Webb maps Uranus's mysterious upper atmosphere

Written by Copernical Team Thursday, 19 February 2026 13:00

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For the first time, an international team of astronomers have mapped the vertical structure of Uranus’s upper atmosphere, uncovering how temperature and charged particles vary with height across the planet. Using NASA/ESA/CSA James Webb Space Telescope's NIRSpec instrument, the team observed Uranus for nearly a full rotation, detecting the faint glow from molecules high above the clouds. The results offer a new window into how ice-giant planets distribute energy in their upper layers.

Led by Paola Tiranti of Northumbria University in the United Kingdom, the study mapped out the temperature and density of ions in the atmosphere extending up to 5000 km above Uranus’s cloud tops, a region called the ionosphere where the atmosphere becomes ionised and interacts strongly with the planet’s magnetic field. These unique data provide the most detailed portrait yet of where the planet’s auroras form, how they are influenced by its unusually tilted magnetic field, and how Uranus’s atmosphere has continued to cool over the past three decades. The measurements show that temperatures peak between 3000 and 4000 km, while ion densities reach their maximum around 1000 km, revealing clear longitudinal variations linked to the complex geometry of the magnetic field.

“This is the first time we’ve been able to see Uranus’s upper atmosphere in three dimensions,” said Paola. “With Webb’s sensitivity, we can trace how energy moves upward through the planet’s atmosphere and even see the influence of its lopsided magnetic field.”

Webb’s data confirm that Uranus’s upper atmosphere is still cooling, extending a trend that began in the early 1990s. The team measured an average temperature of around 426 kelvins (about 150 degrees Celsius), lower than values recorded by ground-based telescopes or previous spacecraft.

Two bright auroral bands were detected near Uranus’s magnetic poles, together with a distinct depletion in emission and ion density in part of the region between two bands (a feature likely linked to transitions in magnetic field lines). Similar darkened regions have been seen at Jupiter, where the geometry of the magnetic field there controls how charged particles travel through the upper atmosphere.

“Uranus’s magnetosphere is one of the strangest in the Solar System,” added Paola. “It’s tilted and offset from the planet’s rotation axis, which means its auroras sweep across the surface in complex ways. Webb has now shown us how deeply those effects reach into the atmosphere. By revealing Uranus’s vertical structure in such detail, Webb is helping us understand the energy balance of the ice giants. This is a crucial step towards characterising giant planets beyond our Solar System.”

The study is based on data from JWST General Observer programme 5073 (PI: H. Melin of Northumbria University in the United Kingdom), which used NIRSpec’s Integral Field Unit on 19 January 2025 to observe Uranus for 15 hours. The research has been published in the Geophysical Research Letters.