Astronomers using the James Webb Space Telescope have detected for the first time the subtle signature of a planet’s rotation during transit, revealing dramatic temperature and chemical asymmetries across the atmosphere of the ultra-hot Jupiter WASP-121 b. The findings, published in Nature Astronomy, open a new window into the three-dimensional structure of exoplanet atmospheres.
WASP-121 b is an ultra-hot Jupiter located approximately 900 light-years from Earth. It orbits its F6-type host star once every 30 hours at a distance of just 1.9 stellar diameters, making it one of the most extreme exoplanets known. The planet is tidally locked, meaning one hemisphere permanently faces its star while the other faces cold space, creating a dayside heated to approximately 2,500 degrees Celsius and a nightside nearly 1,800 degrees cooler.
Led by Cyril Gapp, a PhD student at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, the research team analyzed two transits of WASP-121 b captured with JWST’s NIRSpec and NIRISS instruments. What they found were asymmetric transit light curves caused by the planet’s rotation as it crossed the face of its star.
Rotation reveals longitudes
The principle is straightforward. During a transit, different longitudes of the planet’s atmosphere rotate into view. If the atmosphere were uniform, the transit light curve would be perfectly symmetric. But WASP-121 b’s light curves showed clear asymmetry, indicating that different regions of its atmosphere have different temperatures and chemical compositions.
The team observed increasing carbon monoxide (CO) absorption and slightly decreasing water (H₂O) absorption as the planet rotated. This pattern is consistent with hotter gas rotating into view, where extreme temperatures thermally dissociate water molecules while CO remains stable.
“This suggests a stronger longitudinal temperature gradient across the evening than across the morning terminator, with higher temperatures in the eastern half of the dayside,” the authors report.
The data resolve a long-standing theoretical prediction: that tidally locked ultra-hot Jupiters should exhibit a temperature gradient across their terminators, the boundary zones between day and night sides. JWST’s sensitivity has now made this measurable.
A planet shedding its atmosphere
WASP-121 b is no stranger to the scientific spotlight. Earlier observations with JWST captured the planet’s atmosphere bleeding into space in real time, with two enormous helium tails stretching more than half the orbit: one trailing behind the planet like a comet and the other stretching ahead toward the star. Those observations, published in Nature Communications in December 2025, demonstrated that the planet is undergoing hydrodynamic escape driven by extreme stellar irradiation.
The new rotational transit data complement those findings by mapping the planet’s atmosphere from a different angle, providing a longitudinal view rather than the morning-evening limb asymmetry that previous studies had focused on.
“This is a new probe for constraining atmospheric heterogeneity using JWST, beyond differences between morning and evening terminators from limb asymmetries,” the team writes.
Implications for exoplanet science
The detection of rotational transit signatures represents a methodological advance for exoplanet atmospheric studies. Most transmission spectroscopy treats the planet’s atmosphere as a uniform annulus, averaging the signal across the entire limb. This approach misses the rich longitudinal structure that the new method reveals.
For WASP-121 b, the gradient suggests powerful zonal winds transporting heat from the substellar point toward the evening terminator, consistent with global circulation models of ultra-hot Jupiters. The dissociation of water on the hotter side implies that temperature differences of hundreds of degrees exist across distances of just a few thousand kilometers in the planet’s upper atmosphere.
The study also demonstrates JWST’s ability to resolve time-resolved phenomena during transits, a capability that future exoplanet observations can exploit. With dozens of hot and ultra-hot Jupiters known to transit bright stars, the rotational transit method could become a standard tool for mapping exoplanet atmospheres in three dimensions.