JWST spots vaporized-rock clouds only on one side

JWST puffy – NASA’s James Webb Space Telescope has produced the first detailed “weather report” for the hot Jupiter WASP-94A b, showing that puffy clouds made of vaporized rock appear along the planet’s morning edge but largely clear up by the evening—an uneven pattern tie

Morning on WASP-94A b doesn’t look like morning on Earth.

For the first time. NASA’s James Webb Space Telescope has revealed how clouds shift across a distant gas giant nearly 700 light-years from Earth—without needing to actually see the planet directly. Instead. astronomers used subtle signals from the way the planet blocks and filters starlight as it circles its star. building a picture of one half of the world turning cloudy while the other half stays mostly clear.

WASP-94A b is a “hot Jupiter” that completes an orbit every four days. That short loop produces a stellar wobble that helps scientists pin down the planet’s mass and orbital period. It also creates a repeating transit: every four days. the planet crosses the face of its star. casting a shadow that slightly dims the star’s light. By carefully monitoring these transits. researchers can measure the planet’s size and probe its atmosphere through spectra—looking for which chemicals are present in the gas by analyzing starlight streaming through the planet’s upper atmosphere.

Dense, starlight-blocking clouds can interfere with those measurements. The twist for WASP-94A b is that the planet is tidally locked, like the moon’s relationship with Earth. Its spin and orbit are synchronized. meaning one hemisphere always faces its stellar host—eternal day on one side and eternal night on the other.

That lock becomes an advantage. As the planet moves around its star, astronomers can observe the edges of these eternal day and night halves separately: the leading “morning” side and the trailing “evening” side. In Webb’s observations, those two regions come out strikingly different.

Along the morning dayside edge, the data show puffy clouds. Toward the nightside’s evening edge, the clouds are almost absent, leaving comparatively clear conditions in the early evening.

The clouds aren’t expected to resemble the wispy formations we’re used to. Instead, they’re thought to be composed of magnesium silicate, iron, and magnesium sulfide—essentially vaporized rock.

These findings, aimed at improving how scientists understand the atmosphere and weather of WASP-94A b and other hot Jupiters, appear today in the journal Science.

“It was really surprising how different the two halves of the same planet are. ” said Sagnick Mukherjee. lead author of the study and a postdoctoral fellow at Arizona State University. “What this tells us is: if we don’t know about the weather cycles on these distant planets. we won’t be able to measure their composition well.”.

The reason for the split comes back to the way the planet’s constant day and night translate into temperature contrasts. Even though the planet keeps the same side facing its star. the temperature difference between the dayside and nightside can drive strong winds. In the study’s interpretation. air circulates and allows clouds to build through the night from cooler conditions. then dissipate in the bright morning light.

Heather Knutson. an exoplanetary scientist at the California Institute of Technology who was not involved in the new study. described what the telescope captured as direct and readable. “It’s just beautifully clear in the data,” she said. “You can, at a glance, see the difference” in the hemispheres’ weather.

WASP-94A b’s uneven cloud cover adds to a broader shift in how exoplanet atmospheres are treated. Cloud patterns may be patchy and changing rather than uniform across the whole planet. Knutson emphasized that if astronomers rely too heavily on simplified assumptions, they can end up chasing the wrong answers. “When we use models that are too simple to interpret our data. we can get tricked into thinking we have the right answer when actually we have the wrong answer. ” she said. “We know that many exoplanets have clouds. and clouds really don’t like to be uniform across the whole planet… so we really need to account for the fact that the atmosphere is not globally the same everywhere.”.

The team has already used Webb to study atmospheric dynamics on two other transiting hot Jupiters: WASP-39 b and WASP-17 b. And as next-generation telescopes arrive. the work could eventually push weather-style studies toward smaller. more Earth-like worlds—where the same question. of what the atmosphere is doing in different regions at different times. will matter just as much.

For now, WASP-94A b offers a dramatic lesson in how quickly an exoplanet can “change its sky.” On one side, puffy clouds rise from vaporized rock; on the other, the light clears enough to reveal something else beneath it—if astronomers know where to look.

James Webb Space Telescope JWST WASP-94A b hot Jupiter exoplanet weather tidally locked clouds magnesium silicate magnesium sulfide iron atmospheric spectroscopy Science journal