With a total now of 274 known moons, Saturn leads all of the planets by far.
diagram with tangle of red and blue ovals showing moon orbits
diagram with tangle of red and blue ovals showing moon orbits
A sudden listing by the Minor Planet Center this week nearly doubled the number of known moons of Saturn, and in the process provided strong evidence for at least one major collision between Saturnian moons within the last 100 million years.
Although the data on the 128 new moons was reported all at once on March 11th in a series of Minor Planet Center circulars, the data had been collected intermittently since 1999, and information for some of the moons had been informally reported before. But it took years of careful study to provide enough repeat observations to officially confirm that these objects — which span between 1 and 20 kilometers (0.6-12 miles) — were indeed real moons of Saturn.
“It has definitely been a long process,” says team member Mike Alexandersen (Center for Astrophysics, Harvard & Smithsonian). The discoveries were all made using the Canada-France-Hawaii 3.6-meter telescope on Mauna Kea, Hawai‘i, in a survey recently led by Edward Ashton (Academia Sinica, China). After seeing preliminary evidence for the moons in surveys that had been conducted between 1999 and 2003, Ashton decided to revisit those patches of sky in 2021 and 2023, making a series of observations on consecutive nights to detect the moons’ movements relative to background stars.
The newfound moons are all irregular, meaning their orbits are scattered in relation to the orbital plane of the rings and the larger moons. In addition, the majority of them circle the planet backwards, or retrograde, in the direction opposite to the orbits of the other moons and the planet itself. They also orbit as much as 50 times farther out from the planet than the other, major moons. Taken together, these observations suggests that the new moons are all captured asteroids, or the remains of collisions of such objects, as opposed to objects that formed along with the planet itself.
What’s more, the orbits of the new moons are grouped into a few clumps. Each clump may represent the debris from a particular collision. One of these clumps in particular shows strong evidence that the collision that produced these tiny moons was recent, at lest in astronomical terms — less than 100 million years ago.
Perhaps coincidentally, that’s about the estimated age of Saturn’s dramatic ring system, according to some reports. “Down at the 1-kilometer level, Saturn really does have way more satellites” than the much larger planet Jupiter, Alexandersen says. That may be because more recent collisions produced a plethora of small fragments.
The team used a shift-and-stack technique to find the new moons, a method pioneered in the late 1990s by team member Brett Gladman (University of British Columbia, Canada). “I basically started the modern epoch of CCD detection of irregular satellites back in 1997, when we found the first couple at Uranus,” he tells Sky & Telescope. “And for Saturn, we found the first dozen back in 2000.” Even back then, it was clear that the tiny moons seemed to orbit in clumps.
After a single large smash-up, the proportion of smaller fragments gradually reduces as multiple minor collisions grind them down to invisibility. The relative proportions of moons of different sizes thus provides a way of estimating the age of the precipitating event.
Based on the number and sizes of the moons and their orbital speeds around Saturn, Gladman estimates the major collision occurred not that long ago: “We get 100 million years as an upper limit.”
But since that’s just an upper limit, the collision could have been even more recent, he says. “It could have been a million years ago” he adds. “It could have been a thousand years ago. We have no idea.”
Some scientists have proposed that Saturn's rings, shown nearly edge-on in this recent image, formed within the last 100 million years.
Damian Peach
The same team reported the discovery of 64 new moons of Saturn two years ago, and some news reports have wrongly claimed that these were included as part of the newly announced group of 128. “That’s not correct, the 128 is in addition to the previous 64, and I don’t know why they’re perfect powers of two,” Gladman says. Saturn’s total now stands at 274 moons, handily eclipsing Jupiter’s second-highest total of 95.
Studying these moons may shed light on Saturn’s recent family history. “I’m thinking that these collisions might have been triggered by one common event,” Alexandersen speculates. “It would be weird if there had been multiple large catastrophic events [recently] in the Saturn system and not as much in other parts of the solar system.”
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Saturn's Moons