Key Points
- JWST detected water vapor, carbon monoxide, silicon monoxide, and methane in the atmosphere of WASP-121 b.
- Methane’s presence on the night side suggests the presence of strong vertical winds, which are not accounted for in current models.
- WASP-121b likely formed in a colder region of its system and migrated close to its star.
- The planet’s high carbon-to-oxygen ratio reveals clues about its birth environment and the process of gas accretion.
New observations from the James Webb Space Telescope (JWST) have revealed surprising details about the exoplanet WASP-121b, shedding light on its formation history and complex atmospheric dynamics. The data, gathered using JWST’s NIRSpec instrument, detected key molecules in the planet’s atmosphere, including water vapor, carbon monoxide, silicon monoxide, and methane.
Published in Nature Astronomy, the study led by astronomers Thomas Evans-Soma and Cyril Gapp compiled a detailed inventory of carbon, oxygen, and silicon in the atmosphere. One of the most unexpected findings was the abundant presence of methane on the planet’s nightside — a discovery that challenges existing models of ultra-hot exoplanet atmospheres. Since methane is typically unstable at high temperatures and destroyed on the dayside, its presence on the cooler nightside implies that strong vertical winds are transporting it from lower atmospheric layers.
WASP-121b is an ultra-hot gas giant orbiting extremely close to its star —
just twice the star’s diameter away — and completing a full orbit in 30.5 hours. Temperatures on the dayside can reach 3000°C, while the nightside cools down to 1500°C. The detection of silicon monoxide in gas form suggests that even rocky materials like quartz are vaporized in the intense heat, providing clues to the planet’s chemical origin.
Based on its elemental makeup, scientists propose that WASP-121b formed far from its current position, likely in a cooler region of the star’s protoplanetary disk, similar to the location of Jupiter and Uranus in our solar system. There, water remained frozen, but methane was gaseous, which explains the planet’s high carbon-to-oxygen ratio.
The JWST observations, spanning a complete orbit and including transits across the star, enabled astronomers to study both hemispheres and the transitional regions between them. While methane was abundant on the night side, it was absent in the twilight zones, indicating dynamic atmospheric mixing that will require updates to current exoplanet models.
