Webb Reveals Hidden Layers of Uranus’ Upper Atmosphere

For the first time, astronomers have mapped the vertical structure of Uranus’ ionosphere, revealing unexpected temperature spikes, weakened ion densities and puzzling 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 the NASA/ESA/CSA James Webb Space Telescope, confirm that Uranus’ upper atmosphere has been cooling for decades and offer a rare glimpse into how the ice giant interacts with space in a way unlike any other world in the solar system.

Tiranti and others. mapped the vertical structure of Uranus’ upper atmosphere, discovering how temperature and charged particles vary with height across the planet. Image credit: NASA / ESA / CSA / Webb / STScI / P. Tiranti / H. Melin / M. Zamani, ESA & Webb.

Uranus’ upper atmosphere is one of the least understood in our solar system, although it is essential for understanding how giant planets interact with their space environment.

Using Webb’s NIRSpec instrument, astronomer Paola Tiranti of the University of Northumbria and her colleagues observed Uranus for almost an entire day.

They measured the vertical structure of its ionosphere – the charged layer of the atmosphere where auroras form.

“This is the first time we are able to see the upper atmosphere of Uranus in three dimensions,” Dr Tiranti said.

“Thanks to Webb’s sensitivity, we can trace how energy moves upward in the planet’s atmosphere and even see the influence of its unbalanced magnetic field.”

Measurements show that temperatures peak between 3,000 and 4,000 km above the planet, while ion densities peak at almost 1,000 km and are significantly lower than predicted by models.

Webb also detected two bright bands of auroral emission near Uranus’s magnetic poles, as well as a surprising region where emission and density are depleted, likely related to the unusual geometry of Uranus’ tilted and offset magnetic field.

These findings 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 essential benchmarks for future missions and improve our understanding of how giant planets (in our solar system and beyond) balance energy in their upper atmospheres.

“Uranus’ magnetosphere is one of the strangest in the solar system,” Dr Tiranti said.

“It is tilted and offset from the planet’s axis of rotation, which means its auroras sweep across the surface in complex ways.”

“Webb has now shown us how deeply these effects ripple through the atmosphere. |

“By revealing the vertical structure of Uranus in such detail, Webb helps us understand the energy budget of the ice giants.”

“This is a crucial step toward characterizing giant planets beyond our solar system.”

The results were published in the journal Geophysical research letters.

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Paola I. Tiranti and others. 2026. JWST discovers the vertical structure of Uranus’ ionosphere. Geophysical Research Letters 53 (4): e2025GL119304; doi: 10.1029/2025GL119304