NASA's James Webb Reveals First Look at Neptune's Aurora

NASA's flagship James Webb Space Telescope has captured the first-ever images of auroral activity on Neptune, after years of tantalizing hints.

The images in question of Neptune were captured by Webb's Near-Infrared Spectrograph in June 2023.

"Imaging and confirming the auroras on Neptune has long evaded astronomers despite successful detections on Jupiter, Saturn and Uranus," the European Space Agency (ESA)—which operates the Webb Telescope alongside NASA and the Canadian Space Agency—said in a statement.

"Neptune was the missing piece of the puzzle when it came to detecting auroras on the giant planets of our solar system."

Alongside capturing images of the aurora, the astronomers were also able to probe the composition and temperature of the ice giant's upper atmosphere.

The team detected an "extremely prominent" emission line indicating the presence of trihydrogen cations, which are known to be created in auroras.

They also found that the temperature of Neptune's ionosphere has cooled by several hundred degrees since it was previously measured by Voyager 2 during a flyby in 1989.

This may well explain how the planet's aurora managed to stay hidden for so long—as colder temperatures are predicted to result in much fainter auroras.

Aurorae—whether on Earth or other planets—form when energetic particles from the solar wind excite atoms in an upper atmosphere, causing them to glow.

The resulting light show varies in color, depending on the particular type of atoms being excited.

As Earth's atmospheric composition is dominated by oxygen and nitrogen, aurorae tend to give off a green hue (from the former) or a blue–pink–purple light (from the latter.)

Over Neptune, meanwhile, the observed aurorae are cyan in the Webb images. They also differ from their terrestrial counterparts in that they appear at mid-latitudes, rather than over the poles.

This is because (as Voyager 2 also discovered) Neptune's magnetic field is tilted by 47 degrees off of the planet's axis of rotation—meaning that the field lines along which the solar wind is channeled meet the atmosphere in a different place to those here on Earth.

According to the ESA, the detection of Neptune's aurorae will help scientists understand exactly how the planet's magnetic field interacts with the solar wind.

With this initial study complete, the researchers are now looking to use Webb to study Neptune over a full 11-year solar cycle. The findings could shine a light on the origin of the planet's strange magnetic field, and why it is tilted.

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Reference

Melin, H., Moore, L., Fletcher, L. N., Hammel, H. B., O'Donoghue, J., Stallard, T. S., Milam, S. N., Roman, M., King, O. R. T., Rowe-Gurney, N., Thomas, E. E., Wang, R., Tiranti, P. I., Harkett, J., & Knowles, K. L. (2025). Discovery of H₃⁺ and infrared aurorae at Neptune with JWST. Nature Astronomy. https://doi.org/10.1038/s41550-025-02507-9