Saturn’s tangled moons may rewrite its ring timeline

Saturn’s irregular – Astronomers have uncovered more than 100 previously elusive irregular moons—especially around Saturn—suggesting the outer solar system may still be reshaping itself. The timing of one newly studied group of moons lines up with proposed ideas for the age of Sat

Out beyond Jupiter, the solar system can feel oddly calm. Light takes just 8 minutes to reach Earth from the Sun, but it can take more than an hour just to cross the vast gap between Uranus and Neptune. In that black, distant space, faint points of light are easy to miss.

And yet. in the last year. astronomers have been collecting a flood of those points—more than 100 new moons hiding in plain sight. They don’t look like the grand satellites most people picture from textbooks. These are irregular moons: small, dark, and often following tilted or even backwards orbits. From Earth, they appear as faint, fast-moving dots, easily swallowed by the glare of their planets.

“[We have had this] huge influx in the last year, [including an] eye opener at Saturn,” Marina Brozovic at NASA’s Jet Propulsion Laboratory in California said.

The surprise is not only how many new moons turned up. It’s what they imply: the outer solar system may not have been quiet for billions of years after all.

For decades, the prevailing story was that the dramatic rearranging of planets happened early. The new moons are forcing a different question—whether some of the violence happened much more recently, and whether it could be tied to one of Saturn’s most enduring mysteries: how its rings formed.

The irregular moons are not just numbers on a screen. Their orbits are stretched and skewed, suggesting they didn’t assemble from the same material that built Saturn itself. Many travel in loose families that follow similar paths around the planet. That pattern points toward fragments—broken pieces of larger parent moons hundreds of kilometres across, shattered by collisions long ago.

To Jonti Horner at the University of Southern Queensland, Australia, that makes them valuable relics. “They are relics of the solar system’s formation,” he said.

Catalogues have been filling in for years, too. One of the first irregular moons recognized was Phoebe around Saturn in 1898. It remains the largest of Saturn’s irregular moons at 213 km across. As telescopes and digital cameras improved—especially from the early 2000s—astronomers began spotting smaller and smaller irregular moons around giant planets.

Then came last year’s deluge. In 2025, researchers announced 128 new moons around Saturn alone, pushing the known total number of solar system moons above 450.

Scott Sheppard at the Carnegie Institution for Science in Washington DC has led searches for many of these faint bodies. For him, the bigger picture wasn’t a shock. “Astronomers were always sure there were more moons to be found around the gas giants,” he said. Telescopes simply couldn’t pick up such faint signals until recently.

Still, the pace surprised even researchers. “Everybody was surprised,” Brozovic said. Astronomers expected maybe a few dozen more moons around the outer planets, not hundreds or even thousands waiting to be found. “It really is starting to be pretty busy out there in the solar system,” she said.

Capturing moons requires losing energy. In the early solar system, that was more plausible, but the details have remained thorny. For decades, astronomers leaned toward mechanisms that could account for how irregular moons arrived in the first place. Sheppard explained that there isn’t an easy way for a planet to capture a passing object in a settled solar system. A comet or asteroid can wander into a planet’s gravitational grip. but without something to slow it down. it flies back out.

“The only way to capture a moon is to dissipate energy from its orbit,” he said.

Several capture scenarios were proposed. One involved the gas giant atmospheres being more extended early on. letting passing asteroids or comets be slowed enough to be captured into the wild orbits irregular moons occupy. That may work for smaller bodies. Sheppard said. but it struggles to explain how the larger parent bodies—later smashed apart—ended up captured in the first place.

A more promising route comes from the Nice model, the most accepted picture of solar system evolution. It proposes that Jupiter, Saturn, Uranus, and Neptune did not form where they are now. Instead, they started packed much closer together, with gravitational interactions sending them migrating outward. During that migration, their combined gravity could also have helped slow passing objects, including the larger progenitors of irregular moons.

This approach also fits the way today’s irregular moons don’t share a common origin, instead looking like a “cosmic jumble.” That view is supported by papers published last year using observations from the James Webb Space Telescope.

The violence didn’t have to stop at capture, either. The chaotic early period may have been when once-larger irregular moons crashed together, creating the smaller objects astronomers find today.

Then came a crucial twist in 2025.

A team led by Edward Ashton at Academia Sinica in Taiwan took a closer look at the Mundilfari group—about 100 newly discovered small moons looping around Saturn. At first glance, the family looks like it could be the debris trail of an ancient smash-up. But Ashton’s team modelled their sizes and found the timing didn’t quite add up.

If the fragments had circled Saturn since the early days of the solar system, many of the smaller moons should have fallen into the planet by now. Instead, the team argued that the Mundilfari group may have formed in a collision just 100 million years ago.

“[That might mean] these collisional processes are still alive and well,” Brozovic said.

Ashton’s paper was published in December 2025. It pulled attention from Yifei Jiao at the University of California, Santa Cruz, because the age of the Mundilfari group seems to sit close to another “suspected” number: the suspected age of Saturn’s rings.

Saturn’s rings have been observed for more than 400 years, but explaining how they formed remains stubborn. For a long time, the simplest story was that the rings formed early—either from leftover material around Saturn or from a moon that shattered billions of years ago.

That changed after the Cassini mission.

In Cassini’s final months. before it plunged into Saturn in 2017. the spacecraft repeatedly threaded the narrow gap between the planet and its rings. Those dives revealed rings that were surprisingly low in mass and remarkably clean. The problem was basic chemistry and physics: over billions of years, micrometeoroids should darken and wear down the ring system. Yet photos taken by Cassini made the rings look suspiciously fresh—perhaps only a few hundred million years old.

So what could have made them recently?

The new moons offered a possible route.

Irregular moons can collide with each other or get struck by passing comets and asteroids. Those impacts can produce clouds of dust that gradually fall toward their host planet. Evidence of this kind of process already exists in Saturn’s system. Saturn’s moon Iapetus—its outermost regular moon—shows a two-toned appearance. Its leading side is extremely dark, while its trailing side is bright.

Sheppard described a mechanism that could connect the dots. Iapetus could be “running through the reddish, carbon-rich dust shed by irregular moons ‘like a bulldozer’.”

For years, though, distant irregular moons weren’t thought to have much to do with Saturn’s rings. In April, Jiao and his team published a paper suggesting a link.

They began with the idea that Saturn once had an extra icy moon about 1000 km across called Chrysalis. Over the roughly 4.5-billion-year history of the solar system. it would have fallen into a gravitational rhythm with Titan. Saturn’s largest moon. The mutual tug between them would have stretched Chrysalis’s orbit from a circular shape into an ellipse.

Then. about 100 million years ago. Chrysalis’s distorted orbit carried it past a threshold around Saturn known as the Roche limit—the boundary within which Saturn’s gravity can tear a moon apart. In a catastrophic pass. Saturn stripped away much of Chrysalis’s ice. almost instantly shrinking the moon to half its size.

What happened next was uncertain. The rocky core of Chrysalis could have been cannibalised by Saturn or ejected entirely. Jiao noted that “we haven’t found such a body” orbiting the sun somewhere else.

The ice, meanwhile, would have spread like butter, forming a broad, bright disc over a few thousand years—Saturn’s rings.

But the story may not end at the ring plane. Some chunks could have been flung into Saturn’s outer reaches, where one piece struck another moon and shattered it—forming the Mundilfari group of irregular moons at the same time.

If that chain of events played out, then Saturn’s rings and some of its hidden moons could be two remnants of the same lost world, both formed about 100 million years ago.

“It is hard to imagine that all of these events occurred at the same time by coincidence,” Jiao said. “I am quite excited about the possibility of linking the lost moon Chrysalis with the irregular satellite population.”

The timing is compelling, but not everyone is ready to call it settled.

Horner said the study is “definitely a very cool” way rings might have formed. but he cautioned that linking the destruction of Chrysalis to the formation of the Mundilfari irregular moons would require more evidence. He pointed to impact scars on Saturn’s other regular moons—signs that other moons may also have been struck. A future mission, he said, might be able to look for that.

Brett Gladman at the University of British Columbia in Canada agreed the alignment of ages is intriguing but emphasized caution. “It’s certainly curious that these two wildly disparate events come out to be the same age, but that doesn’t necessarily mean they have a causal connection,” he said.

Jiao said further modelling will be needed to test whether his idea is right.

There is another reason the new discoveries are hitting so hard with planetary scientists: the race to find irregular moons isn’t finished, and Saturn isn’t the only candidate for surprises.

For a long time, astronomers expected Jupiter to dominate the irregular moon inventory. It is the largest planet by far and has the strongest gravitational pull. Last year’s discoveries flipped that assumption. Saturn is now ahead, with 274 known moons compared with Jupiter’s 115.

That still leaves a question mark over Uranus and Neptune.

Uranus and Neptune have far fewer known moons—29 and 16. respectively—but researchers say that gap may reflect how hard those planets are to survey rather than how many moons truly exist. Their distance gives them vast regions of gravitational influence called Hill spheres. Neptune’s stretches some 115 million km—almost twice Saturn’s. Their proximity to the Kuiper Belt, a reservoir of icy debris, may also have fed captures.

Luke Dones at the Southwest Research Institute in the US said he expects that trend to become clearer over time. “I fully expect that someday, a couple of decades away, we will find thousands of these objects at Uranus and Neptune,” he said.

If Uranus and Neptune turn out to have the richest systems, it could show how efficiently the ice giants gathered material from their surroundings. If they don’t, the absence itself could reveal something—either that capture was less efficient or that something stripped those systems bare.

There is also a chance to see an irregular moon up close. The second time a spacecraft has ever visited an irregular moon after Cassini’s brief visit to Saturn’s satellite Phoebe in 2004 could involve Jupiter.

Tilmann Denk at the German Aerospace Center said the European Space Agency is considering adjusting the trajectory of its Jupiter Icy Moons Explorer spacecraft so it passes close to Kallichore. one of Jupiter’s tiny irregular moons. in 2031. It would be a fleeting encounter with a small. dark object—yet worthwhile. researchers say. because these overlooked moons may preserve some of the clearest records of how giant planets ended up where they are today.

The outer solar system has always been distant and faint. But with more than 100 newly revealed moons and fresh disputes about the age of Saturn’s rings, it has become vivid again—busy, violent, and still rewriting its own history.

irregular moons Saturn rings Cassini mission Mundilfari group Chrysalis Nice model James Webb Space Telescope Hill sphere Roche limit Iapetus Phoebe Kallichore Jupiter Icy Moons Explorer