A huge dark vortex has been spotted in the atmosphere of Neptune. The presence of the vortex, which was first seen in September last year, was confirmed by images from Nasa's Hubble Space Telescope. It is the first dark vortex to appear on Neptune in the 21<sup>st century.

Mike Wong, who led the team analysing the Hubble data, announced the discovery in an astronomical telegram. Last year researchers noticed bright clouds on Neptune that are associated with the presence of dark vortices. These high-altitude clouds are created when air moves upwards and the gases freeze into methane crystals.

After spotting the bright clouds, follow-up observations revealed a dark spot close to the location of the clouds, with Hubble images allowing scientists to create a high-quality map of the vortex.

"Dark vortices coast through the atmosphere like huge, lens-shaped gaseous mountains," Wong explained. "And the companion clouds are similar to so-called 'orographic clouds' that appear as pancake-shaped features lingering over mountains on Earth."

Neptune's dark vortices are high-pressure systems first discovered in 1989 thorough images returned from Voyager 2. Another vortex was spotted by Hubble in 1994.

All observed vortices show a great deal of diversity in terms of shape, size and stability. They appear and disappear on fairly short time scales when compared to those seen on other planets. For example, similar features on Jupiter last for decades.

Scientists still do not fully understand how dark vortices originate, how they interact with the environment and why they dissipate. By studying the latest dark vortex, researchers hope to learn more about them and the atmosphere of Neptune.