Astronomers have found a huge new ring around the planet Saturn. The faint dust ring extends up to 7.4 million miles (12 million km) from the planet and could fit over a billion Earths inside it, making it the largest in the Solar System. It could also solve a mystery about one of Saturn's moons that has puzzled scientists for centuries.
The ring was found with the help of NASA's Spitzer Space Telescope, with details published today in the journal Nature. It is thought to consist of ice and dust from Saturn's moon Phoebe, which is kicked up by collisions with comets and then drifts in towards the planet. The ring and moon both orbit in a plane inclined at 27 degrees to the other rings.
"This is one supersized ring," said Dr Anne Verbiscer of the University of Virginia, one of the authors of the paper. Of Saturn's other rings, the largest is the E-ring, a mere 150,000 miles (240,000 km) in diameter. Jupiter also has "gossamer rings" of a similar diameter to the E-ring. If the Phoebe ring was visible from Earth, it would appear twice as large in the sky as the full Moon.
However the newly found ring is extremely faint. It is made up of dust particles around 10 microns (thousandths of a millimetre) in size, and according to Verbiscer, "In a cubic km of space, there are all of 10-20 particles." This explains why it has evaded discovery until now. "If you were standing in the ring itself, you wouldn't even know it."
The Phoebe ring does not reflect much visible light, but the Spitzer telescope was able to pick up the dust's faint infra-red glow. The telescope, launched in 2003, orbits the Sun and is roughly 66 million miles (107 million km) from Earth. It is one of NASA's four Great Observatories.
The discovery could also finally account for the unusual appearance of Iapetus, another of the planet's moons. When Iapetus was first observed in 1671 by astronomer Giovanni Cassini, its leading side was seen to be much darker than the other. Until now scientists had been unsure why this was. Now it is thought that the moon orbits in the opposite direction to the ring, and as Iapetus moves through the ring, dust builds up on its front surface. Verbiscer likens it to "bugs on a windshield."
"Astronomers have long suspected that there is a connection between Saturn's outer moon Phoebe and the dark material on Iapetus," said Douglas Hamilton, another author of the paper. The material has been found to have a similar composition to Phoebe's surface. "This new ring provided convincing evidence of that relationship."
This is the second major discovery for astronomers studying Saturn in the past month. In September, evidence from the Cassini orbiter showed that Saturn's other rings were far less flat than expected.

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New ring discovered around Saturn, could explain dark side of its moon