Neptune, the outermost planet of the Solar System, has always been a challenging world to study due to its distant location. Standing 4.5 billion kilometers away from the Sun, even powerful telescopes like Hubble struggle to capture images with the level of detail scientists hope for.
One thing Neptune shares with the other outer planets is that they are all classified as giant planets. Jupiter and Saturn are gas giants, while Uranus and Neptune are considered ice giants. Among these four, until now, Neptune was the only one on which auroras had never been observed.
Despite this, by knowing the characteristics of giant planets, scientists were confident that a planet like Neptune should have auroras too. But until now, no telescope had been able to image them.
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Webb makes it possible
It is not a secret that the James Webb Space Telescope has revolutionized infrared space observations since it started its science operations in July 2022. In June 2023, Webb pointed its instruments toward Neptune, collecting infrared data. Then, after combining its infrared data with a visible light image from the Hubble Space Telescope, scientists confirmed what they had been searching for: Neptune’s auroras.
These auroras appear in the Webb images as glowing cyan patches. Their detection was possible thanks to Webb’s Near-Infrared Spectrograph, which also collected a spectrum to study the composition and temperature of Neptune’s upper atmosphere, the ionosphere.
That’s where astronomers found a powerful emission line linked to H3+, a molecule that forms during auroral activity. This molecule had already been seen on Jupiter, Saturn and Uranus, so scientists expected it on Neptune too, but until now, none of the ground-based telescopes had been able to detect it.
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A different kind of aurora
Furthermore, Neptune has one of the most unusual magnetic fields in the Solar System. It’s not only tilted by about 47 degrees compared to the planet’s rotation axis, but it’s also offset from the planet’s center. This strange geometry makes it behave very differently from Earth’s or Jupiter’s magnetic fields, and that’s why Neptune is a particularly interesting case for studying how magnetic fields work on giant planets.
Since auroras form where magnetic field lines enter the atmosphere, this offset and tilted field causes the glow to appear far from the poles. So, unlike on Earth, Jupiter or Saturn, where auroras tend to appear near the poles, Neptune’s auroras show up at mid-latitudes.
Thanks to Webb’s data, astronomers also measured the temperature of Neptune’s upper atmosphere, something that hadn’t been done since the Voyager 2 flyby in 1989.
The results were unexpected: in 2023, the temperature was just over half of what it was back then, meaning it had cooled by several hundred degrees. This could explain why Neptune’s auroras had remained hidden for so long. With such a cold ionosphere, the auroras are much fainter, and previous telescopes simply weren’t sensitive enough to detect them.
These findings not only help explain why the auroras were so elusive, but they also raise new questions. Why is Neptune’s upper atmosphere cooling so much? And how does it respond to solar activity, considering the planet is over 30 times farther from the Sun than Earth? Scientists now hope to keep observing Neptune throughout a full solar cycle, about 11 years, to better understand its strange magnetic field and how it affects the planet’s atmosphere.
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