October 7, 2009 — Astronomers at the University of Virginia and the University of Maryland, using an infrared camera aboard NASA's Spitzer Space Telescope, have found the largest planetary ring in the solar system.

Listen to the UVA Today Radio Show report by Fariss Samarrai:

The thin ring swirls around Saturn at about 200 times the radius of the planet and is likely caused by debris – ice and dust – from Phoebe, one of Saturn's most distant moons. Until this discovery, Saturn's E-ring, at about four to 10 times the radius of the planet, was known as its farthest ring.

'This ring is so enormous, if we could see it with our eyes from Earth, it would appear twice the size of our moon,' said Anne Verbiscer, a U.Va. astronomer in the College of Arts & Sciences, and the lead investigator.

The finding is reported online this week in the journal Nature.

Verbiscer and her colleagues, astronomers Michael Skrutskie of U.Va. and Douglas Hamilton of Maryland, used the Spitzer telescope to scan a region around Saturn in the vicinity of Phoebe, suspecting that a very fine ring might be present. By searching with cameras sensitive to infrared light, they were able to reveal the hidden ring.

The bits of ice and dust that make up the ring are widely spaced. 'We estimate that there are only 20 particle grains per cubic kilometer across the length of the ring,' Verbiscer said. 'We used a camera at a mid-infrared range that is sensitive to the light emitted by these cold tiny particles.'

Phoebe is about 200 kilometers across. It orbits Saturn, the second-largest planet in the solar system, at a radius of about 13 million kilometers.

As with all bodies in the solar system, meteoroids and comets regularly strike Phoebe and Saturn's other moons. The impacts excavate ice and dust, which spreads to occupy a ring.

The finding also suggest that another outer moon of Saturn, Iapetus, which has a dark leading face, may be coated with dust from Phoebe and other outer moons of Saturn.

'Saturn lies in a very dusty place in space,' Verbiscer said. 'We basically detected more of that dust, much farther out from the planet than ever seen before.'