About 700 light-years from Earth, a gas giant nearly the size of Jupiter orbits so close to its star that one face is permanently scorched in daylight while the other never sees the sun at all. The planet, WASP-94A b, is tidally locked, meaning it doesn’t spin the way Earth does. Instead, fierce winds whip superheated air from the blazing dayside around to the frigid nightside and back again, creating a weather cycle unlike anything in our solar system. On the morning edge of that world, where the dark side gives way to daylight, temperatures drop low enough for vaporized rock to crystallize into thick mineral clouds. By the time the atmosphere reaches the evening edge, those clouds have cooked away, leaving comparatively clear skies laced with water vapor.
That is not a simulation. It is what the James Webb Space Telescope actually measured.
What JWST recorded at the planet’s edge
When WASP-94A b crosses in front of its host star as seen from Earth, a sliver of starlight passes through the thin ring of atmosphere at the planet’s rim. JWST’s NIRISS/SOSS instrument splits that filtered light into a near-infrared spectrum, essentially a chemical fingerprint of whatever the light encountered on its way through.
A research team led by astronomers who published their results in the journal Science in mid-2025 analyzed the two edges of that atmospheric ring separately: the leading limb (the morning side, where air is rotating into sunlight) and the trailing limb (the evening side, where air is rotating into darkness). The difference was stark. The morning limb was cooler and far more opaque, consistent with dense layers of condensed particles blocking starlight. The evening limb was clearer, and its spectrum showed strong water vapor absorption lines that the morning-side clouds had muted.
The mechanism behind this split follows directly from the planet’s locked orientation. Atmospheric circulation drives superheated gas, with temperatures around 1,500 Kelvin on the dayside, eastward around the planet. As that air passes the evening terminator, it is still hot enough to keep silicate minerals dissolved as vapor. But after crossing the cold nightside, the air cools below the condensation threshold. Mineral particles crystallize out, forming thick cloud decks that pile up along the morning terminator. Once those clouds rotate back into the searing dayside heat, the particles evaporate again, and the cycle restarts.
The result is a permanent weather gradient baked into the atmosphere by physics alone: no seasons, no day-night rotation, just a relentless conveyor belt of heat and chemistry that rebuilds the cloud cover with every orbit.
Corroboration from other hot Jupiters
WASP-94A b is not the only tidally locked giant showing this pattern. WASP-39 b, a well-studied hot Jupiter observed by JWST in earlier programs, displayed analogous morning-evening differences in its own transmission spectrum, with opacity and composition shifting from one limb to the other.
A broader comparative survey using the same NIRISS/SOSS instrument across WASP-94A b, WASP-39 b, and WASP-17 b, posted as a preprint in July 2025, reports limb-to-limb opacity differences in all three targets. The consistency suggests that the morning-cloudier pattern is not a quirk of one planet but a recurring feature among a subset of hot gas giants.
The mineral cloud interpretation also has direct spectral backing. A separate JWST program previously identified suspended quartz nanocrystals in the atmosphere of WASP-17 b, proving that silicate-based cloud particles do form at the temperatures and pressures found in hot Jupiter upper atmospheres. That detection established the instrument’s sensitivity to exactly the kind of mineral signatures now inferred on WASP-94A b’s morning limb.
Earlier modeling work from NASA’s Jet Propulsion Laboratory had predicted this outcome: the hottest tidally locked gas giants should display clouds concentrated on their morning terminators, with clearer skies on the evening side. The new JWST data convert those theoretical forecasts into observational confirmation across multiple systems.
What scientists still don’t know
The exact composition of the morning-side clouds on WASP-94A b has not been pinned down. The transmission spectrum shows high opacity consistent with condensed particles, but whether those particles are silicates, metal oxides, or some mixture remains unresolved. Future observations at additional wavelengths or across multiple orbital phases could isolate diagnostic absorption features unique to particular minerals.
The three-planet sample also raises questions it cannot yet answer. WASP-94A b, WASP-39 b, and WASP-17 b differ in equilibrium temperature, surface gravity, and host-star properties. Whether the strength of the morning-evening asymmetry scales predictably with any single parameter, such as how close a planet’s temperature sits to the silicate condensation threshold, has not been established. Three planets do not constitute a robust statistical trend, and additional hot Jupiters will need to be observed with comparable precision before broader conclusions hold up.
There is also a selection bias to acknowledge. The planets easiest to study with transit spectroscopy are large, puffy gas giants orbiting bright stars. Whether smaller or denser worlds show similar cloud asymmetries is an open question that JWST’s current target list is only beginning to address.
Why mapping alien weather matters
For most of the history of exoplanet science, distant worlds were reduced to a handful of numbers: mass, radius, orbital period. Researchers could say a planet was hot and gaseous, but not much more. The WASP-94A b result, alongside the findings from WASP-39 b and WASP-17 b, represents a qualitative leap. Astronomers can now describe whether a planet’s dawn skies are cloudier than its dusk, whether minerals condense and evaporate over the course of a single orbit, and how those cycles compare from one system to another.
That capability matters beyond hot Jupiters. The same spectroscopic techniques being refined on these large, dramatic targets will eventually be applied to smaller, rockier worlds, including those in habitable zones where clouds play a decisive role in regulating surface temperature. Every hot Jupiter whose weather JWST decodes is, in part, a dress rehearsal for the harder measurements to come.
For now, WASP-94A b stands as one of the clearest examples yet of a world whose climate can be read in starlight: a planet where rock vapor rises, crystallizes into clouds at dawn, and burns away by dusk, 700 light-years from anyone who will ever watch it happen.
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*This article was researched with the help of AI, with human editors creating the final content.
