When scientists announced Webb’s first definitive detection of Neptune’s auroras in March 2025, they revealed another surprise: observations taken in 2023 showed that the planet’s upper atmosphere had
Astronomers using the James Webb Space Telescope have detected auroras on Neptune for the first time while discovering that the planet's upper atmosphere is significantly colder than previously estimated. The findings create a scientific puzzle regarding the atmospheric temperature drop since the 1989 Voyager 2 flyby.
Why it matters
This discovery challenges existing models of planetary atmospheres and demonstrates the advanced capabilities of the James Webb Space Telescope.
Webb's 2023 H3+ spectrum measured Neptune's upper atmosphere at 358 plus or minus 8 kelvin, far below Voyager 2's 1989 estimate, but two snapshots cannot yet explain what drove the change.
By Lachlan Brown · Edited by Lachlan Brown
The first definitive detection of auroras on Neptune came with a quieter discovery concealed in the same infrared spectrum. When the James Webb Space Telescope observed the distant planet in June 2023, it found the molecular signature astronomers had pursued for decades. It also found an upper atmosphere far colder than the one Voyager 2 measured during its 1989 flyby.
The auroras made the image. The temperature made the puzzle.
Get smarter about the news
Sign up free for a feed built around what you actually care about, Dive Deeper research on any story, and the full text of every article.
Create free accountAlready have an account? Sign in