Astronomers using the James Webb Space Telescope (JWST) have shed new light on the formation and origins of an ultra-hot exoplanet, WASP-121b. By detecting multiple key molecules in its atmosphere, including water vapor, carbon monoxide, silicon monoxide, and methane, a team led by Thomas Evans-Soma revealed clues about how the planet formed and where it might have originated.
The study found that WASP-121b likely accumulated most of its gas in a region cold enough for water to remain frozen yet warm enough for methane to evaporate. This suggests that the planet undertook a long journey from icy outer regions to the center of the planetary system, which is remarkable given its close proximity to its host star.
The team detected silicon monoxide, which originated from rocky material such as quartz stored in planetesimals, indicating that the formation process occurred during the later stages of planetary development. They also found a higher carbon-to-oxygen ratio in the planet’s atmosphere than in its host star, which is best explained by the planet continuing to attract carbon-rich gas after oxygen-rich pebbles had stopped flowing.
Furthermore, the detection of methane on the cooler nightside of WASP-121b was unexpected and requires strong vertical currents to replenish it. This challenges current exoplanet dynamical models, which will likely need to be adapted to reproduce the observed vertical mixing.
The JWST’s Near-Infrared Spectrograph (NIRSpec) played a crucial role in observing WASP-121b throughout its complete orbit around its host star, allowing the team to characterize the conditions and chemical composition of its dayside and nightside. The study provides new insights into the formation and evolution of ultra-hot exoplanets and highlights the importance of JWST’s capabilities for understanding these complex celestial bodies.
Source: https://phys.org/news/2025-05-space-pebbles-play-pivotal-role.html