Every morning on WASP-94A b, clouds made of vaporized rock and metal condense out of the atmosphere. Every evening, they’re gone, burned away by the searing heat of a nearby star. And for the first time, astronomers have watched that cycle play out in real data, catching a gas giant in the act of running its own alien weather pattern on repeat.
A research team using NASA’s James Webb Space Telescope (JWST) reported the detection in a study posted to the arXiv preprint server in mid-2025 and under review as of July 2026. By splitting the light filtering through the planet’s atmosphere into two halves, morning and evening, they found a stark and persistent difference: thick mineral clouds dominate one side while the other stays clear, with water vapor absorption standing out sharply. It is the first time a repeating daily weather cycle has been documented on a hot Jupiter.
A planet with permanent jet lag
WASP-94A b is a gas giant roughly 1.7 times the mass of Jupiter, locked in a tight orbit around its host star with a year that lasts just under four Earth days. At that distance, the planet is tidally locked: one face permanently bakes under starlight while the opposite hemisphere sits in perpetual darkness. Surface-level equilibrium temperatures hover around 1,500 Kelvin (roughly 2,240°F), hot enough to vaporize minerals that would be solid rock on Earth.
The planet sits in a binary-star system. Its host star, WASP-94A, has a gravitational companion, WASP-94B, which harbors its own giant planet. The system was first described in a 2014 discovery paper that established the orbital parameters and stellar properties researchers have relied on ever since.
When WASP-94A b crosses in front of its star as seen from Earth, starlight passes through a thin ring of atmosphere along the day-night boundary. That ring has two edges: a morning limb, where cooler night-side air rotates into daylight, and an evening limb, where superheated dayside air flows toward darkness. By isolating each edge in the transit signal, the JWST team could effectively take two separate atmospheric snapshots in a single observation.
What the spectra showed
The morning limb looked hazy. Its spectrum was flattened and muted, consistent with a thick layer of aerosols and cloud particles blocking the starlight. Water vapor features, which should appear as distinct dips at specific wavelengths, were largely smothered.
The evening limb told a different story. Its spectrum was cleaner, with pronounced water-vapor absorption lines standing out against a comparatively transparent atmosphere. The contrast between the two sides was statistically significant and pointed to a single explanation: clouds form on the cooler night side, drift toward the morning terminator on powerful atmospheric winds, and then evaporate as the air crosses the blazing dayside before reaching the evening edge.
Because the planet is tidally locked and its orbit takes less than four days, this cycle repeats continuously. Rock-mineral clouds condense, travel, and vanish on every single orbit, producing what amounts to a permanent, planet-scale weather loop.
How the observation was made
The team used JWST’s NIRISS instrument in its Single Object Slitless Spectroscopy (SOSS) mode, a configuration optimized for capturing the faint atmospheric signatures of transiting exoplanets. NIRISS/SOSS spreads incoming starlight into a spectrum across a wide near-infrared range, giving researchers the wavelength coverage needed to distinguish between cloud opacity and molecular absorption.
A separate, peer-reviewed study had already observed WASP-94A b with a different Webb instrument, NIRSpec/G395H, confirming the planet as an established atmospheric target with independent data. Earlier ground-based work using the NTT/EFOSC2 telescope had picked up sodium absorption and Rayleigh scattering in the planet’s optical spectrum, hinting at aerosol activity but lacking the resolution to separate the two limbs. JWST’s precision made the limb-by-limb split possible for the first time.
NASA has also investigated morning-versus-evening differences on another hot Jupiter, WASP-39 b, building a theoretical framework for why tidally locked worlds should display exactly this kind of asymmetry. That earlier work helped set expectations, but WASP-94A b now provides the clearest empirical case.
A pattern emerging across hot Jupiters
WASP-94A b is not the only planet showing this behavior. A broader comparative analysis covering several hot Jupiters observed with JWST NIRISS/SOSS found that overcast mornings and clear evenings appear to be a recurring atmospheric pattern across this class of planets. The survey examined the physical processes, including downwelling currents and cloud evaporation timescales, needed to produce clear evening limbs, and concluded that the trend is consistent across multiple targets.
That consistency matters. If only one planet showed the asymmetry, it could be dismissed as a measurement artifact or an oddity of that particular world. The fact that the pattern repeats across several hot Jupiters points to a general circulation regime: mineral condensates form preferentially on the cooler night and morning sides and are destroyed as air crosses the intensely irradiated dayside. WASP-94A b happens to be the case where the signal is sharpest.
What scientists still don’t know
The exact composition of the morning-limb clouds has not been pinned down. Silicates, iron-bearing minerals, and aluminum oxides are all plausible condensates at the relevant temperatures, but their spectral signatures overlap heavily in the near-infrared bands JWST observed. Mid-infrared observations targeting vibrational modes specific to individual minerals could help narrow the list, but no such data for WASP-94A b have been published.
The mechanism driving the cloud cycle also remains an open question. One hypothesis points to night-side downwelling, a vertical atmospheric current that pushes gas and particles downward, controlling where and when condensates form. An alternative focuses on straightforward radiative cooling as air moves away from the hottest point on the dayside. Phase-curve observations, which track a planet’s brightness as it rotates through a full orbit, could distinguish between these scenarios by revealing whether there is a measurable lag between peak temperature and cloud dissipation. No phase-curve data for WASP-94A b have been released.
There is also the question of stability over time. The current analysis is based on a limited number of transits, effectively a snapshot. If stellar activity or deeper atmospheric dynamics shift on timescales of weeks to months, the balance between morning clouds and evening clarity could change. Repeated observations across multiple epochs would be needed to confirm whether the weather cycle is truly fixed or subject to longer-term variability.
Both the WASP-94A b study and the comparative hot-Jupiter survey are based on preprints that have not yet completed formal peer review. The underlying data come from a well-characterized instrument on a well-understood telescope, and the methods are consistent with published JWST analyses, but the results should be treated as preliminary until journal publication.
Why a daily weather report on a distant gas giant matters
For most of the history of exoplanet science, atmospheres were detected as bulk averages: a single spectrum smeared across an entire planet. The ability to split that signal into morning and evening components represents a qualitative leap. It turns a distant gas giant from a data point into something closer to a place, one with geography, circulation, and weather that changes from one horizon to the other.
WASP-94A b is not habitable by any stretch. It is a scorching, bloated world with no solid surface and temperatures that would vaporize most known materials. But the techniques used to map its weather are the same ones astronomers plan to apply to smaller, cooler planets in the years ahead. Every refinement in limb-resolved spectroscopy on a hot Jupiter brings the field closer to doing the same thing on a rocky world orbiting in a habitable zone.
For now, WASP-94A b stands as a proof of concept: a planet where astronomers can watch clouds form and disappear on a schedule, tracked from more than 600 light-years away. As JWST continues its survey of hot-Jupiter atmospheres, these limb-by-limb comparisons are shifting from novel discoveries to routine diagnostics, turning abstract points of light into worlds with skies that change by the hour.
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*This article was researched with the help of AI, with human editors creating the final content.
