4 May 2026
Almost 320 light-years away in the Libra constellation lies WASP-189b, an exoplanet known as an ultra-hot Jupiter. Such planets 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.
Using the high-resolution Immersion GRating INfrared Spectrograph (IGRINS) when it was mounted on the Gemini South telescope in Chile, astronomers were able to simultaneously quantify the magnesium and silicon content of the exoplanet’s atmosphere. This is the first time such a measurement has been made. And with it, astronomers have found that WASP-189b shares the same magnesium-to-silicon ratio as its host star, providing the first observational evidence of a widely adopted assumption about planet formation.
“IGRINS is one of the most efficient instruments in use today,” said Gregory Mace, a research scientist at McDonald Observatory, IGRINS’s home institution, and co-author on the recent study about WASP-189b. “The broad range of spectra IGRINS can view, from 1.45 to 2.5 microns, means that it observes many molecular features all at once.”
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 have long assumed 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 but 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,” said Jorge Antonio Sanchez, a graduate student at Arizona State University who led the study on WASP-189b.
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.
“A molten core is very important to life on Earth because it causes surface change as plate tectonics and volcanoes, it heats the surface from within and it drives the Earth's magnetic field. Without a magnetic field, radiation from the Sun would destroy most life on Earth,” said Mace. “So, that one assumption about the planet composition matching the star is a very important assumption in our search for livable planets.”
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 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,” said study co-author Michael Line, Associate Professor at Arizona State University. “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. Once such observatory, the Giant Magellan Telescope, under construction in Chile, will measure four times the wavelengths as IGRINS (1.08-5.4 microns) with its GMTNIRS instrument. Based on IGRINS and built in collaboration by The University of Texas at Austin and the Korea Astronomy and Space Science Institute, it will allow astronomers to run observations like those on WASP-189b for about 100 more planets. Such studies will enable deeper insights into the conditions that govern planetary origins, evolution, and potential habitability.
IGRINS was also built in collaboration by The University of Texas and the Korea Astronomy and Space Science Institute (KASI). After visiting other observatories, it returned to McDonald Observatory in 2024.
Astronomers discovered that a giant planet, WASP-189b, echoes the composition of its host star, providing the first direct evidence of a foundational concept in astrobiology. This discovery was achieved through the first-ever simultaneous measurement of gaseous magnesium and silicon in a planet’s atmosphere. Credit: NOIRLab.
