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NASA’s James Webb Space Telescope Captures First-Ever Auroras on Neptune.

 

On the left, an enhanced-color image of Neptune from NASA's Hubble Space Telescope. On the right, that image is combined with data from NASA’s James Webb Space Telescope, showcasing cyan splotches of auroral activity and white clouds from Webb's Near-Infrared Spectrograph (NIRSpec), overlaid on Hubble’s full image of the planet.



For the first time, NASA’s James Webb Space Telescope has successfully captured bright auroral activity on Neptune, a significant breakthrough in the study of the ice giant. Auroras, typically seen on Earth and other planets in our solar system, occur when energetic particles, often from the Sun, become trapped in a planet’s magnetic field and collide with the upper atmosphere, releasing energy in the form of light.


While tantalizing hints of auroral activity on Neptune were previously observed — notably during NASA’s Voyager 2 flyby in 1989 — imaging and confirming these auroras had long eluded astronomers. This new discovery marks Neptune as the final giant planet in our solar system to reveal its auroras, following successful detections on Jupiter, Saturn, and Uranus.


Lead author Henrik Melin, a researcher from Northumbria University who conducted the work while at the University of Leicester, described the achievement: “It was so stunning to not just see the auroras, but the detail and clarity of the signature really shocked me.”


The data, collected in June 2023 using Webb’s Near-Infrared Spectrograph, provided not only a clear image of the planet but also a detailed spectrum that allowed astronomers to analyze the composition and temperature of Neptune's upper atmosphere, or ionosphere. For the first time, the team detected a prominent emission line of the trihydrogen cation (H3+), which is associated with auroral activity. The auroras themselves appear as glowing cyan splotches in the Webb images.


“H3+ has been a clear signifier of auroral activity on all the gas giants — Jupiter, Saturn, and Uranus — and we expected to see the same on Neptune,” explained Heidi Hammel, Webb interdisciplinary scientist and leader of the Guaranteed Time Observation program. “Only with a machine like Webb have we finally gotten that confirmation.”


Unlike auroras on Earth or Jupiter, which are typically confined to the poles, Neptune’s auroras occur at the planet's mid-latitudes — around the same latitude as South America on Earth. This unusual positioning is due to Neptune’s magnetic field, which was discovered by Voyager 2 to be tilted 47 degrees from the planet’s rotation axis. Since auroral activity forms where magnetic fields converge, Neptune’s auroras are located far from the poles.


This groundbreaking detection opens up new avenues for understanding how Neptune's magnetic field interacts with solar particles. The Webb team’s measurements also revealed an unexpected cooling of Neptune’s upper atmosphere, which has dropped several hundred degrees since Voyager 2’s 1989 flyby. This temperature drop may explain why Neptune’s auroras remained hidden from astronomers for so long, as colder temperatures typically result in fainter auroras.


Looking ahead, scientists hope to observe Neptune over a full solar cycle — an 11-year period of solar activity — to gather more insights into Neptune’s unique magnetic field and its interactions with the Sun’s emissions. This will also help researchers understand the origins of Neptune’s peculiar magnetic field and its extreme tilt.


As astronomers turn their attention to future missions to Uranus and Neptune, Leigh Fletcher of Leicester University, co-author of the study, emphasized the importance of infrared instruments like Webb to continue studying auroral phenomena on these distant worlds.


The study's results, led by Fletcher, were published in *Nature Astronomy*. This marks a major step forward in space exploration, with the James Webb Space Telescope offering unparalleled views of our solar system’s icy giants.

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