For the first time, NASA’s James Webb Space Telescope has captured bright auroral activity on Neptune. Auroras occur when energetic particles, often originating from the Sun, become trapped in a planet’s magnetic field and eventually strike the 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 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. 

“Turns out, actually imaging the auroral activity on Neptune was only possible with Webb’s near-infrared sensitivity,” said lead author Henrik Melin of Northumbria University, who conducted the research while at the University of Leicester. “It was so stunning to not just see the auroras, but the detail and clarity of the signature really shocked me.”

The data was obtained 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 signifying the presence of the trihydrogen cation (H₃⁺), which can be created in auroras. In the Webb images of Neptune, the glowing aurora appears as splotches represented in cyan.

“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,” explained Heidi Hammel of the Association of Universities for Research in Astronomy, Webb interdisciplinary scientist and leader of the Guaranteed Time Observation program in which the data were obtained. “Only with a machine like Webb have we finally gotten that confirmation.”

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 located at the planet’s geographic mid-latitudes — think where South America is located on Earth. 

This is due to the strange nature of Neptune’s magnetic field, originally discovered by Voyager 2 in 1989, which is tilted 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, 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. 

“I was astonished — Neptune’s upper atmosphere has cooled by several hundreds of degrees,” Melin said. “In fact, the temperature in 2023 was just over half of that in 1989.” 

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, an 11-year period of activity driven by the Sun’s magnetic field. Results could provide insights into the origin of Neptune’s bizarre magnetic field, and even explain why it’s so tilted.

“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,” added Leigh Fletcher of Leicester University, co-author on the paper. “This observatory has finally opened the window onto this last, previously hidden ionosphere of the giant planets.”

These observations, led by Fletcher, were taken as part of Hammel’s Guaranteed Time Observation program 1249. The team’s results have been published in Nature Astronomy.

The James Webb Space Telescope is the world's premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).