Gemini South Confirms Long-Suspected Link Between the Composition of Exoplanets and Their Host Stars
Almost 320 light-years away in the Libra constellation lies WASP-189b, an exoplanet known as an ultra-hot Jupiter (UHJ). UHJs have temperatures high enough to vaporize rock-forming elements like magnesium (Mg), silicon (Si), and iron (Fe), 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.
An international team of astronomers led by Jorge Antonio Sanchez, a graduate student at Arizona State University (ASU) observed WASP-189b using the high-resolution Immersion GRating INfrared Spectrograph (IGRINS) when it was mounted on the Gemini South telescope in Chile [1]. This powerful instrument allowed them to simultaneously measure the magnesium and silicon content of the exoplanet’s atmosphere. This is the first time such a measurement has been made, and the data reveal that WASP-189b shares the same magnesium-to-silicon ratio as its host star. 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.
“These discoveries show Gemini’s ability to help us understand the characteristics of the remarkable zoo of exoplanets in our solar neighborhood,” says Chris Davis, NSF Program Director for NOIRLab. “Such discoveries are only possible because of Gemini’s cutting-edge instruments.”
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.
The detected gas for each map is indicated in the upper left of the panel. If that gas is present, a peak occurs near the expected values for the planet’s radial velocity semi-amplitude (KP) and the offset from the star-planet system velocity (dVsys)–indicated by the white dot-dashed lines. The S/N for each detection is indicated in each panel. — Nature Communications
“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,” says Sanchez.
This assumption is not only useful for understanding planet formation, but it is also foundational to the field of astrobiology, which includes the study of habitable environments in the Solar System. By measuring the chemical composition of a star, scientists can infer the abundances of rock-forming elements in the star’s exoplanets, which can dictate the geochemical conditions that make a planet habitable. For instance, the rock-forming elements in Earth are in-part responsible for our protective magnetic field, plate tectonics, and driving the release of life-sustaining chemicals into our atmosphere, oceans, and soil.
As the exoplanet field looks towards the characterization of terrestrial planets, and seeks to elucidate the habitable conditions of rocky worlds, empirical evidence validating the relationship between stellar and planetary compositions represents a fundamental step forward. And the level of spectral resolution necessary for these types of studies is currently only available on ground-based telescopes.
“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,” says study co-author Michael Line, Associate Professor at ASU. “This advancing capability opens an entirely new dimension in our study of exoplanet atmospheres.”
Further multi-wavelength, high-resolution observations to study exoplanet atmospheres like that of WASP-189b will help reveal the larger chemical inventory that exists within distant worlds. Such studies will enable deeper insights into the conditions that govern planetary origins, evolution, and potential habitability.
Notes
[1] IGRINS was a visiting instrument at Gemini South from 2022 to 2023. It has since left the telescope to return to its home institution. The instrument was so successful that a new iteration of it — IGRINS-2 — was commissioned as a facility instrument for the Gemini North telescope in Hawai‘i.
This research was presented in a paper titled “A stellar magnesium-to-silicon ratio in the atmosphere of an exoplanet” to appear in Nature Communications. DOI: 10.1038/s41467-026-69610-x (open access)
Astrobiology, exoplanet,
