Astronomers Stunned by Giant Exoplanet Formation

JWST spectroscopy reveals clues about the origins of massive gas giants

Published on Feb. 12, 2026

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Astronomers have used data from the James Webb Space Telescope to study the HR 8799 star system, which hosts four massive gas giant exoplanets that are five to ten times the mass of Jupiter. The findings, published in Nature Astronomy, suggest these planets formed through core accretion, a process previously thought unable to create such large worlds so far from their host star. The detection of sulfur and other heavy elements in the planets' atmospheres provides key evidence for this unexpected formation pathway.

Why it matters

The discovery challenges existing models of planet formation and raises new questions about the limits of what can be considered a "planet" versus a "brown dwarf." Understanding how these massive exoplanets form is crucial for piecing together the full picture of planetary system evolution beyond our solar system.

The details

The HR 8799 system, located 133 light-years away, hosts four gas giant planets that orbit at distances ranging from 15 to 70 astronomical units from their host star. Using spectroscopy data from the James Webb Space Telescope, astronomers detected signatures of sulfur and other heavy elements in the atmospheres of the three inner planets, indicating they likely formed through core accretion rather than gravitational instability. This was unexpected, as previous models suggested core accretion could not produce planets this massive so far from their star before the surrounding gas disk dissipated.

  • The HR 8799 system is relatively young at around 30 million years old.
  • The research findings were published in the journal Nature Astronomy in 2026.

The players

Jean-Baptiste Ruffio

A research scientist at the University of California San Diego and first co-author of the paper.

Jerry W. Xuan

A 51 Pegasi b Fellow at the University of California, Los Angeles, who built sophisticated atmospheric models to analyze the JWST data.

Quinn Konopacky

A Professor of Astronomy and Astrophysics at the University of California San Diego and co-author of the paper.

HR 8799

A star system located 133 light-years away in the constellation Pegasus, which hosts four massive gas giant exoplanets.

James Webb Space Telescope (JWST)

A powerful space observatory that provided the high-resolution spectroscopy data used in this research.

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What they’re saying

“With its unprecedented sensitivity, JWST is enabling the most detailed study of the atmospheres of these planets, giving us clues to their formation pathways. With the detection of sulfur, we are able to infer that the HR 8799 planets likely formed in a similar way to Jupiter despite being five to ten times more massive, which was unexpected.”

— Jean-Baptiste Ruffio, Research Scientist, UC San Diego (Nature Astronomy)

“The quality of the JWST data is truly revolutionary and existing atmospheric model grids were simply not adequate. To fully capture what the data were telling us, I iteratively refined the chemistry and physics in the models.”

— Jerry W. Xuan, 51 Pegasi b Fellow, UCLA (Nature Astronomy)

“There are many models of planet formation to consider. I think this shows that older core accretion models are outdated. And of the newer models, we are looking at ones where gas giants can form solid cores really far away from their star.”

— Quinn Konopacky, Professor of Astronomy and Astrophysics, UC San Diego (Nature Astronomy)

What’s next

Researchers continue to explore the limits of planet formation, studying other star systems with large exoplanets to better understand the transition between planet and brown dwarf formation.

The takeaway

The discovery of sulfur and other heavy elements in the atmospheres of the massive HR 8799 exoplanets challenges existing models of planet formation and suggests core accretion can produce gas giants much larger and farther from their host stars than previously thought possible. This finding opens new avenues of research into the diversity of planetary systems beyond our own.