Using observations from the James Webb Space Telescope (JWST), researchers have identified cloudy “mornings” and clear “evenings” on a distant gas giant exoplanet. The findings suggest that the planet’s atmospheric aerosols are dominated by condensation-driven clouds that form, circulate, and evaporate as they move through extreme temperature contrasts across the planet. Aerosols play an important role in shaping the appearance, chemistry, and temperature of exoplanet atmospheres. However, there is limited information about the nature of these particles, including their atmospheric distribution or the physical processes that determine their properties. In hot Jupiters – a class of gas giant exoplanets that are physically similar to Jupiter – it has long been debated whether atmospheric aerosols are primarily mineral clouds formed through condensation or photochemical hazes generated by intense stellar radiation. Because they can obscure or distort spectral signals, they also complicate efforts to determine the chemical composition of distant worlds.

Here, Sagnick Mukherjee and colleagues used the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument on the JWST to observe the tidally locked, hot Jupiter exoplanet, WASP-94A b, and analyzed the light passing separately through the planet’s “morning” and “evening” atmospheric horizons. The findings revealed stark differences between the two hemispheres: the cooler morning side appeared heavily shrouded in high-mineral clouds that obscure gaseous signatures, while the hotter evening side is comparatively clear and shows strong water vapor absorption. According to Mukherjee et al., this pattern suggests that the planet’s aerosols are dominated by clouds formed through condensation rather than photochemical processes. Moreover, further analysis using a 3D general circulation model indicates a dynamic cloud cycle driven by extreme temperature contrasts of roughly 450 degrees Kelvin between the planet’s two hemispheres. Clouds appear to form on the cooler night side of the planet, circulate toward the morning side, and then evaporate as they move into the intensely heated day side. According to Mukherjee et al., the findings warn that treating an exoplanet’s atmosphere as uniform, which is a common simplifying assumption, can significantly distort or bias estimates of their chemistry and physical properties, and suggest that previous measurements of exoplanet atmospheres may need to be reconsidered to account for complex, asymmetric weather systems.