Neptune is so far away — at an astonishing 3 billion miles from the sun — that there’s a lot we still don’t know about this planet. From fluctuating temperatures to the discovery of tiny moons there to even debates about the planet’s true color, Neptune continues to surprise. Now, though, researchers have uncovered another distinctive feature there: a bright auroral glow.
Neptune often looks slightly different in observations, as it has dark spots which appear periodically. But it can also be bright at times, as the observations of auroras there show: In the image above, the auroras are shown in the cyan color.
Spotted by the James Webb Space Telescope, auroras occur when energetic particles from the sun interact with a planet’s atmosphere, creating a glowing light show. It’s the same effect that is seen on Earth (and often called the Northern Lights), and it has also been seen on other planets like Mars — but this is the first time it’s been spotted on Neptune.
“Turns out, actually imaging the auroral activity on Neptune was only possible with Webb’s near-infrared sensitivity,” said lead researcher Henrik Melin of Northumbria University. “It was so stunning to not just see the auroras, but the detail and clarity of the signature really shocked me.”
The data was collected using a Webb instrument called the Near-Infrared Spectrograph which can also tell what is present in the planet’s atmosphere. It found indications of a molecule called trihydrogen cation (H3+), thought to be created by auroras.
“H3+ has a been a clear signifier on all the gas giants — Jupiter, Saturn, and Uranus — of auroral activity, and we expected to see the same on Neptune as we investigated the planet over the years with the best ground-based facilities available,” explained Webb scientist Heidi Hammel. “Only with a machine like Webb have we finally gotten that confirmation.”
However, despite the similarities with auroral activites on other planets in the solar system, Neptune’s auroras have a character all of their own. Because of the way the planet orbits — almost entirely tipped over onto its side — its auroras aren’t over its north and south poles like they are on other planets. In researching the phenomenon, the scientists also found that the very top of Neptune’s atmosphere was quite different from what they expected.
“I was astonished — Neptune’s upper atmosphere has cooled by several hundreds of degrees,” Melin said. “In fact, the temperature in 2023 was just over half of that in 1989.”
Now, with Webb, scientists are finally able to study the strange mysteries of Neptune — such as how its atmosphere is cooling so fast.
“As we look ahead and dream of future missions to Uranus and Neptune, we now know how important it will be to have instruments tuned to the wavelengths of infrared light to continue to study the auroras,” said fellow researcher Leigh Fletcher of Leicester University. “This observatory has finally opened the window onto this last, previously hidden ionosphere of the giant planets.”
The research is published in Nature Astronomy.
