NASA's Spitzer Space Telescope has detected a gigantic ring around Saturn that spans a diameter equivalent to 300 Saturns lined up side by side.

“This is one supersized ring,” says Anne Verbiscer of the University of Virginia, Charlottesville. “If you could see the ring, it would span the width of two full Moons’ worth of sky, one on either side of Saturn.”

Until now the largest-known planetary rings were Jupiter’s gossamer rings and Saturn’s E ring — sheets of dust that extend to about 5–10 times the radius of their respective planets. Saturn's newly discovered belt blows these records completely out of the water. Inclined by 27 degrees to the main ring plane, the bulk of its material starts at a distance of about six million kilometers from the planet and extends across another 12 million kilometers – a diameter equivalent to 300 Saturn's lined up side by side. The observations also reveal that the ring is thick – its vertical height reaches 20 times the diameter of Saturn, and it would take one billion Earths stacked together to fill the ring.

One of Saturn’s farthest moons, Phoebe, circles within the newfound ring in the same direction, and is likely the source of its material – ice and dust particles that were easily detected by Spitzer's infrared capabilities. Astronomers had long suspected that Phoebe might be circling around in a belt of dust kicked up from its minor collisions with comets and sure enough they were right – Spitzer was able to sense the cool 80 degree kelvin glow of the dust that is invisible to the human eye. The team also produced computer simulations to demonstrate how repeated impacts on Phoebe can keep the ring supplied with dust.

“By focusing on the glow of the ring’s cool dust, Spitzer made it easy to find,” says Verbiscer. Moreover, the ring particles are so diffuse that it may even extend beyond the bulk of the ring material all the way in to Saturn and all the way out to interplanetary space. “The particles are so far apart that if you were to stand in the ring, you wouldn’t even know it,” adds Verbiscer.

The faint ring may also explain a longstanding mystery why Saturn's moon Iapetus displays a bright trailing hemisphere but a curiously dark leading hemisphere. “Astronomers have long suspected that there is a connection between Saturn’s outer moon Phoebe and the dark material on Iapetus,” says Douglas Hamilton of the University of Maryland. “This new ring provides convincing evidence of that relationship.”

The team suggest that the moon's leading face is coated with dust spiralling in from Saturn’s darker outer moons, including Phoebe, and slamming into Iapetus' surface, over time building up into a thick, dark blanket of debris several metres thick.

The observations were made before Spitzer ran out of coolant in May and the results will be published in tomorrow's edition of the journal Nature.