JWST Finds Hydrogen Sulfide on Exoplanets
JWST detected H2S in the atmospheres of super-Jupiter exoplanets in the HR 8799 system (130 light-years away) during February 2026 observations. This marks the first hydrogen sulfide detection in exoplanet atmospheres, plus earlier CO2 findings, providing evidence these 5–10 Jupiter-mass worlds formed via core accretion like our own gas giants.
The HR 8799 system is a trailblazer in exoplanet studies, being the first multi-planet system to have its planets directly imaged. This was achieved by observing the faint, infrared glow of the young, still-hot planets. The four known gas giants in the system are massive, ranging from 5 to 10 times the mass of Jupiter, and orbit their star at wide separations, from 15 to 69 astronomical units. The host star, HR 8799, is a young A-type star, roughly 1.4 times the mass of our Sun and about 30-60 million years old. Its relative youth means the planets are still radiating heat from their formation, making them luminous enough for direct imaging. The system also contains two debris belts, analogous to our own asteroid and Kuiper belts, suggesting a rich environment for planetary formation. The detection of hydrogen sulfide is significant because sulfur is a relatively heavy element. Its presence, along with previously detected carbon dioxide, indicates that the planets' atmospheres are enriched with heavy elements. This chemical makeup is a key piece of evidence supporting the core accretion model of planet formation. The core accretion model posits that giant planets form in a two-step process: first, a solid core of rock and ice coalesces, and once it reaches a critical mass, it rapidly pulls in a massive envelope of gas from the surrounding protoplanetary disk. This contrasts with the gravitational instability model, where a planet forms directly from the collapse of a dense clump of gas and dust. The heavy elements detected in the HR 8799 planets suggest they formed from solid materials, consistent with core accretion. Prior to the hydrogen sulfide finding, the JWST had already detected carbon dioxide in the atmospheres of the HR 8799 planets. Interestingly, studies have also revealed that the atmospheric compositions of the four planets vary significantly from one another, which is surprising given they likely formed from the same disk. The orbits of the planets in the HR 8799 system may be in a state of resonance, similar to Jupiter's Galilean moons. This means their orbital periods are in a simple integer ratio, which helps to maintain the long-term stability of the system. Dynamical simulations suggest a possible 1:2:4:8 resonance for the four planets.
