Cloudy mornings and clear evenings on a giant extrasolar world

Abstract: Aerosols are common in exoplanet atmospheres, but their formation-whether through gas condensation or photochemical reactions-remains uncertain. We report a 6$\sigma$ detection of limb asymmetry in the transmission spectrum of WASP-94A b, revealing a cloud-covered (11$\sigma$) cooler morning limb and a clear hotter evening limb with strong H$_2$O absorption (10$\sigma$). Models suggest cloud droplets formed near mbar pressures are lofted to 0.01 mbar by strong vertical dynamics in the morning limb. They evaporate when circulated to the hotter evening limb, requiring a minimum 280 K (3$\sigma$) limb-to-limb temperature difference. We confirm that aerosols in hot Jupiters like WASP-94A b can have clouds cycling between day and night sides instead of photochemical hazes. Ignoring these effects severely biases inferred chemical abundances, showing limb-resolved spectroscopy is critical for characterizing the formation mechanisms of transiting exoplanets-from gas giants to terrestrial exoplanets, indicating the need to reassess inferences from a decade's worth of Hubble Space Telescope observations.