JWST Reveals Daily Cloud Cycles on Distant Hot Jupiter, Reshaping Exoplanet Models

Seven hundred light-years away, in the constellation Microscopium, a gas giant called WASP-94A b orbits so close to its star that surface temperatures exceed 1,000 degrees Celsius. For years, astronomers studying this world and others like it have been frustrated by a simple problem: clouds. They obscure the view, making it nearly impossible to measure what an exoplanet's atmosphere is actually made of. Now, using the James Webb Space Telescope, an international team has found a way through the haze—and in doing so, they've discovered something unexpected about how weather works on distant worlds.

The breakthrough came from a technique called transit spectroscopy, which works by watching a planet pass in front of its star. As light from the star travels through the planet's atmosphere, different chemical elements absorb different wavelengths. By analyzing which light gets blocked, astronomers can determine composition. What made this study different was precision. Webb's instruments allowed the researchers, led by Sagnick Mukherjee at Arizona State University, to measure the leading edge of WASP-94A b separately from its trailing edge as the planet crossed in front of its star. This seemingly small distinction revealed something remarkable: the planet's morning and evening skies are completely different.

On the morning side—where the planet faces the star after its night—the atmosphere is thick with magnesium silicate clouds. By evening, those same skies are clear. The team observed air currents flowing from the night side toward the day side at the leading edge, then reversing direction at the trailing edge. This circulation pattern, combined with the extreme heat, appears to drive a daily cloud cycle. The researchers propose two mechanisms. One involves powerful winds lifting clouds on the cooler night side, then pushing them over the terminator into the scorching dayside, where temperatures vaporize them before they can drift back toward evening. The other is simpler: clouds form at night and burn away each morning as the sun's heat intensifies, much like fog evaporating on Earth but at a scale and temperature almost incomprehensible.

What matters most is what the clear evening sky revealed. Previous measurements suggested WASP-94A b had far more oxygen and carbon in its atmosphere than Jupiter itself—a finding that contradicted existing models of how planets form. The new data, freed from cloud interference, tells a different story. The planet has only about five times Jupiter's abundance of these elements, a ratio that aligns with current theory. "I've been looking at exoplanets for 20 years, and general cloudiness has been a thorn in our side," said David Sing, a Johns Hopkins researcher and principal investigator of the observation program. "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. 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."

The team's success depended entirely on Webb's capabilities. The Hubble Space Telescope, despite decades of service, lacked the sensitivity and resolution needed to isolate the trailing edge where nighttime skies were clear. Webb's advanced optics made the difference. The research, published in Science on May 21st, represents one of the first detections of cloud cycles on any hot Jupiter. The team tested their method on eight other hot gas giants and found the same pattern on at least two: WASP-39 b and WASP-17 b. This suggests the phenomenon may be common among planets in this class.

The implications ripple outward. As the exoplanet census grows—now standing at over 6,200 confirmed worlds across nearly 4,700 systems—the field is shifting from discovery toward understanding. Characterizing atmospheres, mapping weather patterns, and testing planetary formation models all depend on seeing through the clouds. The technique developed here offers a template. Harry Baskett, a PhD student from the University of Exeter, emphasized the power of combining observations with three-dimensional atmospheric simulations developed in partnership with the UK Met Office. "It is really exciting to be able to combine observations and 3D simulations to distinguish weather patterns on exoplanets, infer the presence of clouds and constrain their formation mechanisms," he said. The team plans to apply this method across a wider range of exoplanets using data from existing Webb observation programs. Each world studied brings astronomers closer to understanding not just distant atmospheres, but the fundamental physics that shapes planetary climates—knowledge that feeds back into improving models of Earth's own weather and climate.