James Webb Telescope Reveals Daily Sand‑Cloud Weather Cycle on WASP‑94A b

Astronomers using the James Webb Space Telescope (JWST) have detected a remarkable weather pattern on the distant gas‑giant exoplanet WASP‑94A b, about 700 light‑years from Earth. Observations show that clouds made of sand‑like rock particles form in the planet’s “morning” atmosphere and disappear by “evening,” creating one of the clearest examples of a repeating weather cycle on a hot Jupiter.

The discovery was possible because scientists analyzed the planet’s atmosphere in an unusual way—separating the two halves of the planet’s day‑night boundary instead of averaging them together.

How JWST Detected the Weather Cycle

WASP‑94A b periodically passes in front of its star from Earth’s perspective, an event known as a transit. During a transit, some of the star’s light filters through the planet’s atmosphere. By analyzing how different wavelengths of light are absorbed, astronomers can determine the atmosphere’s composition using a method called transmission spectroscopy.

Instead of treating the entire atmospheric edge as uniform, researchers split the data into two segments:

• Ingress: the side of the planet entering the transit first, corresponding to the planet’s eternal morning limb.
• Egress: the side leaving the transit later, corresponding to its eternal evening limb.

Because hot Jupiters like WASP‑94A b are tidally locked—always showing the same face to their star—the two regions experience very different conditions. Comparing them revealed a strong asymmetry: the morning side is cooler and cloudier, while the evening side is hotter and much clearer.

Sand Clouds That Appear at Dawn and Vanish by Dusk

The clouds observed on WASP‑94A b are not made of water. Instead, they are thought to consist of magnesium‑silicate particles, essentially microscopic grains of rock similar to sand.

The observations suggest the following cycle:

1. On the planet’s cooler nightside, temperatures drop enough for silicate material to condense into cloud particles.
2. Strong winds transport these particles toward the dawn side of the planet.
3. By the time the air reaches the morning terminator, thick clouds blanket the atmosphere.
4. As the particles continue toward the intensely heated dayside, they evaporate, leaving the evening atmosphere comparatively clear.

This explains why the evening spectrum shows stronger molecular signatures—such as water vapor—because fewer clouds are blocking the view deeper into the atmosphere.

How Clouds Exist on a World Hotter Than 1,000 °C

Hot Jupiters are extremely close to their stars, with atmospheric temperatures often exceeding 1,000 °C. Yet clouds can still form because conditions vary dramatically around the planet.

Models indicate that:

• Condensate particles may form deeper in the atmosphere where temperatures and pressures allow rock‑forming materials to condense.
• Vertical mixing can loft these particles upward to higher altitudes.
• Fast global winds then transport them around the planet until they reach hotter regions, where they vaporize again.

This continuous process—condensation, circulation, and evaporation—creates a repeating day‑night cloud cycle.

Why Separating Morning and Evening Data Matters

Earlier exoplanet studies often assumed that a planet’s atmospheric edge (its terminator) had uniform properties. For tidally locked planets, that assumption turns out to be too simple.

By isolating the two halves of the terminator, researchers effectively “de‑fogged” the data. The clearer evening side provided a more accurate view of the planet’s atmospheric chemistry, allowing stronger detection of molecular features that clouds might otherwise obscure.

This approach could become a powerful method for improving atmospheric measurements of other exoplanets.

A Pattern That May Be Common on Hot Jupiters

WASP‑94A b may not be unique. Astronomers suspect similar morning‑cloudy, evening‑clear patterns could occur across many hot Jupiters.

Related JWST observations have already revealed atmospheric differences between the morning and evening sides of other worlds, including WASP‑39 b, while studies of WASP‑17 b have detected mineral clouds such as quartz‑like silica particles.

Together, these findings suggest that mineral clouds and dynamic weather systems may be common in the extreme atmospheres of hot gas giants—a reminder that even worlds hotter than molten rock can have surprisingly complex weather.