For the first time, astronomers have observed a daily weather cycle on a planet, showing thick clouds in the morning and a clear sky by evening. Using the James Webb Space Telescope (JWST), scientists gained an unobstructed view of WASP-94Ab’s atmosphere, allowing more precise measurements of its chemical composition.
WASP-94Ab is located about 690 light-years from Earth, orbiting one star in a wide binary system. The planet is a gas giant 1.7 times larger than Jupiter and completes an orbit every four days at a distance of 5.1 million miles (8.2 million kilometers) from its star. Temperatures exceed 2,200 degrees Fahrenheit (1,200 degrees Celsius), making it a typical hot Jupiter, where extreme heat strongly influences atmospheric behavior.
Studying hot Jupiter atmospheres has long been difficult because clouds of vaporized metals and rock obscure direct observations.
Transit Spectroscopy Uncovers Cloud Patterns
David Singof Johns Hopkins University has described these clouds as “a thorn in our side” for decades. To investigate the planet, Sing’s team employed transit spectroscopy, watching WASP-94Ab as it passed in front of its star from JWST’s vantage point.
“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,” he explained in a peer-reviewed publication.
This technique examines starlight filtered through the atmosphere, revealing gases and cloud layers. The study published in Sciencereports that the leading edge, the “morning” side where air moves from night to day, contained substantial magnesium silicate clouds. The trailing “evening” side, by contrast, was mostly clear.
“It was really surprising how different the two halves of the same planet are,” Sagnick Mukherjee, who led the study as a postdoctoral fellow at Arizona State University, stated. “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.”
This separation corrected previous misconceptions. Earlier Hubble observations could not distinguish between the leading and trailing edges, leading to overestimates of oxygen and carbon hundreds of times greater than Jupiter’s levels. JWST measurements revealed that these elements are only about five times more abundant than on Jupiter.
How Water Travels Above Us
The cloud cycle onWASP-94Ab is likely tied to its tidal locking, which keeps one hemisphere in permanent daylight and the other in constant darkness. Strong winds at the boundary between day and night can lift magnesium silicate clouds into the atmosphere on the nightside. These clouds then drift to the dayside, where heat causes them to sink out of view, before returning to the nightside as circulation continues.
Another possibility is that the extreme heat gradually dissipates the clouds during the day, much like morning fog on Earth. The study highlights that either explanation fits the observed data, showing a dynamic system of condensation and evaporation that changes throughout a single day.
These observations demonstrate that hot Jupiter atmospheres are not uniform and that cloud coverage can shift rapidly. Sing noted that:
“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.”
Applying The Technique To Other Exoplanets
Following WASP-94Ab, the study team observed eight more hot Jupiters with JWST. Two of them, WASP-17b and WASP-39b, exhibited similar daily cloud patterns. WASP-17b is a low-density gas giant with a bloated atmosphere and a retrograde orbit, while WASP-39b has a water-rich atmosphere along with carbon and sulfur dioxide.
These findings suggest that daily cloud cycles may be common among hot Jupiters, though their chemical makeup and atmospheric density produce unique patterns on each planet.
The researchers plan to extend the survey to planets with highly eccentric orbits, where dramatic temperature changes could drive extreme weather visible to JWST. The publication from May 21 noted that these observations mark a milestone in exoplanet research, showing that telescopes like JWST can reveal how they behave over time.
Enjoyed this article? Subscribe to our free newsletter for engaging stories, exclusive content, and the latest news.
