Teams of astronomers from Berkeley and the University of Wisconsin-Madison were involved in the research.

"People may think that Uranus is relatively inactive, but these images show that Uranus is definitely changing, and perhaps quite dramatically," said Imke de Pater, professor of astronomy at the University of California, Berkeley, and lead investigator for the team responsible for the Berkeley observations. "What is causing it, no one knows for sure. Only time will tell."

The images were captured at the infrared wavelength, a spectrum of light that human eyes cannot see. Scientists substituted composite images or color representations for the actual data to produce a visible representation of their findings, and those images were released Wednesday at the 36th meeting of the American Astronomical Society's Division for Planetary Sciences.

"We are stunned by the quality and detail of these images," said Frederic Chaffee, director of the Keck Observatory. "These are the best pictures of Uranus that have ever been produced by a telescope, and they are opening new windows of understanding for this unique and special world."

Uranus, the seventh planet from the sun, revolves around the sun once every 84 years. The most recent observations show the planet as it approaches its southern autumnal equinox, which takes place in 2007.

The new observations were made at a distance of about 1.6 billion miles.

Until recently, little was known about the oddball planet Uranus, which gets its name from the Greek word "Ouranos," a mythological god who personifies the heavens. Uranus lies tipped on its side, probably the result of an ancient cosmic collision, and its magnetic field lays strangely off-set from and tilted with respect to the planet's rotational pole.

In 1986, the Voyager 2 spacecraft sent pictures to Earth of Uranus, which appeared as a nondescript ball suspended in space.