Astronomers using the James Webb Space Telescope have discovered a striking weather cycle on a distant so-called “Hot Jupiter” exoplanet, revealing cloudy mornings and clear evenings on a world 689 light-years from Earth. The findings offer one of the clearest looks yet at the atmosphere of a giant exoplanet and could reshape how scientists study alien worlds.

The planet, known as WASP-94A b, is a gas giant orbiting extremely close to its host star in the constellation Microscopium, which in May becomes visible low in the southern sky just before sunrise, as seen from the Northern Hemisphere. Hot Jupiter planets orbit even closer to their stars than Mercury orbits the sun, so they are much hotter and are exposed to more radiation than most exoplanets.

Researchers found that on WASP-94A b, mineral clouds form overnight on the planet's cooler side before dissipating under intense daytime heat. The study, led by researchers from Johns Hopkins University and published in the journal Science , represents one of the first detections of cloud cycling on a Hot Jupiter exoplanet.

According to NASA , WASP-94A b was first discovered in 2014 and has a mass half that of Jupiter, taking 4 days to orbit its star. Its star is part of a binary system, where two stars orbit each other.

A Planet Of Extreme Weather

WASP-94A b is tidally locked, meaning one side of the planet permanently faces its star while the other remains in darkness. This creates enormous temperature differences across the planet, with scientists estimating contrasts of roughly 350 degrees Fahrenheit (177 degrees Celsius) between hemispheres.

Using the Near Infrared Imager and Slitless Spectrograph aboard the James Webb Space Telescope, researchers analyzed the planet as it passed in front of its star. That allowed them to separately examine the planet’s “morning” and “evening” atmospheric regions.

The results revealed a stark difference. The morning side was covered in thick magnesium silicate clouds, while the evening side showed relatively cloud-free skies with strong water vapor signals.

Clouds That Form And Vanish Daily

Scientists believe the clouds form on the planet's cooler nightside before circulating toward the dayside via powerful atmospheric winds. Once exposed to temperatures exceeding 1832 degrees Fahrenheit (1,000 degrees Celsius), the cloud-forming minerals evaporate. Researchers compared the process to morning fog burning off on Earth, although on a much more extreme scale.

Cloudiness has thwarted attempts to study exoplanets for decades. “We’ve known for quite a while that clouds are pervasive on Hot Jupiter planets, which is annoying because it’s like trying to look at the planet through a foggy window,” said David Sing, a Bloomberg Distinguished Professor at Johns Hopkins and co-author of the study. “Not only have we been able to clear the view, but we can finally pin down what the clouds are made out of and how they’re condensing and evaporating as they move around the planet.”

Revising Earlier Assumptions

Earlier observations made with the Hubble Space Telescope suggested WASP-94A b contained hundreds of times more oxygen and carbon than Jupiter, a result that conflicted with current theories of planetary formation. However, the new, clearer JWST data revealed that the planet contains only about five times as much oxygen and carbon as Jupiter, bringing the findings more in line with existing models.

The ability to distinguish between different atmospheric regions marks a major advance in exoplanet science. “With the Hubble telescope, when we used to do this type of observation, we got an average view of the whole planet with data from the clouds and the atmosphere squished together and indistinguishable,” said lead author Sagnick Mukherjee, now a postdoctoral fellow at Arizona State University. “This approach with the JWST lets us localize our observations, which helped us see the cloud cycle.”

The team also identified similar cloud cycles on two additional Hot Jupiters, WASP-39b and WASP-17b, suggesting the phenomenon may be common among gas-giant exoplanets.

Wishing you clear skies and wide eyes.