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The James Webb Space Telescope has captured direct evidence of auroras on Neptune, according to findings published in Nature Astronomy, with lead author Henrik Melin of Northumbria University. The observations were made using Webb’s Near-Infrared Spectrograph in June 2023.The result closes a gap that has sat in the planetary science literature for more than three decades. When Voyager 2 flew past Neptune in 1989, it carried instruments designed in part to look for auroral activity. It found ultraviolet emissions and signatures consistent with auroral processes, but nothing like the clean, mapped auroral ovals seen at Jupiter, Saturn and Uranus. The question of what Neptune’s auroras actually looked like sat unanswered until Webb pointed at the planet.

What the telescope actually saw

Webb detected the trihydrogen cation, H3+, in Neptune’s upper atmosphere. H3+ is the standard auroral fingerprint used at the other giant planets. It forms when charged particles funnel down magnetic field lines and excite the local gas, and it glows in the near-infrared at wavelengths Webb is built to read. According to the Live Science report, the team also measured the temperature of Neptune’s upper atmosphere and found it several hundred degrees cooler than the temperature Voyager 2 measured in 1989. That cooling is one of the reasons the H3+ signal has been so hard to detect from Earth for so long. Cooler gas emits less brightly.

The auroras themselves did not appear where an Earth-based intuition would put them. On Earth, auroras sit roughly over the geographic poles because the magnetic poles sit roughly over the geographic poles. Neptune does not behave that way. Its magnetic field is tilted about 47 degrees from its rotation axis and offset from the planet’s centre, a configuration mapped by Voyager 2’s magnetometer in 1989 and discussed in NASA’s planetary fact sheets. The auroral ovals on Neptune therefore sit well away from the rotational poles, closer to the mid-latitudes in places, which is part of why earlier searches kept looking in the wrong place.

Why Voyager 2 came up short

It is tempting to read the 1989 result as a failure of the mission. It was not. Voyager 2’s instruments were doing what they were designed to do, with the sensitivity available in late-1970s hardware. The trouble was a combination of three things, on our reading of the published material. The auroras were faint. The upper atmosphere was already on its way to becoming cooler than the spacecraft expected. And the auroral regions were not where a Jupiter-trained instinct would have placed them, because Neptune’s magnetic geometry is genuinely odd.

Voyager 2 flew past Neptune in 1989 and detected faint hints of auroras it couldn’t explain — because the magnetic field is tilted 47 degrees off the rotation axis and the auroras were glowing in entirely the wrong place

What the telescope actually saw

Why Voyager 2 came up short