The Voyager spacecraft has discovered a ring around Jupiter, a finding that had eluded the largest telescopes on Earth.

"We have discovered a thin, flat ring of particles surrounding Jupiter," the University of Arizona's Dr. Bradford Smith said today at the Jet Propulsion Laboratory, from which the historic flight to Jupiter has been directed. "Jupiter now joins Saturn and Uranus as planets with rings and leaves Neptune as the only one of the giant outer planets without a known ring."

What is believed to be a single ring circling Jupiter was found last Sunday inside the orbit of Amalthea, the smallest and innermost of Jupiter's 13 known moons.

Smith said scientists do not know the composition of the ring but believe it is no thicker than 20 miles. The ring circles Jupiter's equator 35,000 miles from the planet's could tops.

Whether the ring around Jupiter is an ancient moon that was broken up by Jupiter's enormous gravitational pull, or is primeval matter that never formed into a moon, is unknown. Scientists think the ring might also be large chunks of space debris drawn toward Jupiter and then trapped in orbit around it.

"My own view is that the ring is a lost moon of Jupiter that unhappily got too close to the planet and was pulled apart," said Smith, who leads the team of 30 scientists analyzing the photographs taken by Voyager's two cameras. "If that's true, then I think Jupiter's ring would have a different origin than the rings of Saturn or Uranus, which are probably primordial rings that never formed a moon."

The discovery of a ring around Jupiter was made with more than a little luck, even though the photographs taken Sunday in a search for a ring had been planed for several years.

Voyager's 1,500-millimeter camera shot six photographs, totaling 11.2 minutes' exposure, Fortunately, a cluster of galaxies known as the Beehive Cluster provided an excellent background. In addition, a nodding motion in the spacecraft's movement -- caused by Voyager's 43-foot-long magnetometer boom -- brought the ring into the camera's view.

"This nodding wobble enhanced our look at the ring," said Dr. Tobias Owen of the State University of New York at Stony Brook, who together with Dr. Edward Danielson of the California Institute of Technology was given the most credit for finding the ring. "As the spacecraft nodded along... the ring drifted into the field each time we took a picture, which gave us six images of the ring."

Though the pictures of the ring were taken Sunday, scientists waited until today to announce their finding because they wanted to be absolutely sure that what they had seen in the pictures was a ring and not the blur of a star or an artifact smearing the lens. The scientists involved in the photography spend the last four days analyzing the pictures and even called in outside experts to verify their interpretation

The ring is believed to consist of pieces of debris hundreds of feet across, pieces much larger than the billions of small jagged chunks believed to make up the three dazzling rings of Saturn.