JWST Captures First Daily Cloud Cycle on Hot Jupiter WASP-94A b | Sand Clouds Form at Dawn, Vanish by Dusk

Astronomers using the James Webb Space Telescope have observed a complete, repeating daily weather cycle on a planet outside our solar system for the first time. The study, published May 21, 2026 in the journal Science, documents towering clouds of vaporized silicate rock that form every morning on the gas giant WASP-94A b and are completely destroyed by nightfall, driven by the planet's permanent 1,000-degree Celsius dayside. The findings represent the most detailed atmospheric characterization ever achieved for an exoplanet.

WASP-94A b orbits a Sun-like star in the Microscopium constellation approximately 700 light-years from Earth and belongs to the class of worlds known as Hot Jupiters, gas giants that orbit their host stars at extreme proximity. The research is led by an international team including co-author Harry Baskett and Professor Nathan Mayne of the University of Exeter. For broader context on the JWST science program, see the OzoneNews NASA hub.

WASP-94A b | What Kind of Planet This Is

Hot Jupiters are among the most extreme environments in the known galaxy. WASP-94A b completes a full orbit of its host star in roughly 3.95 Earth days, placing it so close to the stellar surface that it is tidally locked, the same mechanism that keeps one face of Earth's Moon permanently facing us. One hemisphere of WASP-94A b bakes in permanent stellar radiation with surface temperatures exceeding 1,000 degrees Celsius. The opposite hemisphere is in permanent darkness and significantly cooler.

This permanent temperature gradient drives one of the most powerful atmospheric circulation systems known: a continuous eastward jet stream carrying hot air from the dayside toward the nightside and cold air back the other way. It is this circulation that creates the planet's 'morning' and 'evening' edges, called the leading and trailing atmospheric limbs, which JWST was able to isolate for the first time.

How Webb Isolated Morning from Evening Atmosphere

The measurement breakthrough came from Webb's Near-Infrared Spectrograph (NIRSpec), one of the telescope's most precise instruments. As WASP-94A b passed directly in front of its host star in a transit event, NIRSpec captured the starlight filtering through the planet's atmosphere at both edges simultaneously. Because the leading limb, the morning side, and the trailing limb, the evening side, present slightly different orbital geometry during transit, the team developed a new analysis pipeline to computationally separate their chemical signatures from the single combined spectrum.

The result was two independent atmospheric profiles from a single planetary transit: one showing the chemistry of a planet's morning, and one showing its evening.^[1] No telescope has achieved this level of limb-resolved spectroscopy on an exoplanet before. Prior studies using Hubble or early JWST data could only return a single blended spectrum covering both limbs, masking the dynamic differences between them.

The Weather Cycle | Silicate Sand Clouds That Build and Die Daily

The morning limb of WASP-94A b is dominated by dense magnesium silicate clouds. Silicates are the mineral class that makes up most common rocks on Earth, including quartz and olivine. On WASP-94A b, temperatures in the cooler atmospheric regions are still hot enough, roughly 800 to 900 degrees Celsius, to hold silicate minerals in a suspended airborne state, where they condense into cloud particles made of microscopic grains of vaporized rock.

Cold air circulating from the nightside arrives at the morning limb already saturated with these condensed silicate grains, producing thick cloud cover every cycle. As that air continues its journey onto the full dayside, temperatures rise past 1,000 degrees Celsius and the clouds disappear entirely.

The evening limb, where air returns from the dayside back toward the nightside, showed the opposite signature: completely clear skies with strong, unobstructed water vapor absorption lines. The clouds are fully gone before the atmosphere reaches the evening edge, producing transparent conditions that allowed NIRSpec to detect water vapor chemistry with exceptional clarity.

Why Do the Clouds Vanish | Two Competing Theories

The team proposes two mechanisms, which are not mutually exclusive, to explain the complete cloud destruction before the evening limb:

Vaporization: The simplest explanation is thermal. Dayside temperatures are high enough to raise the mineral particles past their vaporization point, converting the solid silicate grains back into gaseous form and eliminating the cloud. This is the most thermodynamically straightforward outcome given the observed temperature profile.

Atmospheric downdraft: The second theory involves fluid dynamics. Supersonic planetary-scale winds may generate powerful downdraft systems on the dayside that physically drag high-altitude cloud layers downward into the deep, hot interior of the planet, where they dissolve under pressure and temperature. This mechanism would also fully remove the clouds from the upper atmosphere visible to NIRSpec without requiring complete vaporization at altitude.

Distinguishing between the two will require additional transit observations with NIRSpec, which the team has scheduled as part of a larger JWST survey of Hot Jupiter atmospheres.

Why This Matters | What Sand-Cloud Separation Unlocks

We have been able to determine what the clouds are made of in the atmosphere of a planet 700 light years from Earth, which is crazy. This work also helps us to test, develop, and improve our modeling approaches leading to improvements in Earth weather and climate prediction.

, Professor Nathan Mayne, University of Exeter

Before this result, clouds were the single biggest obstacle to accurate exoplanet atmospheric chemistry. When both limbs were blended in a single spectrum, cloud cover on one side contaminated or suppressed the chemical signatures from the other, making it impossible to trust abundance measurements of molecules like water, methane, or carbon dioxide. The WASP-94A b result demonstrates that JWST can surgically separate those contributions.

With a clean evening-limb spectrum free of silicate interference, the team was able to calculate the planet's bulk chemical composition with significantly higher confidence than any prior exoplanet measurement. WASP-94A b carries roughly five times the carbon and oxygen abundance relative to its host star, a ratio that closely mirrors Jupiter's enrichment relative to the Sun and strongly supports formation models in which gas giants accumulate their heavy elements by accreting solid planetesimals in the outer protoplanetary disk before migrating inward.

Pattern Confirmed on Two Other Exoplanets

The cloud-cycling signature is not unique to WASP-94A b. Using the same limb-separation technique, the research team identified the identical morning-cloudy, evening-clear pattern on two other previously well-studied exoplanets: WASP-39 b and WASP-17 b. WASP-39 b was the first exoplanet to have its atmosphere chemically characterized in detail by JWST in 2022, making it a natural calibration target for the new methodology.

The confirmation of the same weather cycle across three independent Hot Jupiter systems suggests this is a universal feature of tidally locked gas giants rather than a quirk of WASP-94A b's particular host star or orbital parameters. The implication is that every Hot Jupiter in the galaxy with a similar dayside temperature is likely running this silicate cloud cycle continuously, which will significantly affect how astronomers interpret archival spectral data from Hubble-era exoplanet surveys that lacked the limb-resolution capability to account for it.

Full data tables, NIRSpec transmission spectra, and atmospheric chemistry models from the study are published in Science.^[2] Earlier JWST science coverage at OzoneNews includes the first close-pair supermassive black hole detection in Markarian 501 and the Cygnus X-1 jet power measurement.