Astronomers have spotted a storm system nearly the size of Earth near Neptune's equator, a region where no bright cloud has been seen before.

"Seeing a storm this bright at such a low latitude is extremely surprising," said Ned Molter, an astronomy graduate student at the University of California-Berkeley. 

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"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," Molter said in a statement released by the university on Thursday. 

Molter spotted the storm complex near Neptune's equator during a dawn test run of twilight observing at W. M. Keck Observatory on Maunakea, Hawaii. The findings showed that useful observations can be made during twilight, a time most astronomers consider unusable because it is not dark enough.

This massive storm system, which was found in a region where no bright cloud has ever been seen before, is about 9,000 kilometres in length, or one-third the size of Neptune's radius, spanning at least 30 degrees in both latitude and longitude. 

Molter observed it getting much brighter between June 26 and July 2.

"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," said Imke de Pater, Professor of Astronomy at University of California, Berkeley, and Molter's adviser. 

"Never before has a cloud been seen at or so close to the equator, nor has one ever been this bright," de Pater added.

At first, de Pater thought it was the same Northern Cloud Complex seen by the Hubble Space Telescope in 1994, after the iconic Great Dark Spot, imaged by Voyager 2 in 1989, had disappeared. 

But measurements of its locale do not match, signaling that this cloud complex is different from the one Hubble first saw more than two decades ago, de Pater added.

A huge, high-pressure, dark vortex system anchored deep in Neptune's atmosphere may be what is causing the colossal cloud cover, said de Pater. 

Having a better understanding of Neptune's atmosphere will help give astronomers a clearer picture of this icy giant's global circulation and it might also help shed light on how exoplanet's atmosphere behave.

This has become increasingly more important in the exoplanet realm, as a majority of exoplanets found so far are nearly the size of Neptune. 

While scientists can calculate their size and mass, not much is currently known about exoplanets' atmosphere.