On the giant exoplanet WASP-94A b, clouds of rocky minerals gather in the sky every morning. In the evening, the clouds disappear.
Using observations from the James Webb Space Telescope (JWST), astronomers discovered this dramatic daily weather cycle on a distant world located in the constellation Microscope, nearly 700 light-years from Earth. The discovery marks one of the first times scientists have directly observed cloud circulation on a hot Jupiter exoplanet.
The discovery gives researchers a clearer picture of the planet’s atmosphere, giving them a better understanding of what the world is made of and how its weather changes. The study was published in the journal science.
“I’ve been observing exoplanets for 20 years, and overall cloudiness has been a thorn in our side. We’ve known for some time that hot Jupiter planets have clouds, but it’s annoying because it’s like trying to see the planet through a foggy window,” said co-author and program PI David Singh, Bloomberg Distinguished Professor of Earth and Planetary Sciences at Johns Hopkins University. “Not only were we able to clear the view, but we were also able to finally determine what clouds are made of and how they condense and evaporate as they move around the Earth.”
WASP-94A abnormal weather b
To study WASP-94A b, scientists watched the planet pass in front of its host star. During this pass, JWST was able to separately examine the leading and trailing edges of the planet as it moves across the starlight.
The leading edge represents the morning side of the planet, where atmospheric winds carry air from the cool night side to the intensely hot day side. The trailing edge acts as the evening side, where air moves back toward darkness.
Observations revealed a marked difference between morning and evening conditions. The morning side was filled with clouds made of magnesium silicate, a mineral commonly found in rocks on Earth. However, there seemed to be almost no clouds in the evening.
Researchers believe there are two possible explanations for the cloud’s disappearance. One idea is that powerful winds drag clouds deep into Earth’s atmosphere during the scorching heat of the day, effectively hiding them from view. Another possibility is that clouds evaporate as they move to temperatures above 1,000 degrees, similar to the morning fog that burns off on Earth, but under much more extreme conditions.
“That was a big surprise. People were expecting some differences, such as being cooler in the morning than in the evening, but that’s a natural thing we experience here on Earth,” Singh said. “But what we saw was a true dichotomy between huge differences in weather and cloud cover on both sides of the globe, which changed the entire picture of the planet.”
James Webb looks into an alien cloud
The brighter evening sky gave scientists opportunities not possible with older telescopes such as Hubble. By isolating the cloud-free side of the Earth, researchers were able to directly examine the atmosphere itself, rather than averaging cloudy and clear areas together.
“When we were making these kinds of observations with the Hubble Telescope, we were getting an average view of the entire planet with data that was indistinguishable because the clouds and atmosphere were squashed,” said lead author Sagnik Mukherjee, a postdoctoral researcher at Arizona State University. He was a student at Johns Hopkins University and the University of California, Santa Cruz at the time of the study. “This approach using JWST allowed us to localize observations, which helped us understand cloud cycles.”
The clearer data also solved long-standing mysteries about the planet’s chemistry. Early measurements suggested that WASP-94A b contained hundreds of times more oxygen and carbon than Jupiter, which was inconsistent with existing theories of planet formation.
New observations paint a very different picture. Scientists now estimate that the planet contains only about five times as much oxygen and carbon as Jupiter, and is thought to be much more similar to the giant planets of our solar system than previously believed.
A new window into the alien atmosphere
A hot Jupiter is a giant gas planet that orbits extremely close to its star, even closer than Mercury orbits the Sun. With their intense heat and radiation, these planets provide ideal natural laboratories for scientists to study atmospheric chemistry and cloud behavior under extreme conditions.
After studying WASP-94A b, the research team examined eight more hot Jupiters and identified similar cloud circulation on two more worlds, WASP-39 b and WASP-17 b.
Next, the researchers plan to expand their search with the larger JWST observation program, which will investigate cloud cycles across many different exoplanets, including an unusual gas giant that passes through the habitable zone in an eccentric orbit.
