- The Webb space telescope has finally caught auroras on Neptune. Neptune lies in the frigid, dark, vast frontier of the outer edges of our solar system about 3 billion miles away from the sun.
- Observatories like NASA’s Hubble Space Telescope have tracked the planet’s changing weather. Hubble even discovered a new moon orbiting the planet in 2013.
- In many images, the planet appears as a bluish orb, sometimes with disappearing and reappearing dark spots. NASA’s James Webb Space Telescope has now revealed a different appearance: a bright auroral glow from this ice giant.
NASA’s Webb Space Telescope published this original story on March 26, 2025. Edits by EarthSky.
First views of auroras on Neptune
On March 26, 2025, NASA shared the first image of bright auroral activity on Neptune, thanks to the Webb space telescope. Auroras occur when energetic particles, often originating from the sun, become trapped in a planet’s magnetic field. They eventually strike a planet’s upper atmosphere. The energy released during these collisions creates the signature glow.
In the past, astronomers have seen tantalizing hints of auroral activity on Neptune, for example, in the flyby of NASA’s Voyager 2 in 1989. However, imaging and confirming the auroras on Neptune has long evaded astronomers. That’s despite successful detections on Jupiter, Saturn and Uranus. Neptune was the missing piece of the puzzle when it came to detecting auroras on the giant planets of our solar system.
Lead author Henrik Melin of Northumbria University in the U.K. conducted the research while at the University of Leicester. Melin said:
Turns out, actually imaging the auroral activity on Neptune was only possible with Webb’s near-infrared sensitivity. It was so stunning to not just see the auroras, but the detail and clarity of the signature really shocked me.
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Cyan splotches represent auroras
The scientists obtained the data in June 2023 using Webb’s Near-Infrared Spectrograph. In addition to the image of the planet, astronomers obtained a spectrum to characterize the composition and measure the temperature of the planet’s upper atmosphere (the ionosphere). For the first time, they found an extremely prominent emission line. This emission line signified the presence of the trihydrogen cation (H3+). And auroras can create these ions. In the Webb images of Neptune, the glowing auroras appear as splotches in cyan.
Heidi Hammel of the Association of Universities for Research in Astronomy, Webb interdisciplinary scientist and leader of the Guaranteed Time Observation program which obtained the data were obtained, said:
H3+ has a been a clear signifier on all the gas giants – Jupiter, Saturn and Uranus – of auroral activity, and we expected to see the same on Neptune as we investigated the planet over the years with the best ground-based facilities available. Only with a machine like Webb have we finally gotten that confirmation.
Auroras on Neptune are different
The auroral activity seen on Neptune is also noticeably different from what we are accustomed to seeing here on Earth, or even Jupiter or Saturn. Instead of being confined to the planet’s northern and southern poles, Neptune’s auroras are at the planet’s geographic mid-latitudes. So picture where South America is located on Earth.
Neptune’s magnetic field is responsible for this, which Voyager 2 originally discovered in 1989. It tilts by 47 degrees from the planet’s rotation axis. Since auroral activity is based where the magnetic fields converge into the planet’s atmosphere, Neptune’s auroras are far from its rotational poles.
The ground-breaking detection of Neptune’s auroras will help us understand how Neptune’s magnetic field interacts with particles that stream out from the sun to the distant reaches of our solar system. It is a totally new window in ice giant atmospheric science.
From the Webb observations, the team also measured the temperature of the top of Neptune’s atmosphere for the first time since Voyager 2’s flyby. The results hint at why Neptune’s auroras remained hidden from astronomers for so long. Melin said:
I was astonished; Neptune’s upper atmosphere has cooled by several hundreds of degrees. In fact, the temperature in 2023 was just over half of that in 1989.
A cooling planet
Through the years, astronomers have predicted the intensity of Neptune’s auroras based on the temperature recorded by Voyager 2. A substantially colder temperature would result in much fainter auroras. This cold temperature is likely the reason that Neptune’s auroras have remained undetected for so long. The dramatic cooling also suggests that this region of the atmosphere can change greatly even though the planet sits over 30 times farther from the sun compared to Earth.
Equipped with these new findings, astronomers now hope to study Neptune with Webb over a full solar cycle. The sun’s magnetic field drives this 11-year period of activity. Results could provide insights into the origin of Neptune’s bizarre magnetic field. And it might even explain why it’s so tilted.
Leigh Fletcher of the University of Leicester in the U.K., co-author on the paper, said:
As we look ahead and dream of future missions to Uranus and Neptune, we now know how important it will be to have instruments tuned to the wavelengths of infrared light to continue to study the auroras. This observatory has finally opened the window onto this last, previously hidden ionosphere of the giant planets.
Bottom line: For the first time, the Webb space telescope has revealed auroras on Neptune. See images and read more about it here.
Source: Discovery of H3+ and infrared aurorae at Neptune with JWST
Via Webb Space Telescope