A scorching planet nearly 320 light-years from Earth has provided the first direct evidence of a fundamental assumption about how planets form. Astronomers discovered that WASP-189b shares an important chemical feature with its host star, supporting a long-standing idea that also helps scientists investigate potentially habitable worlds. The breakthrough came with the first simultaneous measurement of gaseous magnesium and silicon in an exoplanet’s atmosphere, using the Gemini South telescope, part of the Gemini International Observatory, partially funded by the US National Science Foundation and operated by NSF NOIRLab. Located nearly 320 light-years away in the constellation Libra, WASP-189b belongs to an extreme category of planets called ultrahot Jupiters (UHJ). These enormous worlds reach temperatures so intense that even materials normally found in solid rock can turn into gas. Therefore, elements such as magnesium (Mg), silicon (Si) and iron (Fe) can be detected in their atmospheres. Astronomers identify these substances using spectroscopy, a method that separates light into different wavelengths to reveal the chemicals present. An international research team led by Jorge Antonio Sánchez, a graduate student at Arizona State University (ASU), investigated WASP-189b with the High Resolution Immersion 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 ratio of magnesium to silicon in 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 of NOIRLab. “These discoveries are only possible thanks to Gemini’s state-of-the-art instruments.” A long-suspected connection between stars and planets Astronomers believe that giant planets like WASP-189b formed within huge disks of gas and dust surrounding young stars. These structures, called protoplanetary disks, provide the raw material from which planets develop. The chemical composition of a giant planet’s outer atmosphere is thought to hold clues to the environment where it originally formed. Because stars and the disks that surround them originate from the same cloud of material, scientists have long assumed that they contain similar proportions of rock-forming elements. In particular, the proportions 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. However, until now this assumption was mainly supported by evidence from our own Solar System. Astronomers had not directly measured the same relationship on a planet orbiting another star. The discovery involving WASP-189b provides missing observational evidence, strengthening the scientific basis for using stellar chemistry to understand the materials from which distant planets form. “WASP-189b provides us with a much-needed observational anchor in our understanding of terrestrial planet formation, as it provides a measurable quantity that validates the supposed similarity of stellar composition and the proportion of rocky material around host stars used to form planets,” says Sánchez. What planetary chemistry could reveal about habitability The implications go 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 can estimate the relative amounts of rock-forming materials available to their planets. These materials influence a planet’s internal structure, geological activity, and chemical environment, all of which can affect the development of conditions suitable for life. 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 to study smaller rocky exoplanets in greater detail, establishing a reliable connection between stellar chemistry and planetary composition becomes increasingly important. The WASP-189b findings represent an important 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 spectral resolution necessary for this type of analysis is currently only available through ground-based telescopes. “Our study demonstrates the ability of high-resolution ground-based spectrographs to constrain critical species such as magnesium and silicon, which are two elemental components from which rocky planets form,” says study co-author Michael Line, an associate professor at ASU. “This breakthrough capability opens up a completely new dimension in our study of exoplanet atmospheres.” The team hopes that future observations using multiple wavelengths and equally precise instruments will discover a broader range of elements in exoplanets’ atmospheres. Building a more complete picture of their chemical compositions could reveal additional clues about how planets form, how they change over time, and what 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 across 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 permanently installed instrument for the Gemini North telescope in Hawai’i.
