WASP-189b’s atmosphere contains clues about planet formation

Located about 320 light-years away in the constellation Libra, WASP-189b belongs to an extreme class of planets known as ultrahot Jupiters. These giant worlds reach temperatures high enough to turn materials normally found in solid rock into gas. As a result, elements such as magnesium (Mg), silicon (Si), and iron (Fe) can be detected in their atmospheres.

Astronomers identify these elements through spectroscopy, a technique that separates light into different wavelengths to reveal the chemicals present.

An international research team led by Arizona State University graduate student Jorge Antonio Sanchez studied WASP-189b using high-resolution immersion grating infrared spectrometry, or IGRINS. At the time of the observations, the instrument was installed at the Gemini South Telescope in Chile.^[1]

For the first time, researchers successfully measured magnesium and silicon together in the atmosphere of a planet outside our solar system. They found that WASP-189b’s magnesium-to-silicon ratio closely matches the ratio in its parent star.

The result offers the first direct observational support for widely used assumptions about the chemical relationship between stars and their planets. It also provides a new way to investigate how planets form and evolve.

“These discoveries demonstrate Gemini’s ability to help us understand the remarkable zoo of exoplanets in our Sun’s neighborhood,” said Chris Davis, NSF program director at NOIRLab. “Such discoveries are only possible thanks to Gemini’s cutting-edge equipment.”

Why a planet’s chemistry can match its star’s

Astronomers believe that giant planets such as WASP-189b formed inside enormous 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 atmosphere can preserve clues about the environment where the planet 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 proportions of these elements in a protoplanetary disk are expected to resemble those measured in its central star. Until now, however, this assumption was supported primarily by evidence from our solar system. Astronomers had not directly measured a similar relationship in a planet orbiting another star.

WASP-189b provides that missing observational evidence. The discovery strengthens the scientific basis for using a star’s chemistry to understand the materials available during planet formation.

“WASP-189b provides a much-needed observational anchor for understanding the formation of terrestrial planets, because it provides measurable quantities to verify the inferred stellar composition similarities and the proportion of rocky material around the host star that is used to form planets,” Sanchez said.

What exoplanet chemistry could reveal about habitability

The discovery has implications beyond the formation of giant planets. The chemical relationship between stars and planets is also important in astrobiology, the study of extraterrestrial life and the environmental conditions that might support it.

By studying the elements present in a star, astronomers may be able to estimate the relative amount of rock-forming material available to its planets. These materials influence a planet’s internal structure, geological activity, and chemical environment—factors that can affect whether conditions suitable for life develop.

Earth provides a familiar example. The elements that make up our planet help determine processes such as plate tectonics, the creation of protective magnetic fields, and the movement of essential chemicals through the atmosphere, oceans, and soil. Together, these processes contribute to an environment that allows life to thrive.

As researchers conduct more detailed studies of smaller, rockier exoplanets, reliable relationships between stellar chemistry and planetary composition will become increasingly important. The findings from WASP-189b represent a significant step toward establishing those relationships.

Gemini South opens a new window into exoplanet atmospheres

Measuring elements in distant planetary atmospheres requires instruments capable of detecting extremely fine details in collected starlight. According to the researchers, the spectral resolution needed for this type of analysis is currently available only through ground-based telescopes.

“Our study demonstrated the ability of ground-based, high-resolution spectrometers to constrain important species such as magnesium and silicon, two elemental building blocks that form rocky planets,” said study co-author Michael Rein, an ASU associate professor. “This advanced capability opens up a whole new dimension in the study of exoplanet atmospheres.”

The researchers hope that future observations across multiple wavelengths, using similarly precise instruments, will reveal a wider range of elements in exoplanet atmospheres. A more complete picture of their chemical composition could provide additional clues about how planets form, how they change over time, and what conditions make them habitable.

By comparing the chemistry of distant planets with the chemistry of their stars, astronomers now have new evidence supporting a principle that could aid the search for potentially habitable worlds across the galaxy.

Instrument note

1. IGRINS operated as a visiting instrument at Gemini South from 2022 to 2023, after which it returned to its home base. Its successful performance led to the development of an updated version, IGRINS-2, which was later commissioned as a permanent facility instrument at the Gemini North telescope in Hawaii.