A planet nearly 700 light-years away has clouds that form on its cooler morning side and disappear by the time the atmosphere reaches its scorching evening side. James Webb has given astronomers their clearest look yet at a world where the clouds may be made from rock-forming minerals rather than water.

WASP-94A b is too distant for the James Webb Space Telescope to photograph as a weather globe. Yet astronomers have separated two narrow slices of its atmosphere and found radically different skies. One edge is cooler and cloud-covered. The other is hotter, clearer and marked by strong water-vapour absorption.

The planet is a hot Jupiter nearly 700 light-years away in Microscopium. It circles its star in roughly four days and is thought to be tidally locked. That creates permanent day and night hemispheres, plus two boundary regions researchers call the morning and evening limbs. Webb’s result suggests mineral clouds form as atmospheric gas cools, travel into the morning boundary, and evaporate after entering the fierce heat of the dayside.

Morning and evening are places, not clock times

On Earth, sunrise and sunset sweep around the rotating planet. WASP-94A b likely keeps the same hemisphere facing its star. Its “morning” is the terminator where winds carry gas from permanent night into permanent day. Its “evening” is the opposite boundary, where heated gas flows back toward darkness.

The atmosphere keeps circulating even though the illumination pattern remains fixed. Clouds can condense in cooler regions, be lofted and transported, then vanish when their particles reach temperatures at which they can no longer remain condensed. The title’s movement from morning to evening describes that flowing air, not a daily sunrise over a solid landscape.

This distinction is central to the Science study. A tidally locked planet lets astronomers sample a weather machine whose hot and cool zones stay in approximately the same places while winds carry material between them.

Webb split one transit into two atmospheric spectra

When WASP-94A b crosses its star, it blocks a small fraction of the starlight. A still smaller amount passes through the atmosphere around the planet’s silhouette. Molecules absorb particular wavelengths, while clouds mute or reshape the pattern.

The team used Webb’s Near-Infrared Imager and Slitless Spectrograph, or NIRISS. The instrument has a mode designed for exoplanet transit spectroscopy. Rather than treat the whole atmospheric ring as one uniform spectrum, the researchers extracted information from the leading edge early in the transit and the trailing edge later.

This is not a resolved photograph of individual clouds. It is limb-resolved spectroscopy, an inference from changes in filtered starlight as different parts of the planet cross the stellar disk. The result was a six-sigma asymmetry between the limbs, strong evidence that a single average atmosphere could not explain the observation.

The cloud is observed; its mineral recipe is modelled

The team’s preprint reports an 11-sigma cloud-covered morning limb and a clear, hotter evening limb with water absorption detected at 10 sigma. Those measurements establish a strong difference in opacity and temperature.

The description “clouds made from rock-forming minerals” goes one step further. Astronomers did not collect a droplet or read a single unambiguous magnesium-silicate line. They compared the spectrum and temperatures with cloud and circulation models. Magnesium silicate, a major constituent of rocky material, is a physically plausible condensate under these conditions.

Mineral clouds are therefore the leading interpretation, not a sample-return result. The broader evidence also favours condensate clouds over a uniform photochemical haze. A haze produced mainly by stellar radiation would not naturally explain why the cooler morning limb is much more obscured than the hotter evening limb.

A temperature contrast drives the disappearing act

The models place initial cloud formation near pressures of about a millibar. Strong vertical motion can then loft droplets to roughly 0.01 millibar, high enough to influence the transmission spectrum. Winds transport them toward the illuminated hemisphere.

To reproduce the observations, the analysis required the evening limb to be at least 280 kelvin hotter than the morning limb at three-sigma confidence. That difference allows a condensate stable on the cooler side to evaporate on the hotter side.

The Johns Hopkins account of the work describes a cloud cycle rather than a one-time storm. Gas cools on the dark side, mineral material condenses and builds into the morning cloud deck, and dayside heat clears it before the flow reaches evening. Webb captured the contrast between two persistent boundaries. Models connect those boundaries into a circulation cycle.

The clear limb exposes water and a chemistry problem

Clouds are scientifically valuable, but they also hide the gases astronomers want to measure. The clear evening limb produced prominent water-vapour absorption, giving the team a less obstructed view of the hydrogen-dominated atmosphere.

Earlier observations that blended cloudy and clear regions had made WASP-94A b appear extraordinarily enriched in heavy elements. Public reporting on the new analysis says the inferred carbon and oxygen abundance moved from implausibly high values to roughly five times Jupiter’s when asymmetric cloud cover was modelled. That is much easier to reconcile with giant-planet formation.

Webb did not simply erase an old error with a sharper camera. The old spectrum mixed unlike atmospheric regions. The new method asked whether the leading and trailing parts of the transit were telling different stories, then fitted them separately.

One averaged atmosphere can manufacture the wrong planet

Transmission spectroscopy usually compresses a planet’s terminator into one profile. That is a practical approximation when signals are faint. On a tidally locked hot Jupiter, however, the ring can include one limb emerging from night and another leaving the dayside, with different temperatures, clouds and molecular feature strengths.

If a retrieval assumes both sides are identical, it may compensate for muted features by assigning the atmosphere the wrong chemical abundance. Clouds are not merely fog in front of the measurement. Their uneven distribution changes the measurement’s meaning.

A Nature report on the result describes clouds streaming and vanishing around a world 690 light-years away. More precisely, Webb measured the contrasting limbs, while atmospheric models infer the path between them. That distinction preserves what is extraordinary without turning spectroscopy into a time-lapse video.

WASP-94A b is becoming a calibration case

SpaceDaily previously covered the discovery as a daily mineral-cloud cycle on WASP-94A b. The deeper consequence is methodological: a familiar, highly irradiated giant has shown how a one-dimensional atmospheric model can misread a three-dimensional planet.

The effect may not be unique. A companion analysis of nine hot Jupiters observed with NIRISS found prominent morning-to-evening opacity differences on WASP-94A b, WASP-39 b and WASP-17 b. The sample is still small, and cloud behaviour will vary with temperature, gravity and circulation, but the pattern gives observers specific targets for testing.

WASP-94A b’s forecast is remarkable: mineral-rich cloud on the cooler morning edge and a clear, hotter evening atmosphere. Webb’s larger achievement is showing that the two edges must not be averaged away. Even when a planet remains an unresolved point beside an unresolved star, carefully timed spectra can turn its silhouette into the beginnings of a weather map.