By Helen Briggs
BBC News science reporter
Astronomers have discovered that the planet Uranus has a blue ring - only the second found in the Solar System.
Schematic view of the outermost rings of Uranus...
Like the blue ring of Saturn, it probably owes its existence to an accompanying small moon.
Scientists suspect subtle forces acting on dust in the rings allow smaller particles to persist while larger ones are recaptured by the moon.
Smaller particles reflect blue light, giving the ring its distinctive colour, the US team reports in Science.
All other rings - those around Jupiter, Saturn, Uranus and Neptune - are made up of both large and small particles, making the rings reddish in appearance.
Astronomers have long known that the gas giant Uranus is surrounded by rings of dark particulate matter up to ten metres in diameter.
But last December, two new rings - the planet's twelfth and thirteenth - were discovered using the Hubble Space Telescope.
Astronomers observed the ring system at infrared wavelengths with the Keck telescope, in Hawaii.
The outermost ring, and its ice-bound moon Mab, could not be observed in infrared light unlike the red inner ring.
A team led by Imke de Pater, professor of astronomy at the University of California, Berkeley, found that the ring was bright blue, something of an oddity in planetary terms.
"The blue colour says that this ring is predominantly submicron-sized material, much smaller than the material in most other rings, which appear red," Professor de Pater said.
The tiny particles - less than a thousandth of the width of a human hair - scatter and reflect predominantly blue light, much like the very small molecules in the air that make the Earth's sky blue.
The more common rings are reddish because they also contain many larger particles, which gives the reflected light its colour, and may be made up of reddish material, perhaps from iron.
It appears that the outer blue rings of Saturn and Uranus are strikingly similar, not least because they are both associated with small moons.
"The moon orbits the planet in the ring," Professor de Pater told the BBC News website.
"It is continuously impacted by very tiny particles [micrometeorites]. On a moon that doesn't have any atmosphere these tiny particles impact the moon at high velocity, and throw stuff up into space.
"Because the moon is so small, it escapes the moon and goes into orbit around the planet.
"The smaller particles stay in orbit around the planet but the larger particles smash back into the moon."
The work was carried out in collaboration with Mark Showalter, of the Search for Extra-Terrestrial Intelligence (Seti) Institute in California; Heidi Hammel, of the Space Science Institute, Colorado; and Seran Gibbard, of Lawrence Livermore National Laboratory in California.
The scientists plan to carry out further observations next year, when the faint rings of Uranus will be more visible.