Webb detects cloud-covered morning limb on hot Jupiter WASP-94A b 700 light-years away
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The James Webb Space Telescope has separated two narrow slices of the atmosphere of WASP-94A b, a hot Jupiter 700 light-years away in Microscopium, and found a cloud-covered morning limb opposite a clear, hotter evening limb. The Science study reports an 11-sigma cloud signal on the morning limb and 10-sigma water-vapour absorption on the evening limb.
Morning and Evening Limbs
On WASP-94A b, morning and evening are physical boundary regions, not times of day. The planet is tidally locked, so its star-facing hemisphere is permanently illuminated while the opposite hemisphere stays dark. Winds carry atmospheric gas from the night side into the morning terminator and from the day side back toward the evening terminator. Webb's data show mineral clouds form as gas cools, travel into the morning boundary, and evaporate in the dayside heat. This circulation persists even though the illumination pattern remains fixed.
Limb-Resolved Spectroscopy
Researchers used Webb's Near-Infrared Imager and Slitless Spectrograph, a mode designed for exoplanet transit spectroscopy. Instead of treating the entire atmospheric ring as uniform, they extracted leading-edge and trailing-edge spectra during a single transit. The result was a six-sigma asymmetry between limbs, too strong for a single average atmosphere to explain. The team measured an 11-sigma cloud-covered morning limb and a clear, hotter evening limb with water absorption detected at 10 sigma. This is an inference from filtered starlight, not a photograph of individual clouds.
Mineral Cloud Model
The cloud is observed; its mineral recipe is modelled. Astronomers propose the cloud particles may be made of rock-forming minerals rather than water. The hot Jupiter lies nearly 700 light-years away in Microscopium and circles its star in roughly four days. Webb's latest observations give astronomers their clearest look yet at such a world. The temperature contrast between the permanent day and night hemispheres creates a weather machine whose hot and cool zones stay approximately in place.
What's Next
Further Webb observations could determine whether the cloud particles are indeed rock-forming minerals and how they survive transit across the morning terminator. It remains unclear whether the asymmetric circulation observed on WASP-94A b is common among other tidally locked hot Jupiters.
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Webb detects cloud-covered morning limb on hot Jupiter WASP-94A b 700 light-years away
