Webb detects two-faced WASP-121b: scorching dusk and cloudy dawn

JWST mapped WASP-121b’s atmosphere longitude by longitude during a single transit as the planet rotated, revealing a hotter, expanded evening limb with water dissociation and a cooler morning limb possibly hosting silicate clouds; this rotational‑transit effect shows strong day–night circulation on this ultra-hot Jupiter (dayside ~2770 K, nightside ~1000 K) and highlights how limb-averaged spectra can miss key chemistry and cloud features.