For the first time ever, astronomers using the James Webb Space Telescope (JWST) have successfully tracked a daily weather cycle on a "hot Jupiter" exoplanet. The forecast for the gas giant WASP-94Ab? Heavy morning sandstorms, clearing up beautifully by sunset.
This discovery doesn't just give us a bizarre weather report—it solves a long-standing problem in how we measure the atmospheres of distant, heavily clouded worlds.
The World: A Scorched Giant
WASP-94Ab is located roughly 690 light-years from Earth, orbiting one of two stars in a wide binary system. It’s a textbook hot Jupiter:
| Feature | WASP-94Ab Specs |
| Size | 1.7 times larger than our Jupiter |
| Orbit | Extremely tight—completes a year every 4 days |
| Distance from Star | 5.1 million miles (8.2 million km) |
| Temperature | Over 2,200 °F (1,200 °C) |
Because it orbits so close to its host star, the heat vaporizes rock and metals. Unlike Earth's clouds, which are made of water vapor, the clouds on WASP-94Ab are composed of magnesium silicate—essentially, giant, flying sandstorms.
The Weather Report: A Tale of Two Limbs
Previously, observing hot Jupiters was frustrating for astronomers because these planets are perpetually shrouded in thick clouds, blocking our view of the gases underneath.
However, a team of astronomers led by David Sing used JWST to perform transit spectroscopy—watching how the star's light filters through the planet's atmosphere as it crosses in front of the star. They looked specifically at the "limbs" (the leading and trailing edges) of the planet and found a stark contrast:
The Morning (Leading Edge): Packed with thick magnesium silicate (sand) clouds.
The Evening (Trailing Edge): Completely clear.
Because the evening skies were cloud-free, JWST was finally able to get an unencumbered look at the hydrogen-dominated atmosphere below, revealing that WASP-94Ab is actually much more similar to our own Jupiter than previous, cloud-obscured measurements suggested.
Why Does It Clear Up?
To understand why the sand clouds vanish by evening, we have to look at the mechanics of a planet orbiting this close to its star. WASP-94Ab is tidally locked.
You can explore how tidal locking creates extreme temperature zones and drives this weather below:
Key insight: Because the planet always shows the same face to its star, it has a hemisphere of permanent, scorching daylight and one of permanent darkness.
Here is how the planetary weather machine likely works:
Creation: Strong winds at the terminator (the dividing line between day and night) blow vaporized magnesium silicate high into the atmosphere, where it cools and condenses into clouds over the nightside.
The Morning Rush: Supersonic winds blow these heavy sand clouds around to the dayside (the leading edge/morning).
The Sinking: As they cross into the intense heat of the permanent dayside, the heavy clouds gradually descend deeper into the planet's atmosphere where our telescopes can no longer see them.
The Clear Evening: By the time the air rotates around to the trailing edge (evening), the clouds have sunk out of view, leaving a clear line of sight.
This clear view has proven that hot Jupiters aren't uniformly cloudy impenetrable spheres. JWST will next aim its mirrors at planets on highly eccentric orbits to see how wild temperature swings affect alien weather patterns as planets dive close to, and retreat from, their stars.