Saturn has solar system’s biggest ring around it

Saturn – planet of rings and moons – now has the largest planetary ring in the solar system, an enormous ring of dust and ice associated with Saturn’s outer moon Phoebe. The new ring was announced today by astronomers at the University of Maryland and the University of Virginia.

Doug Hamilton of the University of Maryland and colleagues Anne Verbiscer and Michael Skrutskie of the University of Virginia used NASA’s Spitzer Space Telescope to find the new ring, which lies in the outer reaches of the planet’s gravitational field. Their findings are published online today in the journal Nature.

Hamilton said the new ring is “huge and very diffuse” and that astronomers didn’t see it before because the ring is so large and far out from the planet that you need a very wide field of view to find it. Furthermore, the ring particles are extremely dark which makes them difficult to see with visible light [telescopes].

According to Hamilton, the ring has a diameter equivalent to 300 Saturns lined up side to side. It’s also thick – some 20 Saturns could fit into its vertical height.

Hamilton explained that one odd fact had long pointed to the possibility of an unseen ring of debris: another of Saturn’s moons, Iapetus, is black on one side and white on the other. The strange coloration of Iapetus was detected more than three centuries ago by astronomer Giovanni Cassini who first spotted the moon in 1671, and some years later figured out it has both dark and light sides.

“Astronomers have long suspected that Saturn’s outer moon Phoebe had a role in this riddle, perhaps as a source for dark material that had impacted one side of Iapetus,” said Hamilton. Finding this new ring provides convincing evidence of that relationship.”

In their Nature article, Hamilton, Verbiscer and Skrutskie say that Phoebe – by far the largest of Saturn’s distant satellites – “is probably the primary source of ejected debris in the outer saturnian system.” Their paper notes Phoebe’s orbit lies within the newly discovered ring and that both ring and moon rotate around Saturn in the same direction, a direction opposite to that of Iapetus and the other inner satellites. “Ring particles smaller than centimeters in size slowly migrate inward and many of them ultimately strike the dark leading face of Iapetus,” they write.

Hamilton and his colleagues used Spitzer’s infrared camera to scan through a patch of sky far from Saturn and a bit inside Phoebe’s orbit. The scientists thought Phoebe might be circling around in a diffuse belt of dust kicked up from its minor collisions with comets – a process similar to that around stars with dusty disks of planetary debris. And, when the scientists took a first look at their data, a broad band of dust stood out.

This ring would be difficult to see with visible-light telescopes because its particles are diffuse and may even extend far beyond main part of the ring material. The relatively small numbers of particles in the ring wouldn’t reflect much visible light, especially out at Saturn where sunlight is weak.