A mysterious dark spot on Neptune has been detected from Earth for the first time – but astronomers don’t know what it is.

Using the European Southern Observatory’s (ESO) Very Large Telescope (VLT), astronomers have seen a large dark spot in Neptune’s atmosphere, with an unexpected smaller bright spot next to it.

Dark spots have been seen on the planet’s surface before, but never from Earth.

These occasional features in the blue background of the planet’s atmosphere are a mystery to astronomers, and the new results provide further clues about their origin.

‘This is an astounding increase in humanity’s ability to observe the cosmos,’ said co-author Michael Wong, a researcher at the University of California, Berkeley, in the US.

‘At first, we could only detect these spots by sending a spacecraft there, like Voyager.

‘Then we gained the ability to make them out remotely with Hubble. Finally, technology has advanced to enable this from the ground.’

Large spots are common features in the atmospheres of giant planets, the most famous being Jupiter’s Great Red Spot.

A dark spot was first discovered on Neptune by Nasa’s Voyager 2 in 1989, before disappearing a few years later.

‘Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are,’ said Patrick Irwin, professor at the University of Oxford and lead investigator of the study.

The researchers used data from the VLT to rule out the possibility that dark spots are caused by a clearing in the clouds.

But instead the observations indicate that dark spots are likely the result of air particles darkening in a layer below the main visible haze layer, as ice and hazes mix in Neptune’s atmosphere.

Because dark spots are not permanent features of Neptune’s atmosphere and astronomers had never before been able to study them in sufficient detail, it was not easy for them to come to this conclusion.

But they were able to do so after Nasa and the European Space Agency’s Hubble Space Telescope discovered several dark spots in the planet’s atmosphere, including one in the planet’s northern hemisphere first noticed in 2018.

Using the VLT’s Multi Unit Spectroscopic Explorer (Muse) instrument, the researchers were able to split reflected sunlight from Neptune and its spot into its component colours, or wavelengths.

This allowed them to study the spot in more detail than was possible before.

The observations also offered up a surprise result – a rare deep bright cloud type that had never been identified before, even from space.

This rare cloud type appeared as a bright spot right beside the larger main dark spot, according to the observations published in Nature Astronomy.

The data showed that the new deep bright cloud was at the same level in the atmosphere as the main dark spot.

This means it is a completely new type of feature compared with the small companion clouds of high-altitude methane ice that have been previously observed.

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