Astronomers using the James Webb Space Telescope have detected clear differences between the morning and evening sides of the ultra-hot exoplanet WASP-121 b. The observations show that the evening terminator is hotter and more expanded than the morning side.
The findings come from measurements of infrared starlight passing through the planet's atmosphere during transits. Researchers observed that the evening terminator absorbs more light, consistent with eastward winds carrying heat from the dayside.
Data from the NIRSpec instrument also indicated reduced water abundance in hotter regions, suggesting molecules break apart at extreme temperatures. Carbon monoxide signals varied as well, though this appears tied to temperature rather than abundance changes.
Computer models reproduced the asymmetry but underestimated its strength. Scientists suggest mineral clouds on the cooler morning side may contribute to additional cooling not captured in current simulations.
The study, led by Cyril Gapp of the Max Planck Institute for Astronomy, appears in Nature Astronomy. It marks the first time such longitudinal atmospheric variations have been mapped on an exoplanet with this level of detail.