Two outer rings, one red the other blue, have been observed around the distant planet Uranus.

While Uranus had been known to have inner rings of neutral color, the newly discovered outer rings show color contrasts that researchers think are caused by light reflected off particles that differ in size from one ring to the other.

And the outermost ring is only the second blue ring to have been observed, a team led by Imke de Pater of the University of California, Berkeley, reports in Friday's issue of the journal Science.

Also blue is Saturn's outermost ring, the researchers said. And they noted that both of the known blue rings have a moon embedded within them, while the red rings do not.

They speculated that the moons swept up larger pieces of debris, leaving only dust and tiny items that reflected more blue light than the red ring which could have larger pieces of debris.