WASP-121 b is split between extremes. One side faces its star nonstop and burns at about 2,770 Kelvin. The other stays in darkness and cools to roughly 1,000 Kelvin. Now astronomers have found that even the narrow boundary zones between those halves are not alike. Using the James Webb Space Telescope, a team led by Cyril Gapp of the Max Planck Institute for Astronomy detected clear differences between the planet’s morning and evening terminators, the regions where day turns to night. The result gives researchers one of their sharpest looks yet at how temperature and chemistry shift across an exoplanet’s atmosphere. The signal appeared during transit, when WASP-121 b crossed in front of its host star. As starlight passed through the planet’s atmosphere, the gases there filtered specific infrared wavelengths. By tracking how that filtering changed over the course of the transit, the team could watch different longitudes rotate into view. “With its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date: By measuring how star light absorption changes …