Astronomers have used the James Webb Space Telescope to create the first three-dimensional map of Uranus's upper atmosphere, revealing details about its temperatures, charged particles, and auroras. The observations highlight the influence of the planet's tilted magnetic field and confirm ongoing cooling in its ionosphere. This data provides new insights into how ice giant planets manage energy.
An international team of researchers, led by Paola Tiranti from Northumbria University in the United Kingdom, employed the James Webb Space Telescope's NIRSpec instrument to observe Uranus on January 19, 2025. Over 15 hours, nearly covering one full rotation of the planet, they captured faint molecular emissions from altitudes up to 5,000 kilometers above the cloud tops. This allowed mapping of the ionosphere, where the atmosphere is ionized and affected by magnetic forces.
The data show temperatures peaking between 3,000 and 4,000 kilometers above the clouds, with ion densities highest around 1,000 kilometers. An average temperature of 426 kelvins (about 150 degrees Celsius) was recorded, lower than previous measurements from ground-based observatories or earlier missions, confirming that Uranus's upper atmosphere has been cooling for the past 30 years.
Webb's observations revealed two bright auroral bands near the magnetic poles and darker regions in between, linked to the planet's unusually tilted and offset magnetosphere. "This is the first time we've been able to see Uranus's upper atmosphere in three dimensions," Tiranti said. "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."
The magnetosphere's odd configuration causes auroras to sweep across the surface in complex patterns, with effects extending deep into the atmosphere. "Uranus's magnetosphere is one of the strangest in the Solar System," Tiranti added. "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."
These findings, from JWST General Observer program 5073 led by principal investigator H. Melin, were published in Geophysical Research Letters in 2026. The James Webb Space Telescope, a collaboration between NASA, the European Space Agency, and the Canadian Space Agency, continues to advance understanding of ice giants and exoplanets.