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.
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And third, this is the big news: We are evolving. Starting this month
one of our releases every month is going to be an interview with a real
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For our fist guest we are thrilled to be joined by Dr. Matthew Caplan,
computational nuclear astrophysicist and long time space time writer.
We'll discuss, debate and argue about the aspect of his work that is closest to my heart,
how real black holes feast on their surrounds and grow to such enormous sizes.
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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.
Hey Everyone. Before we go, I wanted to let you know that today’s episode was part of
Earth Month on PBS. Every April, PBS releases a lot of new episodes celebrating our amazing
planet. There’s going to be lots of great science content and links to their latest
episode and the full Earth Month playlist - are in the description.