A storm nearly the size of Earth is raging on Neptune, astronomers have discovered.
The massive system, marked by bright clouds in an equatorial region where none have been seen before, stretches across 9,000km (5,592 miles).
Scientists believe a huge high-pressure swirling vortex anchored deep in Neptune's atmosphere may be responsible.
Gases rising up in the vortex are thought to be cooling and condensing out to form clouds. Unlike the watery clouds on Earth, those on Neptune consist of methane.
Professor Imke de Pater, from the University of California at Berkeley, US, said: "Historically, very bright clouds have occasionally been seen on Neptune, but usually at latitudes closer to the poles, around 15 to 60 degrees north or south.
"Never before has a cloud been seen at or so close to the equator, nor has one ever been this bright."
Such a large cloud system spanning many latitudes is most likely to have been created by a "dark vortex" super-storm, said the scientists.
An alternative explanation could be a huge convective cloud similar to those sometimes seen on other planets, including one detected on Saturn in 2010. However, a convective cloud would be expected to "smear out" significantly over a period of about a week.
"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," said Prof de Pater.
Neptune, an icy gas giant, is the fourth largest planet in the solar system. It is also the windiest planet with equatorial winds reaching speeds of 1,000mph. In comparison, a Category 5 hurricane has wind speeds of 156mph, and the most powerful tornadoes spin at around 300mph.
The new discovery was made by UC Berkeley graduate student Ned Molter during a dawn test run of the WM Keck Observatory telescope on Maunakea, Hawaii.
"Seeing a storm this bright at such a low latitude is extremely surprising," he said.
"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."
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