side-by-side images of Neptune, with the one at the right showing splotches of cyan (auroras) and white clouds
At the left, an enhanced-color image of Neptune from NASA’s Hubble Space Telescope. At the right, that image is combined with data from NASA’s James Webb Space Telescope to show auroras, represented by cyan coloring. NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Henrik Melin (Northumbria University), Leigh Fletcher (University of Leicester), Stefanie Milam (NASA-GSFC)
Earth’s beautiful northern and southern lights are the result of auroras: when high-energy charged particles from the sun hit gases in our upper atmosphere, making them glow in brilliant colors as they release energy from the collision. Auroras occur where Earth’s magnetic field crosses our planet’s atmosphere near the north and south poles.
While scientists have previously observed auroras on Mars, Jupiter, Saturn and Uranus, NASA’s James Webb Space Telescope (JWST) has finally spotted them on Neptune for the first time. The announcement is detailed in a study published Wednesday in the journal Nature Astronomy.
If JWST “was powerful enough to see the earliest galaxies in the universe, it’d better be powerful enough to see things like aurorae on Neptune,” study co-author Heidi Hammel, an astronomer at the Association of Universities for Research in Astronomy, tells the New York Tim es’ Robin George Andrews. “And by golly, it was.”
Astronomers have been searching for auroral activity on Neptune, the most distant planet from the sun, since NASA’s Voyager 2 spacecraft flew by it in 1989 and picked up “tantalizing hints” of the phenomenon, per a NASA statement.
“Neptune has always been elusive,” James O’Donoghue, a planetary scientist at the University of Reading in England and a co-author of the new study, tells the Associated Press’ Christina Larson. “We’ve been trying to see it again ever since [Voyager].”
In a newly released image featuring data from the Hubble and Webb space telescopes, cyan splotches represent Neptune’s auroral activity. Unlike on Earth, the giant planet’s auroras occur at its geographic mid-latitudes—the equivalent auroras lighting up over South America. This is because Neptune’s magnetic field has an odd orientation, which Voyager 2 revealed is tilted by 47 degrees relative to the planet’s axis of rotation.
The detection offers a long-anticipated conclusion to astronomers’ search for auroral activity on the ice giant planet. “Everyone is very excited to prove that it’s there, just like we thought,” says Rosie Johnson, a space physics researcher at Aberystwyth University in Wales who wasn’t involved in the study, to the New York Times.
JWST collected the exciting data in June 2023 with its Near-Infrared Spectrograph—an instrument that can detect near-infrared wavelengths invisible to the human eye. The researchers analyzed the makeup and temperature of Neptune’s upper atmosphere, detecting the presence of the trihydrogen cation (H3+), a molecule that can result from auroras.
“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,” Hammel says in the statement. “Only with a machine like Webb have we finally gotten that confirmation.”
The telescope’s data on Neptune also revealed a new mystery: Since Voyager 2’s flyby more than three decades ago, the temperature of the planet’s upper atmosphere has dropped by hundreds of degrees Fahrenheit. Since colder temperatures lead to fainter auroral activity, that likely explains why it’s been so difficult to spot Neptune’s auroras—but it doesn’t explain why the ice giant cooled so dramatically.
The answer to that might emerge with further research. Moving forward, astronomers aim to use JSWT to continue investigating Neptune’s magnetic field over a full solar cycle of 11 years.