The James Webb Space Telescope (JWST) has enabled the first clear detection of a repeating, daily cloud cycle on a hot Jupiter: mineral-rich clouds form on the planet’s morning limb and then evaporate as that air circulates into the dayside, leaving the evening limb comparatively clear.
That result,published in Science in May 2026 and led by Sagnick Mukherjee and collaborators,used limb-resolved transit spectroscopy to separate light passing through the planet’s leading (morning) and trailing (evening) horizons, revealing asymmetries that average spectra previously concealed.
How Webb separated morning and evening skies
The key observational advance was limb-resolved transit spectroscopy: by measuring the spectrum while the planet’s leading edge begins to cross the star and then again as the trailing edge finishes the transit, the team isolated the morning and evening atmospheric columns. This geometry lets observers compare two distinct terminator regions rather than a single disk-averaged signal.
JWST’s NIRISS instrument provided the necessary wavelength coverage and signal-to-noise to detect subtle differences in water absorption and continuum opacity between the two limbs. The observing sequence and careful systematics control produced statistically significant limb asymmetries at multiple wavelengths.
Practically, the analysis compared the leading-edge (morning) transmission spectrum to the trailing-edge (evening) spectrum, then used atmospheric retrievals and forward models to infer temperature, cloud opacity, and composition differences that are consistent with a daily cloud-formation-and-evaporation cycle.
The instruments and data pipeline behind the mapping
NIRISS slitless spectroscopy on JWST was the workhorse for this study because it balances spectral resolution with throughput for bright host stars; the team also cross-checked results against established pipeline reductions and modelled instrument systematics. Robust calibration and noise modeling were essential to separate real planetary signals from detector effects.
Data were reduced with transit-specific techniques: time-dependent systematics were modeled and removed, and spectral light curves were fit across narrow wavelength channels to produce limb-resolved transmission spectra. Independent reductions and retrieval codes were used to confirm the atmospheric conclusions.
The study’s methodological rigor,multiple analyses, statistically significant detections, and comparisons to three-dimensional general circulation models (GCMs),is why the result moved quickly from observation to high-confidence physical interpretation.
What the spectra revealed about the clouds
The leading (morning) limb shows muted molecular absorption and a strong gray-ish continuum consistent with high-altitude condensate clouds, while the trailing (evening) limb exhibits clearer water-vapor signatures and lower effective cloud opacity. The contrast is large enough to be detected with high significance.
Composition constraints from the wavelength dependence indicate the aerosols are consistent with refractory, rock-forming condensates,magnesium silicates and sulfides,rather than photochemical hazes that form uniformly across the terminator. Those minerals condense where temperatures drop on the nightside and morning limb.
Spectral retrievals and forward-cloud models suggest the clouds form at high altitudes on the colder morning limb, are lofted by vertical mixing, then evaporate when transported into the much hotter dayside, producing a daily ‘‘formation,evaporation’’ cycle. The amplitude of the effect implies substantial differences in inferred abundances if limb asymmetry is not accounted for.
Atmospheric dynamics and model interpretation
Three-dimensional general circulation models that include condensation physics reproduce the basic pattern: winds advect cool, cloud-laden air from the nightside to the morning terminator; condensates form there and then sublimate on the dayside as temperatures rise by several hundred kelvin. This dynamical behavior explains the observed spectral asymmetry.
Modeling also constrains the timescales involved. The cloud formation and evaporation cycle must operate on timescales comparable to the planet’s circulation times (hours), so particle growth, vertical mixing, and vapor pressure of candidate minerals all influence whether condensates persist long enough to be observed.
Uncertainties remain in particle size distributions and vertical cloud structure; resolving those requires additional multiwavelength phase curves and eclipse mapping. Nevertheless, the combined observational,model picture firmly supports a dynamic, repeating cloud cycle on this tidally locked hot Jupiter.
Why limb asymmetry matters for atmospheric measurements
Most past transmission spectra averaged the two terminators together, implicitly assuming the planet’s limbs were similar. The WASP-94A b result shows that assumption can bias chemical abundance retrievals and inferred metallicities when the two limbs differ substantially in cloud cover.
For example, a cloudy morning limb mutes water absorption and can make the atmosphere appear drier or more metal-rich than it really is if not modeled properly. Limb-resolved techniques decouple cloud effects from gas absorption, producing more reliable composition estimates.
This methodological lesson has immediate implications for the interpretation of archival Hubble and ground-based spectra and for planning future JWST observations: where possible, observers should aim for limb- or phase-resolved measurements to avoid conflating distinct atmospheric regions.
Broader implications and next steps
The team used WASP-94A b as a benchmark and then searched eight additional hot Jupiters; similar cloudy-morning/clear-evening signatures appeared on at least two other planets (WASP-39 b and WASP-17 b), indicating the phenomenon may be common among strongly irradiated, tidally locked gas giants.
Going forward, the field will combine limb-resolved transits, full-orbit phase curves, and eclipse mapping to build three-dimensional, time-resolved pictures of exoplanet climates. Those data will refine cloud microphysics parameters and improve GCM fidelity.
From a policy and funding perspective, the discovery underscores the scientific return of flagship observatories and the need to support sustained, focused surveys that can reveal population-level atmospheric behavior across exoplanet classes. These coordinated observing programs will be essential to turn pioneering case studies into robust statistical science.
The WASP-94A b study is a clear demonstration that JWST can transform exoplanetary science from static spectra into dynamic weather maps,an essential step toward characterizing atmospheres in three dimensions and over time.
Adopting limb-resolved approaches and expanding the sample of well-characterized planets will let researchers quantify how frequently mineral cloud cycles occur, how they scale with irradiation and gravity, and how they affect inferred compositions,key inputs for models of planet formation and evolution.
