A storm 6,000 miles wide has been found raging on Neptune in what scientists say is an “extremely surprising” discovery.
The massive system is marked by bright clouds near the equator where they have never been seen before.
Experts say such clouds are normally closer to the poles.
They believe a huge high-pressure, dark vortex anchored deep in the atmosphere may have caused the storm, which is almost as big as Earth.
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 in the 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.”
But it may not be linked to a vortex and instead could be a huge convective cloud similar to those 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.
UC Berkeley graduate student Ned Molter made the discovery during a dawn test run of the WM Keck Observatory telescope on Maunakea, Hawaii.
Mr Molter, who observed it getting much brighter between 26 June and 2 July, said: “Seeing a storm this bright at such a low latitude is extremely surprising.”
Neptune, the fourth biggest planet in the solar system, is the windiest with wind speeds at the equator reaching up to 1,000mph.
In comparison, a Category 5 hurricane on Earth has wind speeds of 156mph and the most powerful tornadoes spin at around 300mph.