Ultra-Hot Jupiter WASP-189b Mirrors Its Star’s Chemistry, Astronomers Find

Using the Infrared Immersion Grating Spectrometer (IGRINS) on the International Gemini Observatory’s Gemini South Telescope, astronomers directly measured the atmospheric composition of WASP-189b and found that it echoed the elemental composition of its host star, providing the clearest evidence to date that planets inherit their chemical identities from the disks that formed them.

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 Libra.

Also known as HD 133112, the star is larger and 2,000 degrees Celsius hotter than the Sun.

First discovered in 2018, WASP-189b is a transiting gas giant approximately 1.6 times the radius of Jupiter.

The planet is located about 20 times closer to the star than Earth is to the Sun and completes a complete orbit in just 2.7 days.

“Ultra-hot Jupiters have temperatures high enough to vaporize rock-forming elements like magnesium, silicon and iron, providing a rare opportunity to see these elements using spectroscopy – the technique of splitting light into the wavelengths of its components to identify the presence of chemicals,” said Arizona State University graduate student Jorge Antonio Sanchez and colleagues.

Using the IGRINS instrument, 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 discovery provides the first observational evidence for a widely adopted hypothesis about planet formation and opens a new avenue for understanding how exoplanets form and evolve.”

Hot giant planets like WASP-189b are thought to have an outer layer of gas whose chemical composition is influenced by the disk of material in which they formed, called protoplanetary disks.

And researchers assume that the ratio of rock elements in a protoplanetary disk matches that of the host star, since both were born from the same primordial cloud of matter.

This inferred chemical bond between a star and the planets forming around it is commonly used to model the composition of rocky exoplanets.

This link was previously based on measurements within our solar system, and until now it had not been observed directly on other planets.

“WASP-189b gives us a much-needed observational anchor in our understanding of terrestrial planet formation, because 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 ability of high-resolution ground-based spectrographs to constrain critical species like magnesium and silicon, which are two building blocks from which rocky planets form,” added Dr. Michael Line, an astronomer at Arizona State University.

“This advanced capability opens a whole new dimension in our study of exoplanet atmospheres.”

An article on the results was published on February 18, 2026 in the journal Natural communications.

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JA Sánchez and others. 2026. A stellar magnesium/silicon ratio in the atmosphere of an exoplanet. Nat Common 17, 2902; doi: 10.1038/s41467-026-69610-x