JWST finds hydrogen sulfide on HR 8799 c, upending where planets end and brown dwarfs begin

A recent analysis of JWST infrared spectroscopy has identified hydrogen sulfide in the atmosphere of HR 8799 c, part of a multi-planet system orbiting a young, sun-like star. That sulfur-bearing molecule is a diagnostic sign because sulfur initially freezes onto solid grains in protoplanetary disks, so its presence in a planet’s gas envelope points to significant solid-core accumulation prior to runaway gas capture. The planet in question lies in the five-to-ten Jupiter-mass range, a regime that has long sat at the ambiguous interface between classical planets and the substellar brown-dwarf population. By showing heavy-element enrichment and sulfur chemistry inconsistent with a pure, rapid gas collapse, the observation supports a fast but core-driven growth pathway even for very massive gas giants. If this interpretation holds across other systems, it will require models of planet formation to permit efficient core accretion at larger orbital distances and higher final masses than many current theories allow. That shift has consequences for population synthesis, because formation history rather than mass alone would become a principal classifier, complicating the use of a blunt deuterium-burning threshold as a taxonomic boundary. The work leverages JWST’s sensitivity to mid-infrared molecular features that were difficult or impossible to detect with previous facilities, demonstrating the telescope’s ability to probe the chemistry that encodes formation processes. Follow-up observations of additional HR 8799 planets and a broader sample are necessary to determine whether this system is exceptional or representative. The result also feeds back into disk evolution studies: abundant solid material or rapid pebble delivery must be reconciled with disk lifetimes at wide separations. Practically, this finding motivates targeted searches for sulfur-bearing species and other solid-derived volatiles as tracers of core formation. It will prompt theoreticians to revisit accretion timescales, migration histories, and gas/dust partitioning in protoplanetary disks. Overall, the detection reframes how astronomers infer origin stories for massive substellar objects and highlights chemical composition as a decisive observable.