Shutterstock\/Johan Swanepoel<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
In the far reaches of the solar system, the planetary neighbourhood seems quiet. Beyond Jupiter, the sun is no longer a blazing disc, but a cold, white lamp. The planets are separated by gulfs of darkness. Light takes just 8 minutes to get from the sun to Earth, but typically more than an hour to cross the yawning chasm between Uranus and Neptune.<\/p>\n
But in the middle of\u00a0what seems like an uneventful\u00a0part of the solar system, astronomers recently made a\u00a0mammoth\u00a0discovery:\u00a0a hidden population of\u00a0more than 100\u00a0moons that, until recently, remained almost invisible. From Earth, they appear as faint, fast-moving points of light, easily lost in their planets\u2019 glare.<\/p>\n
They aren\u2019t moons as we imagine them \u2013 grand worlds like our own pale satellite, Jupiter\u2019s volcanic Io or Saturn\u2019s haze-wrapped Titan. They are smaller, darker and far more unruly. Astronomers call them irregular moons, and with their numbers now so high, their hidden kingdom has become harder to ignore. \u201cWe have had this huge influx in the last year, [including an]\u202feye\u202fopener at Saturn,\u201d\u202fsays Marina Brozovic\u00a0at NASA\u2019s Jet Propulsion Laboratory in California.<\/p>\n
But it\u2019s not just the discovery of these moons that has astronomers excited. For one thing, they may hint that the outer part of our solar system might not be enjoying a quiet retirement, but instead has seen periods of incredible turbulence surprisingly recently. For another, these hidden moons may help us solve a mystery about one of our solar system\u2019s most iconic sights: how did Saturn get its rings?<\/p>\n
<\/p>\n What is a moon, exactly?\u00a0If you looked up at our night sky\u00a0you\u2019d\u00a0see our own natural satellite,\u00a0more than\u00a03400\u00a0kilometres across, keeping stately company with\u00a0Earth. Many of the solar system\u2019s other moons fit that picture,\u00a0too:\u00a0big, round worlds\u00a0circling\u00a0close\u00a0to their\u00a0host\u00a0planet,\u00a0usually moving with that\u00a0planet\u2019s spin.<\/p>\n But there is another kind of moon.\u00a0These are small, misshapen things, often only\u00a0a few kilometres\u00a0wide,\u00a0following\u00a0distant, tilted and sometimes backwards paths.\u00a0These are irregular moons\u00a0and,\u00a0for a long time, they were easy to overlook.<\/p>\n One of the first irregular moons to be recognised was Phoebe around Saturn in 1898, the largest of the planet\u2019s irregular moons at 213 km across. It was the herald of many more to come. As telescopes and digital cameras improved, especially from the early 2000s, astronomers began seeing smaller and smaller irregular moons around giant planets in droves. Then came last year\u2019s deluge. In 2025, researchers announced 128 new moons\u00a0around Saturn alone,\u00a0pushing the known total\u00a0of\u00a0solar system\u00a0moons\u00a0above 450.<\/p>\n For astronomer\u00a0Scott Sheppard at the Carnegie Institution for Science in Washington DC, who has helped lead many of these searches, the broad pattern didn\u2019t come as a surprise. Astronomers were always sure there were more moons to be found around the gas giants, he says; telescopes just couldn\u2019t pick up such faint signals until recently.<\/p>\n Despite being one of Saturn\u2019s largest moons, Hyperion has a strange non-spherical shape and an unruly orbit<\/p>\n NASA\/JPL\/Space Science Institute<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n Still, the scale of discoveries last year caught many off guard. \u201cEverybody was surprised,\u201d says Brozovic. Astronomers had expected maybe a few dozen more moons to be found around these outer planets, but instead hundreds or even thousands are now thought to be awaiting discovery. \u201cIt really is starting to be pretty busy out there in the solar system,\u201d says Brozovic.<\/p>\n These moons might be small, but their implications are large. Their oddly elongated orbits suggest that they didn\u2019t form in the same place as their host planets, the way larger moons did. Many also travel in loose families, following similar paths around their planet \u2013\u202fa pattern that seems to suggest they are the fragments of larger parent moons hundreds of kilometres across that broke apart in collisions long ago.<\/p>\n Thanks to these irregular moons, astronomers now think they can reconstruct this violent history and its role in shaping the solar system, says Jonti Horner at the University of Southern Queensland, Australia. \u201cThey are relics of the solar system\u2019s formation,\u201d he says.<\/p>\n To reconstruct this history, we need to first ask a pivotal question: how exactly did these irregular moons end up around these gas giants? For decades, astronomers thought the answer lay in the solar system\u2019s first flush of youth, because there just isn\u2019t an easy way for a planet to capture a passing object in the settled solar system we see today, says Sheppard. A comet or asteroid can wander briefly into a planet\u2019s gravitational grip, but unless something slows it down, it would simply fly out again. \u201cThe only way to capture a moon is to dissipate energy from its orbit,\u201d he says.<\/p>\n However, soon after the birth of our solar system \u2013 about 4.5 billion years ago \u2013 there were several possible mechanisms of capture. One involved the atmospheres of the gas giants themselves, which were more swollen and extended back then. Asteroids or comets flying through them could have been slowed enough to be captured into the wild orbits we see today. But while that works for smaller bodies, it struggles to explain how planets captured the suspected parent bodies of irregular moons, which were later smashed apart.<\/p>\n A more promising avenue\u00a0for that\u00a0is\u00a0the\u00a0Nice\u00a0model, the most accepted picture of solar system evolution.\u00a0It says Jupiter, Saturn,\u00a0Uranus\u00a0and Neptune\u00a0didn\u2019t\u00a0originally form\u00a0where\u00a0we see them now. They were packed much closer together when the solar system first took shape, before gravitational interactions sent them migrating outward. Their combined gravitational interactions during their migration could also have helped slow passing objects, including\u00a0the larger progenitors of the irregular moons we see today.<\/p>\n This would help explain\u00a0why\u00a0today\u2019s irregular moons\u00a0don\u2019t\u00a0seem to have a common origin, instead resembling a cosmic jumble\u00a0from across the solar system, according to\u00a0papers\u00a0published\u00a0last year\u00a0that used\u00a0observations from\u00a0the James Webb Space Telescope.<\/p>\n The chaotic nature of this early period was thought to also be when some of the once-larger irregular moons <\/span>might have crashed together, creating the much smaller objects around the planets we see today.<\/p>\n But then came a puzzling discovery in 2025. A team led by\u00a0Edward Ashton at Academia Sinica in Taiwan took a closer look at the Mundilfari group,\u00a0a clutch of some\u00a0100\u00a0newly discovered\u00a0small moons\u00a0looping around Saturn.<\/p>\n \u00a0<\/p>\n At first glance, this\u00a0strange family looks like the debris trail of some ancient cosmic smash-up.\u00a0But when Ashton\u2019s team modelled\u00a0their\u00a0sizes, that timing\u00a0didn\u2019t\u00a0quite add up. If these fragments had been circling Saturn since the early days of the solar system, many of the smaller moons should have fallen into the planet by now, tugged inward by its gravity.<\/p>\n Instead, Ashton\u2019s team argued that the Mundilfari group may have formed in a collision just\u00a0100 million years ago.\u00a0\u201c[That might mean] these collisional processes are still alive and well,\u201d says Brozovic.\u00a0If these\u00a0collisions really were surprisingly recent, that would suggest the outer solar system is still being\u00a0dramatically reshaped today,\u00a0long after the main drama of planet formation was thought to be over.<\/p>\n Such\u00a0collisions could\u00a0be linked to other events, too.\u00a0Ashton\u2019s paper was published in\u00a0December 2025, and\u00a0piqued the interest of\u00a0Yifei\u00a0Jiao\u00a0at the\u00a0University of California, Santa Cruz, because\u00a0the age of the Mundilfari group seems suspiciously close to another number:\u00a0the suspected age of Saturn\u2019s rings. Could the two be related?<\/p>\n For all their fame, Saturn\u2019s rings\u00a0remain\u00a0oddly hard to explain. We have seen them through telescopes for more than 400 years, yet\u00a0the\u00a0most important mystery lingers:\u00a0how did they form?\u00a0For a long time, Saturn\u2019s rings looked like an ancient ornament,\u00a0a bright, almost permanent fixture that had circled the planet since the solar system\u2019s youth. The simplest story was that they formed early,\u00a0perhaps from\u00a0leftover material around Saturn or from a moon\u00a0that\u00a0shattered billions of years ago.<\/p>\n Then measurements from NASA\u2019s Cassini mission complicated things. In its 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. That was hard to square with great age: over billions of years, micrometeoroids should have darkened the ice and worn the system down. Instead, photos taken by Cassini made the rings look suspiciously fresh \u2013 perhaps only a few hundred million years old. That leaves a difficult question: what could have made them so recently?<\/p>\n The new moons may offer a way in. We already know that irregular moons can make a mess as they collide into each other or are struck by passing comets and asteroids. They can explode into clouds of dust that gradually fall towards their host planet. We already see evidence for this on Saturn\u2019s moon Iapetus, the outermost of the planet\u2019s regular moons, which has a strange, two-toned colouration. Its leading side \u2013\u202fthe face that ploughs forward through space \u2013 is extremely dark, while the trailing side is nice and bright. Sheppard says that Iapetus could be running through the reddish, carbon-rich dust shed by irregular moons \u201clike a bulldozer\u201d.<\/p>\n Saturn has a striking two-toned moon called Iapetus. One face is covered in dark, reddish dust<\/p>\n NASA\/JPL\/Space Science Institute<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n Still, those distant,\u00a0irregular moons of Saturn\u00a0weren\u2019t\u00a0thought to have much to do with\u00a0the planet\u2019s rings. But\u00a0in\u00a0April, Jiao and his team\u00a0published a paper\u00a0suggesting a potential link.\u00a0They\u00a0first\u00a0built on the idea\u00a0that\u00a0Saturn once had an extra\u00a0icy\u00a0moon about 1000 km\u00a0across, called Chrysalis. Over the\u00a04.5-billion-year\u00a0history of the solar system,\u00a0it\u00a0fell into a gravitational rhythm with Titan, Saturn\u2019s largest moon. The mutual tug between both\u00a0elongated Chrysalis\u2019s\u00a0orbit from\u00a0a\u00a0circular\u00a0shape\u00a0into an\u00a0ellipse.<\/p>\n Then, about\u00a0100 million years ago, the moon\u2019s\u00a0distorted\u00a0orbit\u00a0carried it\u00a0past a threshold\u00a0around Saturn known as the Roche limit,\u00a0the boundary\u00a0within\u00a0which a planet\u2019s gravity can tear a moon apart.\u00a0In one catastrophic pass, Saturn stripped away much of Chrysalis\u2019s ice,\u00a0almost instantly\u00a0shrinking the moon to half its size.<\/p>\n What\u00a0would have happened next is uncertain.\u00a0The rocky core of Chrysalis\u00a0may have been cannibalised\u00a0by Saturn or ejected entirely, although Jiao notes we \u201chaven\u2019t found such a body\u201d orbiting the\u00a0sun\u00a0somewhere else. The ice, meanwhile, would have\u00a0spread\u00a0like butter, forming a broad, bright\u00a0disc\u00a0over\u00a0a\u00a0few thousand years\u00a0\u2013 Saturn\u2019s rings.<\/p>\n The origin of Saturn\u2019s rings continues to be one of the enduring mysteries of the solar system<\/p>\n Smith Archive\/Alamy<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n But\u00a0not all the debris\u00a0would have\u00a0stayed close in. Some chunks\u00a0could have been flung into\u00a0Saturn\u2019s outer reaches, where one piece struck another moon and shattered it, forming the\u00a0Mundilfari group of irregular moons\u00a0at the same time.\u00a0If so, Saturn\u2019s rings and some of its hidden moons may be two remnants of the same lost world, both formed about 100 million years ago.<\/p>\n \u201cIt is hard to imagine that all of these events occurred at the same time by coincidence,\u201d says Jiao.\u00a0\u201cI am quite excited about the possibility of linking the lost moon Chrysalis with the irregular satellite population.\u201d<\/p>\n While the timing adds up,\u00a0there is some\u00a0scepticism.\u00a0\u201cIt\u2019s definitely a very cool study showing one way that the rings might have formed,\u201d says Horner. But he cautions\u00a0that linking\u00a0the\u00a0destruction of Chrysalis\u00a0to the formation of the\u00a0Mundilfari\u00a0irregular moons\u00a0would require more evidence, such as\u00a0impact scars on\u00a0Saturn\u2019s other\u00a0regular moons, which might also have been struck\u00a0\u2013 something a future mission might be able to look for.<\/p>\n Brett Gladman\u00a0at the University of British Columbia in Canada\u00a0is\u00a0also\u00a0intrigued by the idea,\u00a0although\u00a0similarly\u00a0not completely convinced.\u00a0\u201cIt\u2019s certainly curious\u00a0that these two wildly disparate events come out to be the same age,\u00a0but that doesn\u2019t necessarily mean they have a causal connection,\u201d\u00a0he says.<\/p>\n Jiao says that further modelling\u00a0will be needed to test whether his idea is right.\u00a0But\u00a0Saturn\u2019s rings may be just one example\u00a0where irregular moons\u00a0are\u00a0yielding\u00a0fresh answers to old solar system puzzles. The next surprises may\u00a0lie even further from the\u00a0sun.<\/p>\n For a long time, astronomers expected Jupiter to be the solar system\u2019s great collector of irregular moons. It is the largest planet by far, with the strongest gravitational pull, so it seemed natural that it would have the biggest satellite system. But last year\u2019s discoveries put Saturn ahead, with 274 known moons compared with Jupiter\u2019s 115. That is surprising enough. But there is reason to question whether the solar system\u2019s other two outer planets may have even more.<\/p>\n As far as we know, Uranus and Neptune have far fewer moons \u2013 29 and 16, respectively \u2013 but that may say more about the limits of our surveys than about what is really there. Both are distant, dim targets. Yet their position could make them rich hunting grounds. Their distance from the sun gives them vast regions of gravitational influence, known as Hill spheres, in which small bodies can remain bound; Neptune\u2019s stretches some 115 million km, almost twice Saturn\u2019s. Their proximity to the Kuiper Belt, a reservoir of icy debris, may also have given them plenty of material to capture. \u201cI fully expect that someday, a couple of decades away, we will find thousands of these objects at Uranus and Neptune,\u201d says Luke Dones at the Southwest Research Institute in the US.<\/p>\n If Uranus and Neptune\u00a0end up on top of\u00a0the\u00a0league table, that could reveal how efficiently the ice giants gathered material\u00a0from their surroundings. If they\u00a0don\u2019t, that absence would be just as telling, hinting that something stripped those systems bare or prevented captures in the first place.<\/p>\n And we may soon even have a chance to see an irregular moon up close, the second time a spacecraft has ever visited an irregular moon after Cassini\u2019s brief visit to Saturn\u2019s satellite Phoebe in 2004. Tilmann Denk at the German Aerospace Center says the European Space Agency is considering whether to adjust the path of its Jupiter Icy Moons Explorer spacecraft so that it passes close to Kallichore, one of Jupiter\u2019s tiny irregular moons, in 2031. It would be a fleeting encounter with one of these small, dark objects, but a worthwhile one. These overlooked moons may be among the best records we have of how the giant planets came to be.<\/p>\n Topics:<\/p>\n<\/section><\/div>\nThe discovery of hidden moons<\/h2>\n
![\"Saturn's](\"https:\/\/images.newscientist.com\/wp-content\/uploads\/2026\/05\/26101419\/SEI_298307516.jpg\")
<\/div>A recent and chaotic history<\/h2>\n
![\"New](\"https:\/\/images.newscientist.com\/wp-content\/uploads\/2026\/06\/05151647\/SEI_300189324.jpg\")
<\/div>\n<\/figure>\nThe mystery of Saturn\u2019s rings<\/h2>\n
![\"Saturn's](\"https:\/\/images.newscientist.com\/wp-content\/uploads\/2026\/05\/26101423\/SEI_298307107.jpg\")
<\/div>![\"The](\"https:\/\/images.newscientist.com\/wp-content\/uploads\/2026\/05\/26101422\/SEI_298307097.jpg\")
<\/div>Hidden kingdoms<\/h2>\n