Jupiter is famous for its Great Red Spot, a storm twice the diameter of Earth that rages on the gas giant's surface. Now, researchers have found that it has a second great spot, almost as large — this one, a Great Cold Spot caused by the planet's vibrant auroras.

Researchers first detected the ever-changing Great Cold Spot in data from the Very Large Telescope in Chile, and went back to track its existence over a 15-year period in observations from another telescope. The cool patch stretches up to 15,000 by 7,500 miles (24,000 by 12,000 km) across at its largest, and it's about 400 degrees Fahrenheit (200 degrees Celsius) cooler than the surrounding area in the planet's upper atmosphere. Although it disappears from time to time it seems to always re-form, just offset from the planet's bright aurora.

"The Great Cold Spot is much more volatile than the slowly changing Great Red Spot, changing dramatically in shape and size over only a few days and weeks, but it has reappeared for as long as we have data to search for it, for over 15 years," Tom Stallard, a planetary astronomer at the University of Leicester in the U.K. and lead author on the new work, said in a statement. [2nd 'Great Spot' on Jupiter Discovered by Astronomers (Video)]

The spot is likely formed as a byproduct of the planet's spectacular auroras, researchers said in the statement, and that because of the way the spot always re-forms it might be as old as the auroras themselves — up to many thousands of years in age.

Jupiter's newfound Great Cold Spot appears as a dark patch in this map of emissions from the hydrogen ion H3+, which is present in Jupiter's atmosphere, as measured by NASA's IRTF telescope in Hawaii. The cool spot moves over time and changes shape, but always re-forms. (Image credit: Tom Stallard)Like on Earth, the bright light of Jupiter's auroras comes from electrically charged particles colliding with the planet's atmosphere near its north and south poles, guided by the planet's magnetic field. But Jupiter's auroras are much more constant and intense, and they're powered from particles coming from the planet's moons as well as the sun.

According to the new research, the aurora deposits energy into Jupiter's atmosphere, heating it up so there's a large disparity in heat between the top of the atmosphere and farther below. This seems to whip up a vortex in the atmosphere, creating a patch that is cooler than the surroundings and offset from the aurora.

Stallard said that a similar effect can be found near Earth's aurora, but that it's less of a permanent fixture because the Earth auroras vary so much more, and because Jupiter's spin acts to trap some of its heat in place.

Researchers tracked the Great Cold Spot over time, noting dramatic changes in shape and size from day to day. Here, it is seen in July of 1995 and continues to reappear until 15 years later, in December of 2000. (Image credit: Tom Stallard)"The atmospheric flows generated by Earth's aurora can drive heat quickly across the whole planet, making the upper atmosphere ring like a bell, while Jupiter's fast spin traps this energy nearer the poles," Stallard said.

The researchers were surprised to find the Great Cold Spot, Stallard added, and they will continue to investigate it while looking for evidence of other atmospheric features. Combining their Earth-based observations with those from the Juno spacecraft currently orbiting Jupiter should provide much more insight into the giant planet's weather.

The new work was detailed April 10 in the journal Geophysical Research Letters.

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