JWST reveals shifting clouds on distant tidally locked planet

JWST maps – Using the James Webb Space Telescope, researchers studying WASP-94A b found a planet-wide weather pattern that changes from morning clouds to evening clear skies. The discovery points to a major risk in how astronomers interpret exoplanet atmospheres—when they

On WASP-94A b, the sky doesn’t behave like a single, unchanging blanket of gas. It shifts—cloudy in the morning and clearing by the evening—according to a new study that used the James Webb Space Telescope to read the atmosphere of a hot gas giant about 690 light-years from Earth.

The planet, WASP-94A b, circles close to one of the stars in a binary system. It is a hot, tidally locked gas giant, meaning the same side of the planet stays facing its star. That lock matters: without the usual sweep of day and night across a rotating world. temperature patterns can settle into something different—more static in some ways. more dynamic in others.

“We wanted to understand the atmospheres of such planets,” said Sagnick Mukherjee, an astrophysicist at Johns Hopkins University, who led the research. “Are they static or dynamic? Do they have winds? Do they have clouds?”

Until now. the answers for WASP-94A b weren’t clear enough to say what the weather actually looked like across the planet’s changing conditions. The study’s team argues that not knowing it already means astronomers may have been getting the chemistry of this and other exoplanets “surprisingly wrong.”.

A big reason is how most atmosphere studies are done. When astronomers use transmission spectroscopy, they study the spectrum of light filtering through a planet’s atmosphere as the planet crosses in front of its star. In principle, that lets researchers infer chemical composition.

But transmission spectroscopy has a built-in simplification: the light passing through the planet’s silhouette is effectively averaged across the entire circumference, as if the atmosphere were one homogenous ball.

For tidally locked planets, that average can hide crucial structure. These worlds often experience massive temperature swings between the day side and the night side. Those temperature differences can drive atmospheric density differences across the planet. Add the Coriolis effect from a planet’s slow rotation. and the result can be a phenomenon called equatorial super-rotation—winds on the equator blowing eastward faster than the planet is spinning.

Circulation models predicted that this exact kind of equatorial super-rotation was happening on WASP-94A b.

The planet itself makes the atmosphere easier to study than some gas giants. WASP-94A b has a mass slightly below half of Jupiter but a diameter more than 70 percent wider. That combination points to low density and an atmosphere that extends further outward into space. which can make observation more straightforward.

In one paragraph, the key sequence becomes hard to ignore: the averaging method treats the atmosphere as uniform, while tidally locked planets can produce day-to-night density differences and wind-driven super-rotation; when those moving features aren’t resolved, the chemical signals can be skewed.

For Mukherjee and the team. the new James Webb results provide a missing piece—evidence that what looks like “weather” at one time of day can give way to something else later. And if the clouds can clear on WASP-94A b from morning to evening. the light astronomers collect during a transit may not be painting a simple chemical portrait of a steady atmosphere.

The study’s central warning is plain: if researchers don’t account for how tidally locked atmospheres shift. they may average away the very dynamics that shape what those planets are made of. With WASP-94A b. the telescope has now shown that the sky can change on a world locked in place by gravity—reminding scientists that even distant planets can have rhythms that get lost when observations assume sameness.

JWST James Webb Space Telescope WASP-94A b tidally locked planet exoplanet atmospheres transmission spectroscopy equatorial super-rotation clouds equatorial winds astrophysics