After decades of nondetections and tantalizing maybes, astronomers have definitively detected an aurora on Neptune. Using the James Webb Space Telescope (JWST), researchers detected an infrared auroral glow and the spectral signature of a key tracer of aurorae in Neptune’s upper atmosphere for the first time. The spectrum of this ionized molecule also suggests that Neptune’s upper atmosphere has cooled significantly since Voyager 2’s flyby 34 years ago.
Aurorae have been seen on planets and moons throughout the solar system. Theories predicted that Neptune should have aurorae, too, but previous attempts to detect them failed, said Henrik Melin, a planetary aurora researcher at Northumbria University in Newcastle upon Tyne, United Kingdom (U.K.).
“I’ve spent many, many nights up a mountain trying to detect this stuff using ground-based telescopes. You spend four nights staring at Neptune, and you see nothing,” Melin said.
This auroral detection is “completing the set” of giant planet aurorae, he added. “We have Jupiter, we have Saturn, we have Uranus. We now have Neptune.”
Chilly Aurora
Aurorae occur when charged particles from the solar wind or a nearby volcanic moon, for example, interact with a body’s magnetosphere and upper atmosphere. Some aurorae glow in visible light, like on Earth and some of Jupiter’s moons. Mercury’s aurorae shine in X-ray light.
On planets with hydrogen-dominated atmospheres like Jupiter, Saturn, and Uranus, aurorae typically glow in the infrared or ultraviolet and are traced by the presence of the trihydrogen cation (H3+). Anywhere they occur, aurorae can help scientists understand the inner workings of a planet’s magnetosphere.
“Auroral emissions provide important insight into the space environment of a planet.”
“Auroral emissions provide important insight into the space environment of a planet, and this is particularly important for Neptune, which has a very bizarre magnetic field,” said Jonathan Nichols, a planetary aurora researcher at the University of Leicester in the U.K. who was not involved with the new discovery.
Voyager 2’s brief 1989 flyby suggested that Neptune’s magnetic field is both tilted from its axis of rotation and offset from the center of the planet. The flyby also detected some hints of a possible aurora that astronomers have been hoping to confirm ever since. Models of Neptune’s atmosphere and magnetic field have suggested that Neptune’s aurorae should also be traceable by H3+ and have even predicted the longitudes at which they should appear. But detecting the aurorae proved elusive.
In June 2023, Melin and his colleagues obtained near-infrared JWST spectra of Neptune, originally intending to explore the circulation of Neptune’s middle atmosphere. The observations unexpectedly revealed an infrared auroral glow as well as a shockingly clear infrared spectrum of H3+ emitted by the planet’s upper atmosphere.
The intensity of the H3+ spectrum suggests that the upper atmosphere generating the aurora is 85°C (358 K), a significant cooldown from the 477°C (750 K) temperature measured by Voyager 2.
“It’s great to see this addition to the family portrait of solar system auroras.”
“That was quite a surprise,” Melin said.
Neptune’s seasons are roughly 41 Earth years long, so this dramatic cooling took place faster than the seasonal timescale. The researchers don’t yet understand what might be driving the cooldown, Melin said, though it is likely unrelated to the unseasonably cool summer observed elsewhere in Neptune’s atmosphere.
“The consequence of these really cold temperatures means that the auroral emissions are extremely faint,” Melin said. That explains why Neptune’s aurorae eluded the gazes of ground- and space-based telescopes before. “It was just really, really cold.”
“It’s great to see this addition to the family portrait of solar system auroras,” Nichols said. “Now we know how bright the infrared emission is, we can work out the intensity in other wavelengths such as ultraviolet, and we can run models to see what the upper atmosphere is like.”
The researchers published this discovery in Nature Astronomy.
A Neptunian Day
These JWST data were clear enough to trace aurorae to specific latitudes and longitudes, “producing the first map of the aurora at Neptune,” Melin said.
What’s more, the aurorae appeared at the exact longitudes in the southern hemisphere predicted by long-standing theories.
“This is the tantalizing starting point of really getting to understand Neptune.”
“This was not a given,” Nichols explained, “since the length of the planet’s day was determined more than 3 decades ago, and the uncertainty was such that we were supposed to have lost track of what the time is at any point on Neptune.” (Uncertainty in planetary day lengths is pretty common.)
“But it appears as if it is more accurate than we thought!” Nichols added.
Later this year, the team will point JWST at Neptune several times over the course of a month to learn more about what drives its aurorae and how the planet’s magnetosphere responds to different levels of solar activity.
“By studying the morphology of the aurorae and its changes over time, we can figure out what drives it,” Melin said. The team needs more data to do that, “but this is the tantalizing starting point of really getting to understand Neptune.”
—Kimberly M. S. Cartier (@astrokimcartier.bsky.social), Staff Writer
This news article is included in our ENGAGE resource for educators seeking science news for their classroom lessons. Browse all ENGAGE articles, and share with your fellow educators how you integrated the article into an activity in the comments section below.