Astronomers have mapped the vertical structure of Uranus’ ionosphere for the first time, revealing unexpected temperature peaks, weakening of ion density, and mysterious dark regions shaped by the planet’s extreme magnetic field. The results, made possible by nearly a full day of observations with the NIRSpec instrument aboard NASA/ESA/CSA’s James Webb Space Telescope, confirm that Uranus’ upper atmosphere has been cooling for decades and provide a rare glimpse into how the ice giant interacts with space in ways unlike other worlds in the solar system.
Tyranti others. They mapped the vertical structure of Uranus’s upper atmosphere, revealing how temperature and charged particles change with height across the planet. Image credits: NASA / ESA / CSA / Webb / STScI / P. Tiranti / H. Melin / M. Zamani, ESA & Webb.
Uranus’s upper atmosphere is one of the least understood in the solar system, despite its importance in understanding how the giant planet interacts with its space environment.
Astronomer Paola Tiranti from Northumbria University and her colleagues observed Uranus for almost an entire day using Webb’s NIRSpec instrument.
They measured the vertical structure of the ionosphere, the electrically charged layer of the atmosphere where auroras form.
“This is the first time we’ve been able to see Uranus’s upper atmosphere in three dimensions,” Tiranti said.
“Using Webb’s sensitivity, we can track how energy moves upward through the planet’s atmosphere, and even see the effects of polarized magnetic fields.”
Measurements showed that temperature peaked at about 3,000 to 4,000 km aloft, while ion density peaked around 1,000 km, much weaker than predicted by models.
Webb also detected two bright bands of auroral emission near Uranus’s magnetic poles, as well as a surprising region of depletion of both emission and density, likely related to the unusual geometry of Uranus’ gradient and offset magnetic fields.
These discoveries not only confirm that Uranus’ upper atmosphere has been cooling for decades, but also reveal new structures shaped by its magnetic environment.
Together, they provide important benchmarks for future missions and improve our understanding of how giant planets (both inside and outside our solar system) balance the energy in their upper atmospheres.
“Uranus’ magnetosphere is one of the strangest in the solar system,” Tiranti said.
“It’s tilted and offset from the planet’s axis of rotation, which means its auroras are spread across the surface in a complex way.”
“Webb showed us how deep those effects reach into the atmosphere.|
“By revealing Uranus’s vertical structure in such detail, Webb is helping us understand the ice giant’s energy balance.”
“This is an important step toward characterizing giant planets outside our solar system.”
of result Published in a magazine Geophysical Research Letters.
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Paola I. Tiranti others. 2026. JWST discovers the vertical structure of Uranus’ ionosphere. Geophysical Research Letters 53 (4): e2025GL119304;doi: 10.1029/2025GL119304