ASU team leads discovery linking exoplanet and star composition

An international team of astronomers showed that exoplanets inherit their rocky composition from their host stars, informing models of planet formation and habitability.

Led by ASU doctoral student Jorge Antonio Sanchez, the team studied WASP-189 b, a giant exoplanet about 320 light-years away in Libra, using the IGRINS spectrograph on the Gemini South telescope in Chile. They detected magnesium and silicon in its atmosphere and found its magnesium-to-silicon ratio closely matches that of its host star.

Their findings, which support this scientific link, were recently published in Nature Communications.

“WASP-189 b offers an amazing observational anchor to understand how terrestrial planets form,” explained Sanchez. “For the first time, we can point to a concrete measurement that validates what planetary scientists have long assumed: the planets echo the composition of their stars when forming rocky worlds.”

Astronomers have long believed that stars and their planets share the same ratios of rock-forming elements because they come from the same cloud of gas and dust. Scientists have used this idea to model exoplanet compositions and study their habitability, but until now, it had not been directly confirmed outside our solar system. This study confirms it for the first time.

This research is important for astrobiology. Elements like magnesium, silicon and iron play a key role in how planets work, including driving plate tectonics, supporting magnetic fields, and releasing chemicals needed for life into the atmosphere, oceans and soil. By measuring a distant star’s chemical makeup, scientists can now better guess what its planets are made of and how likely they are to support life.

Michael Line, an associate professor at ASU’s School of Earth and Space Exploration and a co-author of the study, pointed out the wider importance of the team’s observational method.

"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," Line said. "This advancing capability opens an entirely new dimension in our study of exoplanet atmospheres."

WASP-189 b is an ultra-hot Jupiter, a type of planet with temperatures high enough to vaporize rock-forming elements. This makes these elements easier to spot using spectroscopy. Because of this, WASP-189 b was a good choice for these measurements, and the team’s success could lead to new ways of studying the atmospheres of distant planets.

This work also highlights the kind of detailed exoplanet characterization that will become routine with the next generation of giant ground-based telescopes. The Giant Magellan Telescope (GMT), slated to begin operations in the early 2030s, will enable even more precise measurements of planetary atmospheres.

IGRINS itself serves as a pathfinder for GMT’s high-resolution spectrograph, GMTNIRS, demonstrating the power of this technique on today’s telescopes. As a founding partner of GMT, Arizona State University is playing a leading role in shaping this future, where astronomers will be able to probe the chemical makeup of distant worlds with unprecedented sensitivity and test fundamental theories of planet formation and habitability.

Additional ASU co-authors include graduate students Peter Smith and Krishna Kanumalla as well as faculty Luis Welbanks, Steve Desch, Patrick Young and Jennifer Patience.

The team did their research at the International Gemini Observatory, which is partly funded by the U.S. National Science Foundation and operated by NSF NOIRLab.