Astronomers using the James Webb Space Telescope have detected sulfur in the atmospheres of giant exoplanets in the HR 8799 system, suggesting they formed through core accretion similar to Jupiter. This finding challenges previous models, as these planets are five to ten times more massive than Jupiter and orbit much farther from their star. The discovery was led by researchers from the University of California San Diego and published in Nature Astronomy.
The HR 8799 star system, located about 133 light years away in the constellation Pegasus, contains four gas giants each between five and ten times the mass of Jupiter. These planets orbit their star at distances from 15 to 70 astronomical units, far beyond what earlier models predicted for core accretion formation, where a solid core builds up from dust and ice before accreting gas.
A team led by Jean-Baptiste Ruffio, a research scientist at the University of California San Diego, analyzed JWST data to study the atmospheres of three inner planets in the system. Using spectroscopy, they identified signatures of sulfur, including hydrogen sulfide on HR 8799 c, and likely on the other two. This presence of refractory elements like sulfur indicates the planets formed via core accretion rather than gravitational instability, which would resemble brown dwarf formation.
"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," Ruffio stated.
The system is young, at around 30 million years old, allowing clearer detection of formation heat. The planets also show higher levels of heavy elements such as carbon and oxygen compared to their host star, supporting planetary origins.
Quinn Konopacky, a UC San Diego professor of astronomy and astrophysics, noted, "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 I think this shows that older core accretion models are outdated.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."
Ruffio added, "I think the question is, how big can a planet be? 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?"
HR 8799 is the only known directly imaged system with four such massive gas giants, though others with fewer large companions exist. The research was supported by NASA.