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The "Great Dark Spot '94," a vast new patch of black in the blue-green clouds of Neptune, has been detected in the northern hemisphere of the distant gaseous planet.

NASA astronomers announced the discovery Wednesday and Heidi B. Hammel of the Massachusetts Institute of Technology said that the finding proves that Neptune, a great sphere of mostly gas, is a place of frequent weather change, rather like Earth.She said a sequence of photos by the Hubble Space Telescope shows dramatic changes in cloud cover, with bright swirls appearing and disappearing within just a matter of weeks.

The new black spot is a mirror image of one that appeared and then disappeared on the planet earlier, said Hammel.

Steve Maran, a National Aeronautics and Space Administration astronomer, said the findings were surprising because many had thought Neptune was less dynamic.

"Neptune is a much more lively world than most of us imagined," Maran said.

About 17 times as massive as Earth, Neptune is one of the giant gaseous planets, first cousin to Jupiter, Saturn and Uranus. Neptune is 30 times farther from the sun than Earth and was one of the least known of the giants until the visit by Voyager.