Auroras are spotted on Neptune for the first time, and lead to a new mystery

The vermilion, amethyst and jade ribbons of the northern and southern lights are some of Earth’s most distinctive features. But our planet doesn’t have a monopoly on auroras. Scientists have spied them throughout the solar system, weaving through the skies of Mars, Saturn, Jupiter and even on some of Jupiter’s fiery and icy moons.

Lights glow in the skies of Uranus, too. But auroras around our sun’s most distant planet, Neptune, have long eluded astronomers.

That has changed with the powerful infrared instruments aboard the James Webb Space Telescope. In a study published Wednesday in the journal Nature Astronomy, scientists reveal unique auroras that spill over either side of Neptune’s equator, a contrast with the glowing gossamer seen arcing over other worlds’ poles.

Astronomers are thrilled to see the completion of an aurora-hunting quest decades in the making. “Everyone is very excited to prove that it’s there, just like we thought,” said Rosie Johnson, a space physics researcher at Aberystwyth University in Wales who wasn’t involved with the new study.

This discovery will also allow scientists to study aspects of Neptune that have previously been out of reach. “They’re using aurora to understand the shape of the planet’s magnetic field, which is seeing the unseen,” said Carl Schmidt, a planetary astronomer at Boston University who wasn’t involved with the new study.

Each world generates auroras differently, but the basics are the same. Energetic particles (often from the sun, but sometimes from a moon’s volcanic eruptions) slam into an atmosphere and bounce off gases. That particle collision briefly causes flashes of light. And if a world has a magnetic field, that guides the location of the auroras.

Auroras don’t always glow in visible light; Saturn, for example, emits mostly ultraviolet auroras. But they can be observed with the right telescopes.

It hasn’t been possible until now to spot Neptune’s atmospheric lights.

“Astronomers have been trying to detect the aurora of Neptune for decades, and each attempt has failed,” said Henrik Melin, a planetary scientist at Northumbria University in England and one of the study’s authors.

Voyager 2, the only spacecraft to fly by Neptune (in 1989), found hints of an aurora. But all follow-up observations — even with the Hubble Space Telescope — failed to spy telltale shimmering.

Fortunately, the Webb telescope, launched in 2021, has come to the rescue.

Heidi Hammel, an astronomer at the Association of Universities for Research in Astronomy and another of the study’s authors, has been studying Neptune since the 1980s. She thought that if Webb “was powerful enough to see the earliest galaxies in the universe, it’d better be powerful enough to see things like aurorae on Neptune,” she said. “And by golly, it was.”

Using the telescope’s Near-Infrared Spectrograph, astronomers caught Neptune’s infrared auroras in June 2023. And unlike Earth’s, they dance not above the poles, but its mid-latitudes. That’s because Neptune has a wonky magnetic field that is tilted by 47 degrees from the planet’s spin axis.

The new Webb observations also reveal why Neptune’s auroras have been invisible until now. Nearly 40 years ago, Voyager 2 recorded a temperature of around 900 degrees Fahrenheit for Neptune’s upper atmosphere. But the Webb telescope shows that the temperature has dropped, to close to 200 degrees. That lower temperature means the auroras are dimmer.

In fact, Neptune’s aurora is glowing “with less than 1 per cent of the brightness we expected, explaining why we haven’t seen it,” said James O’Donoghue, a planetary astronomer at the University of Reading in England and one of the study’s authors. “However, that means we now have a new mystery on our hands: How has Neptune cooled down so much?”

With the detection of Neptune’s strange light show, answers may be forthcoming.

“Auroras are like a TV screen,” said Leigh Fletcher, a planetary scientist at the University of Leicester in England and one of the study’s authors. They are “allowing us to watch the delicate dance of processes in the magnetosphere — all without actually being there.”