Emissions from the upper-atmospheric trihydrogen cation (H3+) have been used to study the global-scale interactions of Jupiter, Saturn and Uranus with their surrounding space environments for over 30 years, revealing the processes shaping the aurorae. However, despite repeated attempts, and contrary to models that predict it should be present, this ion has proven elusive at Neptune. Now, using observations from the NASA/ESA/CSA James Webb Space Telescope, astronomers have detected the trihydrogen cation at Neptune as well as distinct infrared southern aurorae.
This composite image, made using data from the NASA/ESA Hubble Space Telescope and the NASA/ESA/CSA James Webb Space Telescope, shows auroral activity (cyan splotches) on Neptune. Image credit: NASA / ESA / CSA / STScI / Heidi Hammel, AURA / Henrik Melin, Northumbria University / Leigh Fletcher, University of Leicester / Stefanie Milam, NASA-GSFC.
“In the past, astronomers have seen tantalizing hints of auroral activity on Neptune,” said Northumbria University astronomer Henrik Melin and his colleagues.
“However, imaging and confirming aurorae on Neptune have long evaded astronomers despite successful detections on Jupiter, Saturn, and Uranus.”
“Neptune was the missing piece of the puzzle when it came to detecting aurorae on the giant planets of our Solar System.”
In the study, the authors analyzed the data obtained by Webb’s Near-Infrared Spectrograph (NIRSpec) in June 2023.
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 (ionosphere).
They found an extremely prominent emission line signifying the presence of the trihydrogen cation.
“In the Webb images of Neptune, the glowing aurora appears as splotches represented in cyan,” the astronomers said.
“The auroral activity seen on Neptune is 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 aurorae 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 NASA’s 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 aurorae are far from its rotational poles.”
“The ground-breaking detection of Neptune’s aurorae 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.”
The researchers were also able to measure the temperature of the top of Neptune’s atmosphere for the first time since Voyager 2’s flyby.
Their results hint at why Neptune’s aurorae remained hidden from astronomers for so long: Neptune’s upper atmosphere has cooled by several hundreds of degrees.
Through the years, astronomers have predicted the intensity of Neptune’s aurorae based on the temperature recorded by Voyager 2.
“A substantially colder temperature would result in much fainter aurorae,” the scientists said.
“This cold temperature is likely the reason that Neptune’s aurorae 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.”
The results appear today in the journal Nature Astronomy.
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H. Melin et al. Discovery of H3+ and infrared aurorae at Neptune with JWST. Nat Astron, published online March 26, 2025; doi: 10.1038/s41550-025-02507-9
