A planet nearly 700 light-years from Earth now has one of the strangest weather forecasts in astronomy: cloudy mornings, clear evenings, and clouds made not of water, but of mineral material that can condense and disappear within the same planetary circulation cycle.

The planet is WASP-94A b, also written WASP-94Ab, a hot Jupiter in the constellation Microscopium. It is a swollen gas giant, larger than Jupiter and much closer to its star than Mercury is to the Sun. One orbit takes only about four Earth days, and the planet is likely tidally locked, with one hemisphere held in permanent starlight and the other facing space.

That means “morning” and “evening” do not mean quite what they mean on Earth. There is no rocky surface where a sunrise moves across continents. Instead, astronomers are talking about the planet’s limbs: the boundary regions between day and night that pass in front of the star during a transit. On one limb, air is moving from the cooler nightside toward the blazing dayside. On the other, it is moving back after being heated.

In May 2026, a team led by Sagnick Mukherjee and David K. Sing reported in Science that the James Webb Space Telescope had separated those two limbs well enough to show that they are not the same atmosphere. The morning limb is cooler and cloudy. The evening limb is hotter and clearer, with stronger signs of gaseous water. The simplest explanation is a repeating cycle in which silicate cloud droplets form in the cooler morning region and then evaporate as winds carry them into stronger heat.

Weather made of minerals

On Earth, clouds usually mean liquid water or ice crystals. On WASP-94A b, the temperatures are so extreme that the relevant condensates are closer to rock. Public reports on the study described the clouds as likely magnesium silicate, the kind of mineral chemistry associated with sand or stone rather than rainwater.

That does not mean boulders are floating through a blue sky. WASP-94A b is a gas giant, and the clouds are atmospheric aerosols: tiny droplets or particles suspended high above any solid surface the planet may or may not possess. The striking part is the chemistry. Materials that would be solid rock on Earth can become vapour in the intense heat of a close-in giant planet, then condense again where the gas cools.

The published analysis found a strong difference between the planet’s leading and trailing limbs. In the team’s interpretation, cloud droplets form near millibar pressures in the morning limb, where temperatures are lower, and strong vertical motion can loft them to still higher altitudes. As that same atmospheric flow circulates toward the hotter evening limb, the droplets evaporate. By the time the gas reaches the clearer side, Webb sees a hotter, less cloud-obscured atmosphere.

The researchers describe the morning side as cloud-covered and the evening side as clear enough for water vapour absorption to stand out. The preprint reports a six-sigma limb asymmetry, an eleven-sigma temperature difference between a cooler cloudy morning limb and a hotter clear evening limb, and a ten-sigma detection of water absorption on the clear side. It also argues that the two limbs differ in temperature by at least 280 kelvin.

How Webb saw a planet it cannot image

WASP-94A b is not resolved as a disk in a telescope image. Astronomers studied it using transit spectroscopy. When the planet passes in front of its star, a tiny fraction of the starlight filters through the planet’s atmosphere. Molecules, clouds and hazes absorb or scatter different wavelengths, leaving a fingerprint in the starlight that reaches Webb.

The technical advance here is limb-resolved transmission spectroscopy. Instead of treating the whole edge of the planet as a single averaged ring of gas, the team compared the morning and evening sides separately. That distinction matters because a tidally locked hot Jupiter can have ferocious atmospheric contrasts. Its dayside is constantly roasted. Its nightside cools. Winds move gas between those regimes, so the limb that has just emerged from night can look very different from the limb that has just endured the dayside.

Earlier Hubble observations of the same planet had blurred these regions together. If cloudy and clear limbs are averaged into one spectrum, the result can make the atmosphere look chemically stranger than it really is. One consequence discussed in public reporting is that the planet’s inferred heavy-element abundance looked much higher when the two sides were not separated. Webb’s cleaner split brought the composition estimate back toward a more plausible value for a giant planet.

That is a warning for exoplanet science. Clouds are not just a nuisance that hides useful chemistry. They are part of the atmospheric system being measured. If they form on one side of a planet and vanish on the other, then a single averaged spectrum can mix two different weather states into one misleading answer.

Not just haze

Many hot-Jupiter spectra have been complicated by aerosols, a broad term that includes clouds and hazes. Hazes can be produced by photochemistry, as radiation from a star breaks molecules apart and drives new reactions. Clouds can form when gases cool enough to condense. Distinguishing those possibilities matters because they point to different physics.

The WASP-94A b result supports the cloud interpretation. The dominant aerosols are not best explained as a uniform photochemical haze spread around the planet. They look like condensate clouds cycling between cooler and hotter regions. They appear where the temperature allows them to exist and disappear where the gas becomes too hot for them to survive.

That makes the planet a natural laboratory for weather under conditions that have no Earthly analogue. Its atmosphere is hydrogen-rich, its orbit is extremely tight, and its thermal contrast is set by permanent day and permanent night rather than by a 24-hour rotation. Yet the underlying idea is familiar: temperature, winds and condensation combine to make clouds. The ingredients are alien, but the atmospheric logic is recognisable.

Why one cloudy planet matters

Hot Jupiters are not candidates for life, but they are valuable test cases because they are large, hot and relatively easy to study. If instruments and models cannot correctly interpret the spectrum of a giant planet like WASP-94A b, they will struggle even more with smaller worlds where the signals are fainter.

That is why the daily rock-cloud cycle is more than a curiosity. It shows that exoplanet atmospheres can change sharply across the very ring of gas that astronomers use to identify molecules. A planet can have one limb that is cloudy and cool, and another that is clear and hot, while standard observations may try to compress both into a single atmospheric profile.

Webb is beginning to loosen that compression. By separating different slices of a transiting planet’s atmosphere, it can turn an unresolved point of light into something closer to a weather map. Not a map with continents and oceans, but a map of circulation, temperature and cloud formation around a world hundreds of light-years away.

For WASP-94A b, that map is stark. In the morning, mineral clouds form high in the atmosphere. By evening, the heat has erased them. The forecast repeats not because the planet is Earthlike, but because its locked orbit and violent winds keep feeding the same atmospheric machine: rock vapour, cold limb, cloud, heat, clear sky.

Sources

Mukherjee et al., Cloudy mornings and clear evenings on a gas giant exoplanet, Science, 2026
arXiv preprint: Cloudy mornings and clear evenings on a giant extrasolar world
Space.com report on JWST’s WASP-94Ab weather observations
The Times report on charting weather on WASP-94A b
Times of India summary of the May 2026 Science study