JWST maps the weather on a hot gas giant 700 light-years away

WASP-94A b is a hot, tidally locked gas giant in a binary system about 690 light-years away. James Webb Space Telescope observations reveal cloudy conditions in the morning and clearer skies in the evening. The planet’s low density and extended atmosphere make it easier to observe. Atmospheric studies often use transmission spectroscopy, which measures starlight filtered through the planet’s silhouette during transit. That method averages light across the entire circumference, treating the atmosphere as uniform. For tidally locked planets, day-night temperature differences and resulting density variations can produce non-uniform atmospheric properties, so averaging can lead to incorrect chemical interpretations.

"WASP-94A b is a hot, tidally locked gas giant orbiting close to one of the stars in a binary system roughly 690 light-years away from Earth. In a new Science study, scientists led by Sagnick Mukherjee, an astrophysicist at Johns Hopkins University, used the James Webb Space Telescope to learn what the weather looks like out there."

"His team found that, on WASP-94A b, it's cloudy in the morning, but the skies clear in the evening. The fact that we didn't know this already means we might have gotten the chemistry of this and many other exoplanets surprisingly wrong."

"When astronomers study atmospheres like this, they usually rely on transmission spectroscopy. By analyzing the spectrum of light filtering through the planet's atmosphere as it crosses in front of its star, they can figure out its chemical composition. The problem with this approach is that the light filtering through the entire circumference of the planet's silhouette was averaged out, as though its atmosphere was one homogenous ball of gas."

"On tidally locked worlds, there are massive temperature swings between day and night sides, which usually lead to differences in atmospheric density between the day side and the night side. These differences, combined with the Cori"

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