JWST Observations Reveal Asymmetric Atmospheric Conditions on WASP-121 b

Astronomers utilizing the James Webb Space Telescope (JWST) have identified significant atmospheric disparities between the dawn and dusk regions of the ultra-hot exoplanet WASP-121 b. By analyzing infrared light as the planet transited its host star, researchers observed that the evening side of the planet is notably hotter and more expanded than the morning side. This study provides the most definitive evidence to date of longitudinal atmospheric asymmetry on a tidally locked gas giant.

The observed differences are primarily driven by intense, high-speed winds that circulate heat from the planet's permanent dayside toward the nightside. Because these winds travel in the direction of the planet's rotation, they disproportionately heat the evening terminator. This thermal expansion increases the atmospheric cross-section, which in turn alters the way starlight is absorbed as it passes through the planet's limb.

Beyond temperature variations, the data suggests complex chemical shifts. Researchers noted a decrease in water molecule abundance in the hotter regions, indicating that the extreme temperatures are likely causing water to dissociate into its constituent elements. Additionally, the presence of carbon monoxide and potential mineral clouds further complicates the atmospheric profile.

These findings are significant because they validate long-standing theoretical models regarding the dynamics of hot Jupiters. By mapping the atmosphere longitude by longitude, scientists are gaining a deeper understanding of how extreme radiation and global wind patterns shape the climates of distant worlds. This level of detail, made possible by the JWST’s NIRSpec instrument, marks a major milestone in exoplanetary science, moving beyond simple characterization toward a nuanced understanding of planetary weather systems.