Jupiter's signature "Great Red Spot" is the strongest storm in the universe and it is shrinking, but scientists cannot determine the cause.

According to Space.com, the rate of its shrinking has sped up as it gets smaller and is now contracting at a rate of 580 miles per year. Scientists said it could have fit three Earths in it when it was its largest, but can now only fit our planet once.

"One possibility is that some unknown activity in the planet's atmosphere may be draining energy and weakening the storm, causing it to shrink," Hubble officials wrote in a statement.

The Great Red Spot has been Jupiter's defining trait since it was first observed in the 1600s, the time of the first telescopes. In the 1800s, it was estimated at 25,500 miles wide and astronomers first noticed it was getting smaller in 1930. In 1979, the Voyager 1 and 2 spacecraft imaged the Great Red Spot in flybys and scientists measured it at 14,500 miles across.

The Hubble Telescope began observing Jupiter in the 1990s and, in 1995, NASA obtained an image showing the Great Red Spot to have shrunk to 13,020 miles. By a recent estimation in 2009, Jupiter's giant storm was 11,130 miles.

"In our new observations it is apparent that very small eddies are feeding into the storm," Amy Simon, associate director for strategic science at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a press release. "We hypothesized that these may be responsible for the accelerated change by altering the internal dynamics and energy of the Great Red Spot."

After the most recent observations, made in 2012, the Great Red Spot is now 10,250 miles across, less than half its size from more than two centuries ago. The shrinkage has also caused the spot to change from an oval to a circle.