Almost 400 years after Galileo spied Saturn's iconic ring system, a University of Maryland astronomer has discovered another gigantic but invisible ring around the planet. More than 14 million miles in diameter, it is by far the largest in the solar system.
Maryland's Douglas P. Hamilton and two University of Virginia astronomers spotted the dark, dusty ring using NASA's Spitzer Space Telescope, which is orbiting the sun 66 million miles from Earth. They reported their find Wednesday in the journal Nature.
The scale of the new ring is staggering.
Saturn's well-known inner rings of water ice extend 4,300 miles to 50,000 miles from the planet's equator. The bulk of the newly discovered ring of carbon-rich dust orbits Saturn in a band 3.7 million to 7.4 million miles out.
Saturn's inner ring system is wafer-thin, while the new one is 20 times thicker than the planet. It is tilted at a 27-degree angle from the other rings and is rotating in the opposite direction. The ring surrounds the orbit of a 60-mile-wide moon called Phoebe, which is thought to be the source of its dust.
"It's a thrill," Hamilton said, speaking by phone from a meeting in Puerto Rico of the planetary science division of the American Astronomical Society. The ring is "the largest in the solar system, 10 to 20 times larger" than the largest previously known.
The find might be the beginning of a series of discoveries of giant planetary rings.
"The prediction is that any time you have small moons anywhere around planets, they ought to be continuing to produce debris," as collisions kick rocks and dust off the moons, he said. "So, we expect to find big rings around Jupiter, Uranus and Neptune."
Saturn looks like a star to the naked eye, and its familiar rings orbit so close to the planet that they can be seen only with a telescope. If the new ring were visible from Earth, Hamilton said, it would span the width of a full moon on each side of the planet. Unfortunately for stargazers, the new ring is invisible. Its dust particles are so diffuse, and the light at Saturn's distance from the sun is so dim, that the ring cannot be seen in visible light wavelengths.
Hamilton and University of Virginia astronomers Anne Verbiscer and Michael Skrutskie went looking for the ring because astronomers have theorized for decades that collisions among small outer moons like Phoebe should spread dust into a vast ring. It was also suspected that dust in the region would resolve a 340-year-old mystery about another small outer Saturnian moon, called Iapetus.
The Italian astronomer Giovanni Cassini discovered Iapetus in 1671 and noted that one side of the moon looked very bright while the other side was dark. Scientists suspected that Iapetus is a bright, icy moon that has been smudged on one side by dust tossed out by Phoebe, which circles Saturn in the opposite direction.
But "nobody's ever looked before," Hamilton said. So, after years of proposals, he and his team finally won time to aim Spitzer that way last February.
Spitzer images the sky in infrared wavelengths. It detected the ring by sensing the feeble "warmth" that its particles have absorbed from the sun against the deep cold of interplanetary space behind it.
It wasn't until summer that Hamilton and his colleagues got their chance to see the data, but they saw the ring immediately.
"It was in the right place, the right thickness, and it looked like a ring," Hamilton said. "But we had to be sure it wasn't an artifact" - some sort of electronic "noise" in the telescope.
It wasn't. And now Hamilton and his team are planning new observations. Because Spitzer has exhausted the coolant it needs to repeat such infrared observations, they will try to use ground-based telescopes and very long exposures to spot giant rings around the outer planets.
"Now that we know this exists," he said, "the obvious thing to do ... is to take telescopes and spend a lot of time looking."
The discovery is "a very nice verification of what we on the Cassini team have suspected ... [that] the leading hemisphere of Iapetus is darkened by particles from an exterior moon. Most figured it had to be Phoebe," said Carolyn C. Porco of the Space Science Institute in Boulder, Colo., and part of the science team on NASA's Cassini mission to Saturn. "It's a lovely confirmation of an idea that's been floating around for some time."