Jupiter's Great Red Spot is the most famous storm in the solar system, but similar ones are known to crop up on the other gas and ice giant planets too. Voyager 2 saw one dubbed the "Great Dark Spot" on Neptune in 1989, with several others following in the years since. Now, for the first time Hubble has watched one of these huge storms develop, shedding new light on their origins.

Spanning roughly 11,000 km long and 5,000 km wide (6,835 x 3,100 mi), the new Great Dark Spot was first detected in the planet's northern hemisphere by Hubble in September 2018. A technical fault with the space telescope postponed further observations, but after a fix was made astronomers could look at it again in early November. Over a period of just under 20 hours, the team watched the storm drift across the Neptunian sky at a speed of about 972 km/h (604 mph).

This isn't the first dark spot ever seen – in fact, it's the sixth such storm observed on Neptune since Voyager 2's visit in 1989 – but it does mark the first time astronomers were able to watch one form.

To do so, the researchers looked back at past images of the region. As part of the Outer Planet Atmospheres Legacy (OPAL) project, Hubble has been snapping shots of Neptune every year since 2015. The team analyzed images taken between 2015 and 2017, and found that the Great Dark Spot was nowhere to be seen. Instead, several small, bright white clouds were spotted in the area.

A composite image of Neptune taken by Voyager 2 in 1989, with the "Great Dark Spot" visible in the center NASA/JPL

The researchers determined that these clouds were made of methane ice crystals, and hover at high altitudes. They seem to gather above the developing storm before it becomes visible to our telescopes, suggesting that the dark spots are forming deeper in the atmosphere than astronomers thought.

According to the team's calculations, new storms appear on Neptune every four to six years. While two years is the average lifespan, six years seems to be the maximum. The researchers also conducted simulations of 8,000 dark spots and matched them to 256 archival images, to get an estimate of how many Hubble would detect. They concluded that the telescope would be able to notice about 70 percent of storms that last a year, and up to 95 percent of those with two-year lifespans.

The team plans to continue observing the progress of Neptune's newest Great Dark Spot, and study how the planet's changing winds affect them.

The research was published in Geophysical Research Letters.

Sources: NASA, American Geophysical Union