"This one is weird, because it's really big and it's not dark. It's bright, and the bright stuff is probably sort of like cirrus clouds on top of a thunderstorm that's underneath," Bryan Butler of the National Radio Astronomy Observatory told National Geographic. "And it seems to be staying steady, for it's been a month or something now."
In the past, Neptunian storms have only been observed on the planet when it was in total darkness. But this storm system was 5,600 miles in length — about the distance from Chicago, IL, to Ankara, Turkey. The massive storm covered a circular area 30 degrees wide.
"Seeing a storm this bright at such a low latitude is extremely surprising. Normally, this area is really quiet and we only see bright clouds in the mid-latitude bands, so to have such an enormous cloud sitting right at the equator is spectacular," said Ned Molter, a graduate student of astronomy at University of California, Berkeley, in a statement. Molter discovered the storm system by accident while observing Neptune with a telescope at the W. M. Keck Observatory in Mauna Kea, Hawaii.
The storm system, which is three-quarters as large as the Earth, could be caused by "a huge, high-pressure, dark vortex system anchored deep in Neptune's atmosphere," according to a statement issued by the observatory.
The clouds are probably methane clouds that exist in multiple strata of Neptune's atmosphere — it is extremely rare for Earth storms to exist in multiple atmospheric strata, with only 44 recorded in the last 100 years.
The size of the storms suggest that a vortex of some kind is holding the storm in place, according to Molter. "This big vortex is sitting in a region where the air, overall, is subsiding rather than rising. Moreover, a long-lasting vortex right at the equator would be hard to explain physically. … This shows that there are extremely drastic changes in the dynamics of Neptune's atmosphere, and perhaps this is a seasonal weather event that may happen every few decades or so," de Pater said in the observatory's statement.
Impressive as the new storm system on Neptune may be, it still pales compared to Jupiter's 10,000 mile long, seemingly immortal Great Red Spot, a storm system that has raged for at least 350 years.
However, don't count Neptune out. Winds of 1,000 miles per hour have been known to sweep across Neptune, making it the solar system's windiest planet. By comparison, even the most intense Earth hurricanes hardly break 160 mph.
In 1989, the Voyager 2 Spacecraft (the last human ship to swing by Neptune) observed a stupendously large storm on Neptune's surface. The "Great Dark Spot" captured the imagination of astronomers for a time, but by the time the Hubble Space Telescope trained itself on Neptune in 1994, the storm had seemingly dissipated.