A gas giant orbiting a star 320 light-years from Earth carries the same ratio of rock-forming minerals in its atmosphere as its host star, and that single measurement just validated a working assumption planetary scientists have been quietly relying on for decades without ever being able to prove it.
Findings published in Nature Communications in February 2026 quantify the magnesium-to-silicon ratio in the atmosphere of WASP-189b, an ultra-hot Jupiter in the constellation Libra, and place it within one standard deviation of the ratio measured in the planet’s own host star.
To understand why that matters, you have to understand the problem scientists face every time they try to model the inside of a planet they cannot visit. When astronomers find a rocky exoplanet, an Earth-sized world orbiting a distant star, they want to know what it is made of. Is it mostly iron? Does it have a thick silica mantle? Is its geology active enough to drive plate tectonics and release gases that chemistry needs to sustain life? None of those questions can be answered by just measuring the planet’s size and mass, which are the only two things most current instruments can directly clock for small worlds that far away. So scientists fall back on a shortcut. They measure the star. Stars and the planets around them are born from the same original cloud of gas and dust, and the assumption has always been that the mineral ratios in that cloud are preserved in both. Measure the star’s magnesium and silicon content, and you have an estimate of what proportion of those same elements ended up baked into the rocky planets nearby. That assumption has been built into virtually every model of rocky exoplanet interiors produced over the past decade. Until WASP-189b, nobody had ever checked it against an actual planet outside our solar system.
WASP-189b is not a rocky planet. It is a gas giant, roughly twice the mass of Jupiter, and it orbits its star so tightly that its dayside atmosphere reaches 3,354 Kelvin, a temperature roughly three times the melting point of steel. That extreme heat is precisely what makes it useful as a measuring tool. At those temperatures, elements that everywhere else in the universe stay locked inside solid rock, specifically magnesium, silicon, and iron, boil off into gas. Once they are in gas form, they become detectable using a technique called spectroscopy. Spectroscopy works by splitting light into its individual wavelengths, the same way a prism breaks sunlight into a rainbow, except each chemical element absorbs and emits light at a unique, fixed set of wavelengths. Read those wavelengths from the light a planet radiates and you can identify exactly what chemicals are floating in its atmosphere, and in what quantities. WASP-189b runs hot enough to keep all three rock-forming elements in gas form simultaneously, which is the only condition under which all three can be read at once from a single planet.
The instrument that recorded those readings is the Immersion Grating Infrared Spectrometer, or IGRINS, which was mounted on the Gemini South telescope in the Atacama region of Chile at the time of the observations. IGRINS scans a near-continuous stretch of infrared light from 1.4 to 2.5 micrometres, a range wide enough to catch the chemical fingerprints of multiple elements in a single sweep. The team ran two separate observation sessions, one on 7 May 2022 and one on 2 April 2023, collecting 155 exposure pairs on the first night and 84 usable pairs on the second, each individual exposure lasting 28 seconds. As the planet moved through its orbit during each session, its spectral fingerprints shifted in wavelength in a predictable pattern driven by the planet’s own motion, and the team used that shifting pattern to pull the planet’s signal apart from the light of its star and from interference introduced by Earth’s own atmosphere sitting above the telescope.
Six separate chemicals registered clearly enough to count as confirmed detections. Iron came in at a signal-to-noise ratio of 8.51, meaning the signal was more than eight times stronger than the background noise, a reading so clean it leaves no statistical room for doubt. Carbon monoxide registered at 6.22, silicon at 5.91, hydroxyl at 5.79, magnesium at 4.71, and water at 4.36, all comfortably above the formal detection threshold of 4. The water reading is weaker than expected for a planet with that atmospheric chemistry, and the reason is straightforward. At 3,354 Kelvin, water molecules get torn apart by heat before they can accumulate in large quantities. The hydroxyl detected at 5.79 sigma is actually a fragment of those shattered water molecules, confirming that water was present but constantly being destroyed faster than it could build up.
From those confirmed detections, the team calculated the ratios between the three rock-forming elements. Magnesium and silicon were present in almost exactly the same proportion to each other as they are in the host star, landing within one standard deviation of the stellar measurement. Magnesium-to-iron and silicon-to-iron both tracked the star’s values within one to two standard deviations. Taken together, those three ratios produced a Mg:Si:Fe proportion of approximately 1.2 to 1.0 to 0.7 in the planet’s atmosphere. For context, that mixture sits between the mineral composition of two types of primitive rocky material that fell to Earth from space during the early solar system, providing a direct chemical link between a planet 320 light-years away and the same geological processes that shaped our own neighbourhood.
There is one number from the same measurement that does not follow the stellar pattern, and it tells a separate story about how WASP-189b formed. While the ratios between magnesium, silicon, and iron precisely matched the star, the total quantity of all heavy rock-forming elements combined, measured against the planet’s carbon and oxygen content, came in at roughly 2.4 times the stellar level. The planet holds more rocky material overall than its star contains proportionally, even though it holds that material in perfectly preserved internal ratios. The working explanation is that during its early formation, the planet swept up and absorbed a large quantity of rocky planetesimals, which are the small solid bodies that eventually clump together to form planets, adding bulk without scrambling the relative balance of individual minerals.
Every future model of a rocky exoplanet’s interior, its core size, its mantle chemistry, its capacity for the kind of geological activity that on Earth drives volcanism, magnetic field generation, and the cycling of carbon through oceans and atmosphere, now rests on observational ground it did not stand on before. The IGRINS instrument has since returned to its home institution, but its successor, IGRINS-2, is now permanently installed on the Gemini North telescope on Maunakea in Hawaii. Further high-resolution spectroscopy campaigns targeting WASP-189b and comparable ultra-hot Jupiters are currently active, with wider wavelength coverage expected to add more detectable elements per target and tighten the precision on ratios already recorded.
Primary Source
Sanchez, J. A., Smith, P. C. B., Kanumalla, K., Welbanks, L., Line, M. R., Pelletier, S., Desch, S., Young, P., Patience, J., Bean, J., Brogi, M., Jaffe, D., Mace, G. N., Mansfield, M. W., Panwar, V., Parmentier, V., Pino, L., Savel, A. B., Van Sluijs, L., & Wardenier, J. P. (2026). A stellar magnesium-to-silicon ratio in the atmosphere of an exoplanet. Nature Communications, 17, 2902. https://doi.org/10.1038/s41467-026-69610-x
Press Release Source
NSF NOIRLab. (2026, April 1). Gemini South confirms long-suspected link between the composition of exoplanets and their host stars: New observations provide the first direct evidence that exoplanets inherit rocky-element ratios from their host stars. NOIRLab Science Release. https://noirlab.edu
