text: Saturn's biggest ring just broke its own record. Scientists have found that the planet's Phoebe ring is more than twice the thickness once thought – even though it can’t be seen with the human eye. 
   
   The study, published in the journal Nature, also finds that this oversized ring is made mostly of tiny particles. The findings could shed fresh light on the evolution of the Saturnian ring system. 
   
   “I was giving talks saying Saturn has a giant invisible ring, which makes you sound just perfectly crazy,” said lead author Douglas Hamilton, a planetary scientist at the University of Maryland, College Park. “It is 10 to 20 times larger than the second-biggest ring, so this thing is absolutely gargantuan.” 
   
   Many of Saturn’s rings seem to hug the planet, forming the beautiful striped disc for which the gas giant is so well known. But in 2009, NASA’s infrared-sensing Spitzer Space Telescope picked up another ring so large that the entire planet and its main rings appear as a small dot in the middle. 
   
   The ring is difficult to detect in visible light (NASA’s Cassini spacecraft did so a few years later), but it can be clearly seen as a ghostly halo at infrared wavelengths. 
   
   Hamilton and colleagues first went looking for this "giant invisible ring" because of the strange two-tone coloring of Iapetus, one of Saturn’s moons. One side is pale, but the other side of the moon is dark, as if covered with soot. The scientists theorized that unseen particles from farther out were hitting Iapetus and blackening one side. (Iapetus is tidally locked with Saturn and can't rotate fast enough to get an all-around coating.) 
   
   “If that material was coming inward and coating the face of Iapetus, then maybe we could see the ring,” Hamilton said, and the team started searching. “Usually those things don’t pay off, but this one did.” 
   
   At the time, they estimated that the ring stretched from 4.8 million to 7.76 million miles, though with Spitzer's relatively narrow field of view, they couldn't say how thick it really was. 
   
   Now, with NASA’s Wide-field Infrared Survey Explorer telescope, they have found that it stretches from about 3.75 million to 10.1 million miles – a little beyond Phoebe, a far-out moon that is the same color as the stuff in the ring, and is probably the source for the dark material. 
   
   While the Phoebe ring is enormous, the particles within are surprisingly tiny – the scientists estimate that the vast majority of them, roughly 90%, are smaller than the size of a soccer ball. 
   
   Understanding why the particle size distribution is so skewed could offer researchers “a window into the history of the Saturn system,” Hamilton said. 
   
   Follow @aminawrite for more science news. ||||| WISE Band 4 mosaic of the Phoebe ring. a, Individual WISE frames manually combined: scattered light from Saturn forms the large white overexposed blob with a black centre while four diagonal diffraction spikes radiate outward. Bright reflections of Saturn are visible as smaller white lumps with black centres at the six and twelve o’clock positions. Iapetus (black dot) and the more distant Phoebe (white dot) are visible at nine o’clock. The Phoebe ring is the white, horizontally oriented 550RS × 40RS rectangle. b, We subtract ±90° rotations of the top frame from itself, yielding clean and cluttered ring ansae (the apparent ends of edge-on rings); here we stitch the two clean ansae together to significantly reduce scattered light and reveal the ring’s inner regions. Distance scale applies to a and b. Credit: (c) Nature 522, 185–187 (11 June 2015) doi:10.1038/nature14476 (Phys.org)—A small team of researchers with members affiliated with the University of Maryland, the University of Virginia and Caltech, has found that the outermost ring of Saturn is much bigger than had been previously thought. In their paper published in the journal Nature, the team describes how they studied the ring using an infrared telescope and what they believe caused its immense size to come about. 
   
   Back in 2009, a team of researchers working with NASA's infrared Spitzer Space Telescope discovered that there was an outer ring circling Saturn that had not been known to exist—made mostly of extremely tiny dark dust particles, the ring was difficult to see. They estimated at the time that the ring was likely over two hundred times the radius of its host planet. In this new effort, the researchers studied images taken from NASA's WISE probe, which has also been taking infrared pictures. Those pictures, the team reports, suggest the ring is actually over 270 times the radius of Saturn, which mean it stretches from almost four million miles from the planet to just over ten million miles, making it approximately ten times the size of the E ring, until this new discovery, Saturn's largest ring. Putting it into perspective, if Saturn were merely the size of a basketball, this new outer ring would extend nearly two thirds the length of a football field away from it. 
   
   The ring has only been revealed by infrared cameras because the dust particles in it absorb heat from the sun—such particles the researchers report are extremely small, just microns in size. They also report that there are some other much larger, though far less in number particles, some as large as soccer balls. Because of the huge distance between the ring and planet, the researchers theorize that the material in the ring is probably very old, perhaps in the billions of years. They also suspect the tiny particles are material ejected from the distant moon Phoebe. 
   
   Prior to the discovery of Saturn's largest ring, it was believed material ejected from moons at such a distance would coalesce into a new moon, rather than form a ring—now it appears such theories will have to be rethought. Also, because of similarities between Saturn's moons and Jupiter's moons, the researchers suspect Jupiter likely has a massive unseen ring around it as well. 
   
   Explore further: Image: Groovy rings of Saturn 
   
   More information: Small particles dominate Saturn's Phoebe ring to surprisingly large distances, Nature 522, 185–187 (11 June 2015) DOI: 10.1038/nature14476 
   
   Abstract 
   
   Saturn's faint outermost ring, discovered in 2009 (ref. 1), is probably formed by particles ejected from the distant moon Phoebe2, 3. The ring was detected1 between distances of 128 and 207 Saturn radii (RS = 60,330 kilometres) from the planet, with a full vertical extent of 40RS, making it well over ten times larger than Saturn's hitherto largest known ring, the E ring. The total radial extent of the Phoebe ring could not, however, be determined at that time, nor could particle sizes be significantly constrained. Here we report infrared imaging of the entire ring, which extends from 100RS out to a surprisingly distant 270RS. We model the orbital dynamics of ring particles launched from Phoebe, and construct theoretical power-law profiles of the particle size distribution. We find that very steep profiles fit the data best, and that elevated grain temperatures, arising because of the radiative inefficiency of the smallest grains, probably contribute to the steepness. By converting our constraint on particle sizes into a form that is independent of the uncertain size distribution, we determine that particles with radii greater than ten centimetres, whose orbits do not decay appreciably inward over 4.5 billion years, contribute at most about ten per cent to the cross-sectional area of the ring's dusty component. 
   
   ||||| Rings around planets are supposed to stay close to home, as any Astro 101 textbook will tell you. Once they venture too far afield from their gravitational overlord, conventional astronomical wisdom dictates that they will collapse and form new satellites. 
   
   “That does a really good job of explaining rings—except for this one,” says Douglas Hamilton, an astrophysicist at the University of Maryland. Hamilton and colleagues describe Saturn’s biggest, strangest, most recently-discovered ring in a study published today in Nature. The so-called Phoebe ring is not only bigger than the researchers thought, it appears to be made of unusually fine particles—particles that continually collide with Saturn’s moon Iapetus as it circles the planet, turning the moon’s leading face black. 
   
   Hamilton was part of the team that discovered the Phoebe ring in 2009, with help from the Spitzer Space Telescope. They wouldn’t have found it except for that weird pattern on Iapetus’ surface: one face an icy white, the other inky black. Like Earth’s own moon, Iapetus keeps one constant face toward its planet. So one side—the darkened one—is forever leading the charge through space. Phoebe, a dark-as-coal moon further away from Saturn than Iapetus, was the second piece of the puzzle. 
   
   “You can imagine something big smashed into Phoebe a billion years ago, and all of this debris was flung out,” Hamilton says. When he calculated how long it would take for this debris to be cleaned up (either by collapsing into a new satellite or to fall to Saturn) he found that it would take a shockingly long time: about 10 billion years. That’s longer than the solar system has existed. “We realized that all of the debris coating the face of Iapetus, that didn’t happen long ago. It’s going on now,” he says. So his team started searching for the source of the material painting the moon black. “Finding it was really gratifying.” 
   
   That was six years ago. Back then, when his team first wrote about their findings, the only available data on this vanishingly faint ring came in the form of a small cross-section. For Hamilton the picture felt “not much bigger than a postage stamp.” Now, using data from NASA’s Wide-field Infrared Survey Explorer spacecraft launched in 2009, they’ve gathered a complete view of Phoebe’s enormous ring. “It was immense beforehand, and we just made it even bigger,” Hamilton says. 
   
   The Phoebe ring, they now know, is 30 percent larger than previous estimates, covering a section of sky equal to more than 500 times the size of Saturn. That makes it the largest ring—and possibly the slowest developing—ever discovered. “The thing that makes this unique is that, because it’s far from the planet, time scales are really slow,” Hamilton says. “Anything that comes off Phoebe just stays out there.” 
   
   No other rings like this have been found, though theoretically they may exist. Jack Lissauer, a planetary scientist at NASA Ames, says that although the evidence is strong that much of the material in the ring comes from Phoebe, he’d like to see closer imaging: Perhaps some of Saturn’s more distant satellites (the planet has 53 moons in total) are contributing material as well. 
   
   Phoebe’s ring is “fantastically faint,” in the words of Luke Dones, astronomer at the Southwest Research Institute. While its mere detection is impressive, it may also point the way toward understanding debris disks around other stars, which could contain unseen planets. Even in our own solar system, Jupiter, Uranus and Neptune may have similar, albeit fainter and more spread out, rings. “There we expect much wider rings,” Hamilton says. “Maybe ring isn’t even the right word anymore. Maybe we should call them clouds. Those are much harder to detect and convince yourself that they’re real.” 
   
   For Hamilton, the Phoebe ring’s quirks are is a reminder that the universe still holds a lot of surprises. “We see it over and over again with exoplanets where you find them doing something you’d never expect, just because planets in our solar system don’t do that,” he says. “This is another reminder that there are lots of mysteries out there—we don’t understand everything yet.”
target: – About six years ago, scientists discovered a huge ring around Saturn that also happened to be invisible to the naked eye. Now, they've learned that it's much bigger than they thought, and their theory on how it came to be might change some thinking about planetary rings and moons. Scientists previously estimated the ring to be about 200 times the radius of Saturn, but now they say it's 30% larger, or about 270 times the radius, they write in Nature. It stretches more than 10 million miles, which a post at Phys.org puts into perspective: "If Saturn were merely the size of a basketball, this new outer ring would extend nearly two thirds the length of a football field away from it." Or as lead author Douglas Hamilton puts it in the Los Angeles Times: “It is 10 to 20 times larger than the second-biggest ring, so this thing is absolutely gargantuan.” Researchers also found that the ring is made of up surprisingly small particles, with only about 10% of them larger than a soccer ball. They suspect the material, which is quite spread out within the ring, comes from the Saturn moon Phoebe, but it was previously believed that debris ejected at such a distance would form a moon rather than a ring. "Now it appears such theories will have to be rethought," says the Phys.org post. The ring can be detected only in infrared wavelengths. Based on all this, it's possible that Jupiter, Uranus, and Neptune have similar rings, reports Wired. “Maybe ring isn’t even the right word anymore," says Hamilton, "Maybe we should call them clouds. Those are much harder to detect and convince yourself that they’re real.” (Another of Saturn's moons might be able to support life.)