Hundreds of new moons are revealing our solar system’s violent history

Within the far reaches of the photo voltaic system, the planetary neighbourhood appears quiet. Past Jupiter, the solar is now not a blazing disc, however a chilly, white lamp. The planets are separated by gulfs of darkness. Mild takes simply 8 minutes to get from the solar to Earth, however sometimes greater than an hour to cross the yawning chasm between Uranus and Neptune.

However in the midst of what looks like an uneventful a part of the photo voltaic system, astronomers just lately made a mammoth discovery: a hidden inhabitants of greater than 100 moons that, till just lately, remained virtually invisible. From Earth, they seem as faint, fast-moving factors of sunshine, simply misplaced of their planets’ glare.

They aren’t moons as we think about them – grand worlds like our personal pale satellite tv for pc, Jupiter’s volcanic Io or Saturn’s haze-wrapped Titan. They’re smaller, darker and way more unruly. Astronomers name them irregular moons, and with their numbers now so excessive, their hidden kingdom has develop into more durable to disregard. “We’ve got had this enormous inflow within the final yr, [including an] eye opener at Saturn,” says Marina Brozovic at NASA’s Jet Propulsion Laboratory in California.

However it’s not simply the invention of those moons that has astronomers excited. For one factor, they might trace that the outer a part of our photo voltaic system may not be having fun with a quiet retirement, however as a substitute has seen intervals of unimaginable turbulence surprisingly just lately. For an additional, these hidden moons might assist us clear up a thriller about one in all our photo voltaic system’s most iconic sights: how did Saturn get its rings?

The invention of hidden moons

What’s a moon, precisely? For those who regarded up at our night time sky you’d see our personal pure satellite tv for pc, greater than 3400 kilometres throughout, protecting stately firm with Earth. Lots of the photo voltaic system’s different moons match that image, too: massive, spherical worlds circling shut to their host planet, often shifting with that planet’s spin.

However there may be one other type of moon. These are small, misshapen issues, typically solely a couple of kilometres large, following distant, tilted and generally backwards paths. These are irregular moons and, for a very long time, they have been simple to miss.

One of many first irregular moons to be recognised was Phoebe round Saturn in 1898, the most important of the planet’s irregular moons at 213 km throughout. It was the herald of many extra to return. As telescopes and digital cameras improved, particularly from the early 2000s, astronomers started seeing smaller and smaller irregular moons round big planets in droves. Then got here final yr’s deluge. In 2025, researchers introduced 128 new moons round Saturn alone, pushing the recognized complete of photo voltaic system moons above 450.

For astronomer Scott Sheppard on the Carnegie Establishment for Science in Washington DC, who has helped lead many of those searches, the broad sample didn’t come as a shock. Astronomers have been all the time positive there have been extra moons to be discovered across the fuel giants, he says; telescopes simply couldn’t decide up such faint alerts till just lately.

Nonetheless, the size of discoveries final yr caught many off guard. “All people was stunned,” says Brozovic. Astronomers had anticipated possibly a couple of dozen extra moons to be discovered round these outer planets, however as a substitute lots of and even 1000’s at the moment are regarded as awaiting discovery. “It truly is beginning to be fairly busy on the market within the photo voltaic system,” says Brozovic.

These moons is perhaps small, however their implications are giant. Their oddly elongated orbits recommend that they didn’t kind in the identical place as their host planets, the way in which bigger moons did. Many additionally journey in unfastened households, following related paths round their planet – a sample that appears to recommend they’re the fragments of bigger mother or father moons lots of of kilometres throughout that broke aside in collisions way back.

Thanks to those irregular moons, astronomers now suppose they will reconstruct this violent historical past and its position in shaping the photo voltaic system, says Jonti Horner on the College of Southern Queensland, Australia. “They’re relics of the photo voltaic system’s formation,” he says.

A latest and chaotic historical past

To reconstruct this historical past, we have to first ask a pivotal query: how precisely did these irregular moons find yourself round these fuel giants? For many years, astronomers thought the reply lay within the photo voltaic system’s first flush of youth, as a result of there simply isn’t a straightforward means for a planet to seize a passing object within the settled photo voltaic system we see at the moment, says Sheppard. A comet or asteroid can wander briefly right into a planet’s gravitational grip, however until one thing slows it down, it will merely fly out once more. “The one approach to seize a moon is to dissipate vitality from its orbit,” he says.

Nevertheless, quickly after the start of our photo voltaic system – about 4.5 billion years in the past – there have been a number of potential mechanisms of seize. One concerned the atmospheres of the fuel giants themselves, which have been extra swollen and prolonged again then. Asteroids or comets flying by way of them may have been slowed sufficient to be captured into the wild orbits we see at the moment. However whereas that works for smaller our bodies, it struggles to elucidate how planets captured the suspected mother or father our bodies of irregular moons, which have been later smashed aside.

A extra promising avenue for that is the Good mannequin, essentially the most accepted image of photo voltaic system evolution. It says Jupiter, Saturn, Uranus and Neptune didn’t initially kind the place we see them now. They have been packed a lot nearer collectively when the photo voltaic system first took form, earlier than gravitational interactions despatched them migrating outward. Their mixed gravitational interactions throughout their migration may even have helped sluggish passing objects, together with the bigger progenitors of the irregular moons we see at the moment.

This might assist clarify why at the moment’s irregular moons don’t appear to have a standard origin, as a substitute resembling a cosmic jumble from throughout the photo voltaic system, in line with papers revealed final yr that used observations from the James Webb Area Telescope.

The chaotic nature of this early interval was thought to even be when a few of the once-larger irregular moons might need crashed collectively, creating the a lot smaller objects across the planets we see at the moment.

However then got here a puzzling discovery in 2025. A workforce led by Edward Ashton at Academia Sinica in Taiwan took a better take a look at the Mundilfari group, a clutch of some 100 newly found small moons looping round Saturn.

At first look, this unusual household seems to be just like the particles path of some historical cosmic smash-up. However when Ashton’s workforce modelled their sizes, that timing didn’t fairly add up. If these fragments had been circling Saturn because the early days of the photo voltaic system, most of the smaller moons ought to have fallen into the planet by now, tugged inward by its gravity.

As an alternative, Ashton’s workforce argued that the Mundilfari group might have fashioned in a collision simply 100 million years in the past. “[That might mean] these collisional processes are nonetheless alive and effectively,” says Brozovic. If these collisions actually have been surprisingly latest, that may recommend the outer photo voltaic system remains to be being dramatically reshaped at the moment, lengthy after the primary drama of planet formation was regarded as over.

Such collisions may be linked to different occasions, too. Ashton’s paper was revealed in December 2025, and piqued the curiosity of Yifei Jiao on the College of California, Santa Cruz, as a result of the age of the Mundilfari group appears suspiciously shut to a different quantity: the suspected age of Saturn’s rings. Might the 2 be associated?

The thriller of Saturn’s rings

For all their fame, Saturn’s rings stay oddly onerous to elucidate. We’ve got seen them by way of telescopes for greater than 400 years, but the most vital thriller lingers: how did they kind? For a very long time, Saturn’s rings regarded like an historical decoration, a vibrant, virtually everlasting fixture that had circled the planet because the photo voltaic system’s youth. The best story was that they fashioned early, maybe from leftover materials round Saturn or from a moon that shattered billions of years in the past.

Then measurements from NASA’s Cassini mission difficult issues. In its closing months, earlier than it plunged into Saturn in 2017, the spacecraft repeatedly threaded the slim hole between the planet and its rings. These dives revealed rings that have been surprisingly low in mass and remarkably clear. That was onerous to sq. with nice age: over billions of years, micrometeoroids ought to have darkened the ice and worn the system down. As an alternative, photographs taken by Cassini made the rings look suspiciously contemporary – maybe only some hundred million years previous. That leaves a troublesome query: what may have made them so just lately?

The brand new moons might supply a means in. We already know that irregular moons could make a multitude as they collide into one another or are struck by passing comets and asteroids. They’ll explode into clouds of mud that steadily fall in direction of their host planet. We already see proof for this on Saturn’s moon Iapetus, the outermost of the planet’s common moons, which has a wierd, two-toned colouration. Its main facet – the face that ploughs ahead by way of house – is extraordinarily darkish, whereas the trailing facet is good and vibrant. Sheppard says that Iapetus might be working by way of the reddish, carbon-rich mud shed by irregular moons “like a bulldozer”.

Nonetheless, these distant, irregular moons of Saturn weren’t thought to have a lot to do with the planet’s rings. However in April, Jiao and his workforce revealed a paper suggesting a possible hyperlink. They first constructed on the thought that Saturn as soon as had an additional icy moon about 1000 km throughout, referred to as Chrysalis. Over the 4.5-billion-year historical past of the photo voltaic system, it fell right into a gravitational rhythm with Titan, Saturn’s largest moon. The mutual tug between each elongated Chrysalis’s orbit from a round form into an ellipse.

Then, about 100 million years in the past, the moon’s distorted orbit carried it previous a threshold round Saturn referred to as the Roche restrict, the boundary inside which a planet’s gravity can tear a moon aside. In a single catastrophic go, Saturn stripped away a lot of Chrysalis’s ice, virtually immediately shrinking the moon to half its dimension.

What would have occurred subsequent is unsure. The rocky core of Chrysalis might have been cannibalised by Saturn or ejected solely, though Jiao notes we “haven’t discovered such a physique” orbiting the solar elsewhere. The ice, in the meantime, would have unfold like butter, forming a broad, vibrant disc over a few thousand years – Saturn’s rings.

However not all of the particles would have stayed shut in. Some chunks may have been flung into Saturn’s outer reaches, the place one piece struck one other moon and shattered it, forming the Mundilfari group of irregular moons on the identical time. In that case, Saturn’s rings and a few of its hidden moons could also be two remnants of the identical misplaced world, each fashioned about 100 million years in the past.

“It’s onerous to think about that each one of those occasions occurred on the identical time by coincidence,” says Jiao. “I’m fairly enthusiastic about the potential of linking the misplaced moon Chrysalis with the irregular satellite tv for pc inhabitants.”

Whereas the timing provides up, there may be some scepticism. “It’s positively a really cool examine exhibiting a technique that the rings might need fashioned,” says Horner. However he cautions that linking the destruction of Chrysalis to the formation of the Mundilfari irregular moons would require extra proof, akin to impression scars on Saturn’s different common moons, which could even have been struck – one thing a future mission may be capable of search for.

Brett Gladman on the College of British Columbia in Canada is additionally intrigued by the thought, though equally not fully satisfied. “It’s definitely curious that these two wildly disparate occasions come out to be the identical age, however that doesn’t essentially imply they’ve a causal connection,” he says.

Jiao says that additional modelling will probably be wanted to check whether or not his concept is true. However Saturn’s rings could also be only one instance the place irregular moons are yielding contemporary solutions to previous photo voltaic system puzzles. The subsequent surprises might lie even farther from the solar.

Hidden kingdoms

For a very long time, astronomers anticipated Jupiter to be the photo voltaic system’s nice collector of irregular moons. It’s the largest planet by far, with the strongest gravitational pull, so it appeared pure that it will have the most important satellite tv for pc system. However final yr’s discoveries put Saturn forward, with 274 recognized moons in contrast with Jupiter’s 115. That’s stunning sufficient. However there may be motive to query whether or not the photo voltaic system’s different two outer planets might have much more.

So far as we all know, Uranus and Neptune have far fewer moons – 29 and 16, respectively – however that will say extra concerning the limits of our surveys than about what is admittedly there. Each are distant, dim targets. But their place may make them wealthy searching grounds. Their distance from the solar offers them huge areas of gravitational affect, referred to as Hill spheres, wherein small our bodies can stay sure; Neptune’s stretches some 115 million km, virtually twice Saturn’s. Their proximity to the Kuiper Belt, a reservoir of icy particles, might also have given them loads of materials to seize. “I totally count on that sometime, a few a long time away, we’ll discover 1000’s of those objects at Uranus and Neptune,” says Luke Dones on the Southwest Analysis Institute within the US.

If Uranus and Neptune find yourself on prime of the league desk, that might reveal how effectively the ice giants gathered materials from their environment. In the event that they don’t, that absence could be simply as telling, hinting that one thing stripped these methods naked or prevented captures within the first place.

And we might quickly also have a likelihood to see an irregular moon up shut, the second time a spacecraft has ever visited an irregular moon after Cassini’s temporary go to to Saturn’s satellite tv for pc Phoebe in 2004. Tilmann Denk on the German Aerospace Heart says the European Area Company is contemplating whether or not to regulate the trail of its Jupiter Icy Moons Explorer spacecraft in order that it passes near Kallichore, one in all Jupiter’s tiny irregular moons, in 2031. It could be a fleeting encounter with one in all these small, darkish objects, however a worthwhile one. These missed moons could also be among the many finest data we’ve got of how the large planets got here to be.