Jupiter-like uniform metal enrichment in a system of multiple giant exoplanets

The accretion of icy and rocky solids during the formation of a gas-giant planet is poorly constrained and challenging to model. Refractory species, like sulfur, are present only in solids in the protoplanetary disk where planets form. Measuring their abundance in planetary atmospheres is one of the most direct ways of constraining the extent and mechanism of solid accretion. Here, using the unprecedented sensitivity of NASA’s James Webb Space Telescope, we measure in detail the chemical make-up of three massive gas giants orbiting the star HR 8799, including direct detections of H₂O, CO, CH₄, CO₂, H₂S, ¹³CO and C¹⁸O. We find that these planets are uniformly and highly enriched in heavy elements compared with the star, irrespective of their volatile (carbon and oxygen) or refractory (sulfur) nature, which strongly indicates that the accretion of solids was efficient during their formation. This composition closely resembles that of Jupiter and Saturn and demonstrates that this enrichment also occurs in systems with several gas-giant planets orbiting stars beyond the Solar System. This discovery hints at a shared origin for the heavy-element enrichment of giant planets across a wider range of planet masses and orbital separations than previously anticipated.