Nearly 700 light-years from Earth, a gas giant called WASP-94A b experiences something that looks like clockwork weather: clouds thick with magnesium silicate mineral rise every morning on its nightside, drift across the planet with the wind, and vanish completely by evening, leaving clear skies in their wake. For the first time, astronomers using the James Webb Space Telescope have captured this daily cloud cycle on a Hot Jupiter exoplanet, and the discovery is rewriting how scientists understand these distant worlds.
For two decades, clouds have been one of the most frustrating obstacles in exoplanet research. "I've been looking at exoplanets for 20 years, and general cloudiness has been a thorn in our side," said David Sing, a Bloomberg Distinguished Professor of Earth and Planetary Sciences at Johns Hopkins University and co-author of the research published in the journal Science. "It's like trying to look at the planet through a foggy window." But by isolating when and where clouds appear on WASP-94A b, Sing's team has finally pierced that fog—literally and metaphorically.
The breakthrough came from a clever observational strategy. As WASP-94A b passed in front of its star, the researchers took separate measurements of the planet's leading edge (its morning side, where air flows from night to day) and its trailing edge (its evening side, where air flows from day to night). The contrast was stark. The morning showed clouds everywhere; the evening revealed clear skies. This extreme dichotomy between dawn and dusk suggests one of two processes at work. Powerful winds might carry clouds high into the cooler nightside atmosphere, then plunge them downward into the scorching 1,000-degree heat of the dayside, burying them deep inside the planet before sunset. Or, in a phenomenon Sing likens to morning fog burning off Earth but on an extreme scale, the clouds might simply vaporize as the chemicals that compose them boil away in the intense daytime heat.
The practical value of these clear evenings cannot be overstated. With the older Hubble Space Telescope, researchers could only see an averaged view of the entire planet, with clouds and atmosphere blended together indistinguishably. "This approach with the JWST lets us localize our observations, which helps us see the cloud cycle," said Sagnick Mukherjee, the first author of the study and a postdoctoral fellow at Arizona State University. By focusing on the cloudless evening sky, the team could finally measure WASP-94A b's true atmospheric composition—and the results surprised them. Earlier observations suggested the planet contained hundreds of times more oxygen and carbon than Jupiter, a finding that contradicted planet formation theory. But with clouds removed from the equation, the real data showed only five times the amount of these elements. WASP-94A b, it turns out, is far more like Jupiter than anyone had realized.
This discovery matters beyond WASP-94A b. Hot Jupiters—planets orbiting so close to their stars that they experience extreme heat and radiation—serve as natural laboratories for understanding cloud chemistry and atmospheric physics in extreme conditions. With this new method in hand, astronomers can now begin clearing the fog on countless other distant worlds, revealing what has been hidden beneath the clouds all along.