Neptune dark spot spotted from Earth for first time

When there’re planets, there’re spots. There’s the Great Red Spot, Pluto’s heart, and according to new research, even Neptune is not immune.

Both a dark and bright spot on Neptune were discovered by the European Space Observatory’s (ESO) Very Large Telescope (VLT). It is the first time a dark spot on a planet has been observed with an Earth-based telescope.

“This is an astounding increase in humanity’s ability to observe the cosmos,” says University of California, Berkeley researcher Michael Wong.

“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.”

The spots are a mystery. The researchers used data to rule out the possibility that the dark spots are actually the planet below the clouds. Instead of this, it seems that the spots could be from air particles darkening below the haze layer.

The types of transient spots on Neptune were first discovered in 1989 with the Voyager 2 mission. However, this original spot disappeared a few years later. Next Hubble was able to find several dark spots in Neptune’s atmosphere, including one in the planet’s northern hemisphere first noticed in 2018.

This image from the VLT combines all colours captured by MUSE into a “natural” view of Neptune, where a dark spot can be seen to the upper-right. Credit: ESO/P. Irwin et al.

It was this 2018 spot – called NDS-2018 – that the researchers began studying from the ground.

“Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are,” says University of Oxford astronomer Patrick Irwin.

“I’m absolutely thrilled to have been able to not only make the first detection of a dark spot from the ground, but also record for the very first time a reflection spectrum of such a feature.”

The key was using the VLT’s Multi Unit Spectroscopic Explore to split reflected sunlight from Neptune into different wavelengths, which correlate with levels of the planet’s clouds.  This allowed the researchers to better identify where in the layers NDS-2018 was coming from.

“In the process we discovered a rare deep bright cloud type that had never been identified before, even from space,” says Wong.

They called this bright spot DBS-2019.

“This bright feature is much deeper than previously studied dark-spot companion clouds and may be connected with the circulation that generates and sustains such spots,” the researchers write in their new paper.

This probably won’t be the last spot that astronomers can find from Earth, with Wong joking “this could put me out of work as a Hubble observer!”

The research has been published in Nature Astronomy.

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