Planet Earth is the jewel of the solar system—the  shimmery blue oceans, the verdant green forests, the wispy whimsical cloud formations.

Saturn  would be the only competitor for most gorgeous planet with that giant ring system.

But you know  what?

If we could put the jewel of the Earth in its own ring?

There'd be no contest.

Well,  there’s an extremely good chance that Earth once did have a ring system.

And, you know, if   we had a ring once maybe we can have one again.

Ever since they were discovered by Galileo and  then seen for their true majesty by Christiaan Huygens, Saturn’s rings have been a symbol of  serene celestial order.

They seem so timeless and eternal.

But they are more transient than they  look.

Saturn will lose its own ring system in a few hundred million years, and a hundred million  years or so ago an astronomer with a time machine, or I guess a caveman with a telescope  would  probably have seen Saturn as a ringless yellow ball.

Jupiter, Uranus and Neptune all have  vestigial rings—perhaps the remnants of greater systems.

There’s even a hypothesis that Mars  goes through cycles of having rings versus moons.

So, if planetary rings come and go, it stands  to reason that Earth may have once had one.

And now we have evidence that it really  did, at least according to a 2024 study by scientists from Monash University, in my  own home town actually, Melbourne, Australia.

But before we get to that evidence, let’s learn a bit about how planetary  rings can form and how they can go away.

We’ll start by imagining a hypothetical planet  that already has a ring system.

Those rings are just collections of particles; rocks, ice,  or dust with circular orbits in a flat plane.

There are a couple of ways this  ring could be lost.

One is that the particles fall onto the planet due to  various factors, including gravitational, magnetic and even atmospheric interactions.

That’s actually inevitable, but can take a really long time.

A quicker way to  remove a ring is to turn it into moons.

Even though the particles of a ring system  particles are small, they attract each gravitationally and can slowly clump together into  massive bodies.

Because the force of gravity is so weak between these particles, this can only happen  if they’re moving very slowly relative to each other.

Larger relative speeds they’ll just wizz  past each other before gravity can do its thing.

We say that a system of particles with large  and random relative speeds is dynamically hot, otherwise it’s dynamically cold.

But  even if a ring system starts out hot, collisions between particles can slow  relative speeds to the point where they do start to coalesce gravitationally.

At that point they can build moons.

But if collisions can’t reduce particles speeds then  the system remains hot and also remains a ring.

But even if a ring system cools, there’s  still a way for it to stay a ring.

For example, Saturn’s rings  are dynamically cold in fact, they may have the minimum possible internal  energy that they could possibly have.

The other thing that can keep a ring a  ring is the gravitational field of the planet itself.

Let’s Imagine some particles just  starting to clump together into a ring system, with the grand ambition of forming a new moon.

As the clump grows, the mutual gravitational field holding it together gets stronger.

But  the planet also has a gravitational field, and that field is stronger the closer you get to  the planet.

That means particles in our growing clump on the side nearer to the planet feel a  stronger inward pull than those on the far side.

As long as we place this body far from the planet,  that gravity differential is be weaker than the gravity force holding the clump together.

But the  closer to the planet we place it, the stronger this differential becomes.

Within a certain  range, the difference in the force of gravity across the body’s width is stronger than the force  of gravity holding the body together.

That causes the growing clump to breaks apart—or to not grow  in the first place.

This differential force is called a tidal force and our Moon’s differential  pull on Earth’s oceans is the cause of tides.

So we can have rings if the material is  dynamically hot OR if the material is inside this critical distance for tidal disruption.

That  distance is called the Roche limit, and it depends on a few factors, like the density of material  forming the moon and whether the material is held together by anything other than gravity.

Within  the Roche limit for a given planet and a given material, Moons aren’t possible and so orbiting  particles will persist as rings.

Beyond the Roche limits, these particles will eventually form moons  as long as they can also become dynamically cool.

For Saturn, the Roche limit for ice is around  140 thousand kilometres from the planet’s centre.

There are no moons inside this boundary, and there  is only very thin ring material outside of it.

There are two main ways for a planet to gain  rings.

One is to capture small particles that arrive elsewhere in the solar system,  which then fail to coalesce into moons, for example due to tidal forces.

Or a larger  object could be shattered into smaller particles while orbiting or on a near fly-by—again  perhaps by tidal forces or through a collision.

There are other possible scenarios, like a  giant impact with the planet itself, spraying particles from the planet into a ring system.

Supposedly that’s how Earth’s moon was formed, although in this case the “ring system” didn’t  last long because it was outside Earth’s Roche limit.

But in principle, Earth could acquire  a persistent ring system as long as it was within its Roche limit.

So let’s look at the new  evidence that this was, in fact, once the case.

We’ve known for some time that around 4 and half  Million years ago a bunch of stuff fell to Earth.

Space rocks are peppering our planet constantly,  and we see their remains—both in impact craters, but also in more broadly dispersed meteorite  material in sedimentary rock, such as limestone.

In fact, from these rocks we can trace the  flux of meteorites—the rate of infall.

And for a period of around 40 million years  starting around 466 million years ago, sediment became enriched with a type of space  rock called L-chondrite.

Enriched by a factor of 100-1000.

So, for this stretch of some tens of  millions of years during the Ordovician period, the sky blazed with shooting stars … for some  reason.

This is the Ordovician impact spike Perhaps the most obvious explanation is that  there was some massive influx of material into the inner solar system from the asteroid belt.

You  expect that sort of thing when two large asteroids collide.

In fact, that was, and perhaps, for the  time being, still is the leading hypothesis for the excess L-chondrite deposits.

Except there’s  something that doesn’t quite hang together.

If this event really did occur in the asteroid belt,  we’d expect all inner solar system bodies to be affected.

But there’s no evidence of an increased  impact rate on Mars or the Moon from this period.

This incongruity is part of what led  the researchers from Monash university to look into this idea that the many, many  smallish meteorites that mark the Ordovician impact spike were produced close to the  Earth instead of in the asteroid belt.

So here’s their story: about 466 million years  ago, a large asteroid – about 10 kilometres across – had a near miss with the Earth.

It  passed just thousands of kilometers above the planet’s surface, below the Roche limit,  where extreme tidal forces disrupted the body.

Some of the resulting cloud of debris  was then captured by the Earth’s gravity.

At first, these particles would have slung around  the Earth on very elliptical orbits and with a range of inclinations.

At first, the scattered  orbital orientations led to frequent collisions, but those collisions circularized the  orbits and helped flattened them into a disk.

Earth’s slight equatorial bulge  also creates a preferential orbital plane around the equator, further stabilizing  orbits into a very flat ring system.

Now some particles ended up outside  the Roche limit may have temporarily formed small moons out there.

But tidal  forces would have caused them to migrate inwards until they were once again tidally  disrupted and rejoined the inner ring.

But that ring could not last forever.

Earth’s  Roche limit is a few thousand kilometers for solid rock asteroid and up to 15 or so thousand  clicks for a fragmented asteroid—a “rubble pile”.

Either way, an object that breaks up with this  range is also largely inside Earth’s exosphere.

That’s the extremely diffuse region where the  atmosphere transitions into interplanetary space.

The tiny drag caused by interactions with the  exosphere would have caused these ring orbits to slowly decay.

One by one, ring particles  would find themselves in an accelerating spiral into the thickening atmosphere,  ending in a fiery plummet to the surface.

So, if this story is right, then  for about 40 million years the Earth had a spectacular ring stretching  thousands of kilometers above the equator, and also a constant rain of shooting stars  from the base of that slowly draining ring.

Well cool story.

But how do we test something  like this?

How do we tell that this stuff that fell from space came from a ring rather than from  a collision in the asteroid belt, given that it would all be the same type of material?

Well the  trick is to see where the stuff landed.

Material coming direction from the asteroid belt should  land relatively randomly over the entire surface of the globe.

On the other hand, material falling  from a ring should land close to the equator.

It’s not exactly easy to say where space rocks  landed half a billion years ago.

The tiny bits of L-chondrite found in sedimentary layers have  dispersed widely from their atmospheric entry points.

But there are several actual craters  thought to be caused by larger impacts from this event.

And these are scattered across the  globe, from Europe to North America to Australia, so definitely not equatorial.

But remember that  these craters formed a half billion years ago, when the continents were nowhere near their  current locations.

So the researchers traced the tectonic motion of these impact craters  back in time to see where they were located when they were formed.

And yes, it’s  crazy that we can do that.

Back then, most of Earth’s solid surface was clustered in  the southern hemisphere supercontinent Gondwana.

And the impact locations?

All of them within  roughly 30 degrees latitude of the equator.

Just what you’d expect from a collapsed planetary  ring.

But let’s not get ahead of ourselves.

It’s only 21 craters, and the range is around 60  degrees latitude.

Maybe there really was an influx of rocks from the asteroid belt, and all  the bigger ones just got lucky and landed in the tropics by accident.

What are the chances?

Well  the team calculate the chances.

The likelihood of getting all of these impacts within this  band around the equator is reported as one in 25 million.

If they really were from a  global random distribution that is.

Therefore, these researchers argue that it’s much  more likely that these impacts came from a reservoir of space rocks localised above the  equator of the Earth.

AKA a planetary ring.

It seems … plausible.

Not super easy  to test,  unfortunately and I'll come back to how we might test this hypothesis.

But it does feel reasonable.

And the idea of a planetary ring does at  least seem consistent with the general state of affairs 2.6 hundred million years  ago.

So then animal life was all marine at that time during the Ordovician period,  was busily complexifying in preparation for its foray onto land.

And that land was  clustered in the Gondwana supercontinent, which spent most of this era covered in  primitive plantlife.

But towards the end became a colossal ice-cap as Earth plunged  into the Hirnantian glaciation.

This ice age was so bad that it led to the second  largest mass extinction in Earth’s history.

The Ordovician impact spike came just before  the Hirnantian glaciation, and others have proposed that the asteroid influx precipitated  the cooling by depositing a lot of reflective dust in the atmosphere.

But the planetary ring  hypothesis may provide an even more compelling explanation for the cooling.

That’s because  an equatorial ring would provide a sunshade for whichever hemisphere happened to be in  winter.

That would result in harsher winters, and potentially initiate the runaway growth of  Gondwana glaciers that precipitated the ice age .

And by the way, if you want a deeper look  into what would happen to the modern Earth if it had rings, check out Joe Scott’s episode  where he explores exactly that.

And in the process produced amazing visualizations  of what a ringed Earth would look like, some of which we borrowed for this episode.

So did the ancient Earth have a ring  system?

We can’t say one way or the other just yet.

The authors suggest various ways  to strengthen the hypothesis or refute it.

The most obvious thing is to find  more impact craters from this era, and refine our dating of the meteorite material  in those craters, and of the mineral similarity of this material.

But if we found craters of the  same type of meteorite and time very far from the equator it would put this hypothesis  in doubt, but if we keep finding sibling craters that were close to the Ordovician  equator then things look more promising.

And if it does turn out that Earth had a ring  system a half billion years ago, then there’s no reason we can’t have one again.

All we need  is a horrifyingly near-miss with an asteroid followed by millions of years of intense meteor  activity and a cataclysmic ice age.

But it's a small price to pay to firmly establish Earth as  the most spectacular world in local space time.