A scorching hot planet nearly 320 light-years from Earth has provided the first direct evidence for a fundamental assumption about how planets form. Astronomers discovered that WASP-189b shares an important chemical characteristic with its host star, supporting a long-standing idea that also helps scientists investigate potentially habitable worlds. The breakthrough came from the first simultaneous measurement of gaseous magnesium and silicon in an exoplanet’s atmosphere, using the Gemini South telescope, part of the International Gemini Observatory, which is partially funded by the U.S. National Science Foundation and operated by NSF NOIRLab.
Located nearly 320 light-years away in the Libra constellation, WASP-189b belongs to an extreme category of planets called ultra-hot Jupiters (UHJ). These enormous worlds reach temperatures so intense that even materials normally found in solid rock can turn into gas. Elements such as magnesium (Mg), silicon (Si), and iron (Fe) can therefore be detected in their atmospheres. Astronomers identify these substances through spectroscopy, a method that separates light into different wavelengths to reveal the chemicals present.
An international research team led by Jorge Antonio Sanchez, a graduate student at Arizona State University (ASU), investigated WASP-189b with the high-resolution Immersion GRating INfrared Spectrograph (IGRINS). At the time, the instrument was installed on the Gemini South telescope in Chile.[1]
For the first time, researchers were able to measure magnesium and silicon together in the atmosphere of a planet beyond our Solar System. Their observations revealed that the proportion of magnesium to silicon on WASP-189b closely matches the ratio found in its parent star. This result provides the first direct observational support for a widely used assumption about the chemical relationship between stars and their planets, offering a valuable new way to investigate planetary origins and evolution.
“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.”
A Long-Suspected Connection Between Stars and Planets
Astronomers believe that giant planets such as WASP-189b formed within enormous disks of gas and dust surrounding young stars. These structures, called protoplanetary disks, provide the raw materials from which planets develop. The chemical makeup of a giant planet’s outer atmosphere is thought to retain clues about the environment where it originally formed.
Because stars and their surrounding disks originate from the same cloud of material, scientists have long assumed that they contain similar proportions of rock-forming elements. In particular, the ratios of these elements in a disk are expected to resemble those measured in its central star.
This relationship has become an important tool for predicting the composition of rocky exoplanets. Until now, however, the assumption was supported primarily by evidence from our own Solar System. Astronomers had not directly measured the same relationship in a planet orbiting another star.
The discovery involving WASP-189b provides that missing observational evidence, strengthening the scientific basis for using stellar chemistry to understand the materials from which distant planets form.
“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.
What Planetary Chemistry Could Reveal About Habitability
The implications reach beyond understanding how planets are assembled. This chemical relationship is also important in astrobiology, which investigates the possibility of life beyond Earth and the environmental conditions that could support it, including habitable environments within our Solar System.
By studying the elements present in a star, astronomers may be able to estimate the relative amounts of rock-forming materials available to its planets. These materials influence a planet’s internal structure, geological activity, and chemical environment, all of which can affect whether conditions suitable for life might develop.
Earth provides a familiar example. The elements that make up our planet help determine processes such as plate tectonics, the generation of its protective magnetic field, and the movement of essential chemicals into the atmosphere, oceans, and soil. Together, these processes contribute to the environment that allows life to thrive.
As researchers work toward studying smaller, rocky exoplanets in greater detail, establishing a reliable connection between stellar chemistry and planetary composition becomes increasingly important. The WASP-189b findings provide a significant step in that direction.
Powerful Telescopes Open a New Window Into Alien Atmospheres
Obtaining these measurements requires instruments capable of distinguishing extremely fine details in the light collected from distant planetary atmospheres. According to the researchers, the necessary spectral resolution for this kind of analysis is currently available only through 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.”
The team expects future observations using multiple wavelengths and similarly precise instruments to uncover a wider range of elements in exoplanet atmospheres. Building a more complete picture of their chemical compositions could reveal additional clues about how planets form, how they change over time, and which conditions might favor habitability.
By connecting the chemistry of a distant planet to that of its star, astronomers now have new evidence supporting a principle that could help guide the search for potentially habitable worlds throughout the galaxy.
Notes
- IGRINS operated as a visiting instrument at Gemini South between 2022 and 2023 before returning to its home institution. Its successful performance led to the development of an updated version, IGRINS-2, which was subsequently commissioned as a permanent facility instrument for the Gemini North telescope in Hawai’i.
