Gas giants in HR 8799 formed by core-accretion, study finds

Using JWST spectra, researchers report the three inner gas giants in HR 8799 show heavy-element enrichment consistent with core-accretion formation, and hydrogen sulfide was detected on at least one planet.

The University of Michigan joined a team that used NASA's James Webb Space Telescope to study three inner gas giants in the HR 8799 system, more than 130 light-years away. The team analyzed near-infrared spectra with NIRSpec to search for chemical signatures in the planets' atmospheres. The results, published in Nature Astronomy by Jean‑Baptiste Ruffio, Jerry Xuan and colleagues, show enrichment in elements heavier than hydrogen and helium. That pattern is reported as consistent with formation by core-accretion rather than by direct gravitational collapse. Key findings: - JWST NIRSpec spectra indicate enrichment of heavy elements (such as carbon and oxygen) in the atmospheres of the three inner HR 8799 gas giants, a signature often linked to core-accretion. - The team detected hydrogen sulfide (sulfur) on HR 8799 c and reports sulfur is likely present on all three planets. - The three planets are reported as about five to ten times the mass of Jupiter and lie in the HR 8799 system roughly 130 light-years from Earth. - Some earlier models had favored formation like brown dwarfs because of the planets' large masses and distances, but refined atmospheric models and JWST data supported a bottom-up formation pathway. - The study was led by Ruffio and Xuan with collaborators from NASA and several universities, and the results appear in Nature Astronomy. Summary: These results indicate that core-accretion can produce gas giants much more massive and more distant than some earlier models assumed, which adjusts how researchers frame the question of maximum planet mass. The team reports differing sulfur concentrations among the three planets and describes the overall heavy-element enrichment as unexpectedly efficient and puzzling. The researchers say examining additional planetary systems will be required to locate the transition between planet formation and brown dwarf formation. Meyer and colleagues plan further analysis within the HR 8799 system.