed resemblance of stellar composition and the proportion of rocky material around host stars used to form planets,” Sanchez said.
“Our study demonstrates the capability of ground-based, high-resolution spectrographs to constrain critical species like magnesium and silicon, which are two elemental building blocks from which rocky planets form,” added Dr. Michael Line, an astronomer at Arizona State University.
“This advancing capability opens an entirely new dimension in our study of exoplanet atmospheres.”
A paper on the findings was published on February 18, 2026 in the journal Nature Communications.
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J.A. Sanchez et al. 2026. A Stellar magnesium to silicon ratio in the atmosphere of an exoplanet. Nat Commun 17, 2902; doi: 10.1038/s41467-026-69610-x
