The discovery provides new insights into the question, 'how large can a planet be?'
Study: Jupiter-like uniform metal enrichment in a system of multiple giant exoplanets (DOI: 10.1038/s41550-026-02783-z)
The University of Michigan was part of a research team that used the world's most powerful space telescope to provide new insights into a longstanding question in astronomy.
Namely, how do gas giants like Jupiter form in other planetary systems when they're larger and farther away from their star? Do they, like Jupiter and its gas giant neighbors, start out with rocky cores and build up gas? Or, as predicted by some models, do they form more like celestial objects known as brown dwarfs? That is, do they come from perturbations in a giant gas-rich disc that collapse under their own gravity?
The team has now used NASA's JWST to determine an answer for the three innermost gargantuan gas giants in the HR 8799 system more than 130 light-years away. The study, led by Jean‑Baptiste Ruffio of the University of California San Diego and Jerry Xuan of the California Institute of Technology, was published in the journal Nature Astronomy.
"The empirical answer is in," said Michael Meyer, a U-M professor of astronomy and co-author of the new report. "These gas giants are formed through core-accretion. It's a bottom-up process."
Meyer has been involved with the JWST for more than two decades and was part of the team that developed the Near-Infrared Camera, or NIRCam. For this study, their science team used another instrument, the Near Infrared Spectrograph, or NIRSpec, to analyze light from HR 8799's gas giants for signatures of chemicals in the planets' atmospheres.
In our solar system, the gas giants are enriched in elements like carbon and oxygen that are heavier than the hydrogen and helium that dominate the sun. This enrichment is indicative of core-accretion formation and was observed in the HR 8799 gas giants. The team also found clear evidence of sulfur in the third planet in the HR 8799 system, HR 8799 c, and believes it is likely present on all three planets.
"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," said Ruffio, a research scientist at UCSD.
Some earlier models had favored the brown dwarf formation path because of the large size of these planets and their distance from their star. But thanks to JWST and atmospheric models developed by Xuan, who is now a 51 Pegasi b Fellow at UCLA, the team found a more familiar planet formation pathway is still possible.
"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," Xuan said. "In the end, we detected several molecules in these planets-some for the first time, including hydrogen sulfide."
Researchers from NASA, UC Santa Cruz, University of Hawaii, Johns Hopkins University, University of Arizona, University of Victoria and Herzberg Astronomy and Astrophysics Research Center also contributed to the study.
While the research provides new answers for how planets form, some questions still loom large for exoplanet researchers.
For example, "how big can a planet be?," Rufio asked. "Can a planet be 15, 20, 30 times the mass of Jupiter and still have formed like a planet? Where is the transition between planet formation and brown dwarf formation?"
Cracking that case will require examining more systems beyond HR 8799. But the team's finding also leads to new questions within the system, some of which will be explored by Meyer and U-M graduate student William Meynardie, who was not involved in this study.
For example, the three planets observed in this study appear to have different concentrations of sulfur, which could suggest something important about how efficiently different planets form. And, on the whole, the concentration of heavy elements in HR 8799's gas giants shows that they were incredibly efficient at forming, Meyer said-in fact, too efficient.
"There's no way planetary formation should be that efficient," Meyer said. "It's a conundrum. We're really left with a mystery here."
