Astronomers using the James Webb Space Telescope have discovered that the dawn and dusk regions of the exoplanet WASP-121 b are dramatically different in temperature and atmospheric composition. This ultra-hot gas giant, locked in a permanent day-night cycle, shows stronger light absorption at its evening terminator, indicating higher temperatures and expanded atmosphere due to powerful eastward winds transporting heat from the dayside. The findings confirm long-standing theoretical models but also reveal discrepancies that suggest missing elements in current simulations, such as cloud formation from mineral particles like silicates.

Water molecules break apart in the hotter regions of the atmosphere, particularly near the evening side, providing direct evidence of temperature-driven chemical changes. Meanwhile, carbon monoxide signals shift due to thermal effects rather than changes in abundance. These insights were made possible by analyzing infrared light during the planet's transit, capturing subtle changes as different longitudes rotated into view. The planet rotates about 30 degrees during each transit, allowing unprecedented longitudinal mapping of an exoplanet’s atmosphere.

Current atmospheric models successfully predict general asymmetry but underestimate the observed effect, pointing to gaps in how we simulate exoplanet weather. Including cloud physics—especially cooling from silicate clouds on the morning side—brings models closer to reality. Future studies of similar ultra-hot Jupiters could refine these models further, helping scientists understand the 3D dynamics of extreme alien atmospheres. The observations were part of JWST programs focused on phase curves and atmospheric diversity in transiting exoplanets.