Astronomers using the James Webb Space Telescope have made the first direct observation of dramatically different atmospheric conditions between the morning and evening sides of a distant exoplanet. Led by Cyril Gapp of the Max Planck Institute for Astronomy in Heidelberg, the team studied WASP-121 b — an ultra-hot gas giant 900 light-years from Earth — and confirmed a stark atmospheric asymmetry long predicted by theory but never directly observed.
WASP-121 b is a tidally locked "hot Jupiter," meaning the same face always points toward its host star. The dayside scorches at around 2,770 Kelvin (roughly 2,500°C), while the nightside sits at about 1,000 Kelvin (725°C). Powerful eastward winds carry this heat outward, expanding the evening-side atmosphere while leaving the morning side comparatively cool and compact. JWST’s infrared sensitivity let researchers track how water vapour and carbon monoxide signals shift between these two zones during a single planetary transit.
The results, published in Nature Astronomy, mark a landmark for exoplanet science: for the first time a telescope has resolved the fine longitudinal structure of an alien atmosphere down to a scale of 30 degrees of planetary rotation. Researchers say the technique opens the door to detailed climate modelling of exoplanets, potentially including worlds closer to habitable conditions.