Green blotches show where auroras light up Neptune’s skies
NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Henrik Melin (Northumbria University), Leigh Fletcher (University of Leicester), Stefanie Milam (NASA-GSFC)
For the first time, researchers have spotted infrared auroras swirling in Neptune’s atmosphere, verifying decades of scientific speculation.
When NASA’s Voyager 2 mission flew by Neptune in 1989, it found tantalising hints of aurora activity in the ice giant’s clouds. However, scientists were unable to verify the phenomenon at the time, as existing instruments were too weak. Now, the James Webb Space Telescope (JWST) has finally provided the power to detect them.
“This was really a fulfillment of years’ worth of anticipation,” says Heidi Hammel at the Association of Universities for Research in Astronomy in Washington DC.
Hammel and her colleagues used JWST’s NIRSpec, a powerful infrared imaging tool, to capture spectroscopic images of Neptune and analyse the different wavelengths of light emitted by the planet. In 2023, researchers used the instrument to detect infrared auroras on Uranus. This time, it found them on Neptune as well.
The images also allowed Hammel and her team to begin constructing a map of Neptune’s magnetic field. This is particularly exciting as the planet is known to have some of the most unusual magnetic poles in the solar system.
Unlike Earth, Jupiter or Saturn, Neptune’s magnetic poles aren’t centred at its rotational poles. Instead, “they’re offset by almost half the planet’s radius”, says Hammel. As a result, its auroras appear as irregular blobs much closer to its equator, over roughly the region where South America sits on Earth.
In addition to detecting auroras, the JWST observations indicate that Neptune’s ionosphere – the layer of charged particles blanketing some planets – is cooling down. It is now, on average, about 10 per cent colder than when Voyager 2 passed by nearly 36 years ago. Similar changes have been detected on Uranus.
While the authors of the new study aren’t sure why this cooling occurred, they hope their next JWST observation period, scheduled for 2026, will offer more clues.
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