James Webb found a planet where rock clouds form each morning and clear by night

The morning sky of WASP-94A b is thick with cloud. By the time the same air drifts around to the planet’s evening side, the cloud is gone. And the cloud is not water or ice. It is rock — magnesium and silicon, vaporized by heat and condensed into a mineral haze that forms at dawn and burns off by dusk.

That split is the discovery. Astronomers using the James Webb Space Telescope have, for the first time, read the weather on the two opposite faces of a single planet beyond our solar system and found them sharply different. One hemisphere builds clouds; the other clears them.

WASP-94A b is a hot gas giant about 700 light-years away, in the southern constellation Microscopium. It belongs to a class of planets nicknamed hot Jupiters: huge, gaseous, and parked so close to their star that a year lasts only a few Earth days. The day side runs well past 1,000 degrees Fahrenheit, hot enough to keep heavier elements aloft as vapor. Where that vapor cools, it condenses into specks of silicate — the stuff of sand and rock — and those specks form the clouds.

Webb did not photograph any of this. As the planet crossed in front of its star, the telescope split the starlight that filtered through the atmosphere at its edges, once where the world turns from night into day and once where it turns from day into night. The morning edge carried the fingerprint of mineral cloud; the evening edge came back clearer. The same readings showed the air holds roughly five times more oxygen and carbon than Jupiter, a clue to how and where the planet formed.

A reading like this is an inference, not a snapshot. The cloud map depends on atmospheric models that turn a spectrum into temperatures, chemistry, and particle sizes, and different models can disagree at the edges. This is one planet, seen across a handful of transits, and the lopsided weather will need repeat looks before it counts as a settled portrait. What the data show plainly is that the two sides are not the same.

The result matters beyond one strange world. Most measurements of exoplanet air treat a planet as a single uniform ball. WASP-94A b shows that assumption can hide a world split into contradictory halves, which means the weather maps astronomers are starting to draw for other planets may be blending climates that have little in common.

The team behind the work plans to turn the same method on other hot Jupiters, comparing morning and evening skies across a set of planets to learn whether this one-sided cloud cover is a quirk of WASP-94A b or a common feature of worlds that orbit too close to their stars.

Tags: astronomy, exoplanet, James Webb Space Telescope