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NARRATOR: Our solar system is home to eight planets,

near-perfect spheres

spinning through the darkness of space.

But the more we explore,

the more weird and wonderful worlds we discover.

ANJALI TRIPATHI: Our solar system is filled

with these strange worlds that defy all expectations.

NAOMI ROWE-GURNEY: Patchwork worlds that look like they've been

stuck together like Frankenstein monsters.

AMY BARR MLINAR: The images were shocking.

Nobody expected to see enormous ice cliffs

on a moon in the outer solar system.

NARRATOR: Worlds that don't seem to play by the rules.

HAKEEM OLUSEYI: There is so much strange activity

out there in the solar system that we don't understand.

Sometimes it does feel like being a detective,

trying to find out why it's there, what it's doing.

NARRATOR: Each discovery

offers new clues to how our solar system works.

A lot of the weirdness that we find

has something to do with gravity.

NARRATOR: But other forces are also at play.

LYNNAE QUICK: Each planet and each moon in our solar system

are examples of how the rules of physics

can play out differently.

GEOFFREY COLLINS: And sometimes we don't understand

which rules can be broken.

NARRATOR: "Solar System: Strange Worlds."

Right now on "NOVA."

♪

NARRATOR: On the edge of the solar system is the Kuiper Belt.

As we travel farther from the warmth of the sun,

we find a world like no other.

Haumea was a really exciting discovery

because it's a Kuiper Belt object

that doesn't seem to follow any of the rules.

NARRATOR: One of the most basic rules

is that gravity usually shapes planetary bodies like this

into spheres.

The force of gravity is always pulling mass

toward a central point.

A sphere is the shape that packs the most material

closest to the center as possible.

NARRATOR: But Haumea's shape is a little harder to define.

Haumea is shaped sort of like a football.

It's a bit of a stretched egg.

COLLINS: Haumea looks like a rounded pebble

that you would find on the beach.

Haumea is definitely one of the strangest worlds out there.

NARRATOR: And the solar system has even more worlds

that defy our expectations.

COLLINS: As we explore the solar system,

we find more and more bizarre objects out there.

Oh man, I just love everything weird,

and what's weirder than outer space?

No two worlds are the same,

and there's always something new to find.

NARRATOR: Oddly-shaped worlds,

moons that look like they've been torn apart

and strange water worlds.

TRIPATHI: How did we get so many different

and unique worlds in our solar system?

That's the million-dollar question.

♪

[eerie static whirring]

♪

NARRATOR: From a cloud of gas and dust...

...gravity, the great sculptor of our universe,

fashioned our star

and the planets and moons around it...

...creating the solar system.

And gravity has continued

to shape these myriad worlds ever since.

But how?

What exactly is gravity?

Michele DOUGHERTY: It depends who you ask.

If you ask Newton, he would say that gravity is a force

that helps pull things together.

It's what's keeping me seated on the Earth at the moment.

QUICK: It builds planets, stars and galaxies

by pulling together the dust and the gas and the rocks

that make them up.

NARRATOR: But Einstein saw it a little differently.

DOUGHERTY: If you asked Einstein, he said

it wasn't a force but that it curved space and time.

Very simply put, space and time were linked to him,

as if they were a fabric.

So any kind of object with mass, uh, would, um,

kind of bend that fabric and, um, things would fall into it.

NARRATOR: At the center of our solar system

is the most massive object in it, our sun,

curving the fabric of space-time around it.

The planets follow this curvature,

creating their orbits.

The sun binds everything in the solar system together

within its gravity, and without that,

uh, there wouldn't be anything.

There wouldn't be a solar system,

and we wouldn't exist, either.

NARRATOR: But gravity is not alone

when it comes to shaping our solar system.

The solar system would be a really boring place

if it was only gravity that was acting on all of these things.

It would mean that everything was spherical and, uh, the same.

QUICK: We see such a variety of shapes

because gravity is not the only force at play.

TRIPATHI: Even though it's counter to what we think about

when we talk about the planets and gravity,

it's actually the weakest force in day-to-day life.

PROUDFOOT: So, although gravity likes making spherical planets,

sometimes rocks are just strong enough to resist gravity.

We don't live in a universe of marbles

because you have to have enough mass

for gravity to pull everything into the spherical shape.

NARRATOR: So how much mass does gravity need

to overcome the strength of rock

and make planets spherical?

James Dottin is a planetary scientist

who studies rocks to understand the evolution of planets.

DOTTIN: Gravity is directly proportional to mass.

The more mass an object has,

the stronger the gravitational force.

So, in order to iron out

all the lumps and bumps

of rocky objects in our solar system,

it requires a lot of gravitational force.

NARRATOR: For a planet's gravity

to overcome the strength of rock,

it must reach a critical size.

We think that, in order for them to form into a sphere,

they need to have a radius of about 200 miles,

so that they're massive enough for gravitational forces

to be strong enough to form them into a sphere.

It's called the potato radius because objects that are smaller

end up looking like potatoes.

NARRATOR: A rocky world with a radius under 200 miles...

...will tend to be oddly-shaped.

While everything with a radius larger than about 200 miles

is a sphere.

That is, almost everything.

Haumea, out in the depths of the solar system,

is a rule breaker.

This is a world about 1,300 miles long,

1,000 miles wide,

and less than 700 miles high pole to pole...

...and it isn't alone.

Two icy moons in a thin ring of rock and ice orbit Haumea,

making an unexpected and odd system...

...the first like it ever discovered.

Haumea was such an exciting discovery

because it's large enough to be round,

but for some strange reason, it's shaped like a football.

NARRATOR: So, if its odd shape

is not due to its size, then what is it?

There is a clue we can see at work here on Earth,

if you know how to look.

We've left this camera out all morning,

fixed on a single point in the sky.

NARRATOR: In this case, the sun,

and that helps to visualize the rotation of the planet.

DOTTIN: Wow. I mean, how cool is that?

Now that's no camera trick,

that's literally the Earth rotating

at about 1,000 miles per hour.

And although that's superfast, I can't feel it,

and that's because everything around me

is rotating with the Earth.

NARRATOR: But even if we cannot feel Earth's rotation

we can still feel the forces created

by a rapidly spinning object.

MLINAR: So a merry-go-round on a playground

is this big spinning disc, it has handles,

you can hold onto it, you get on,

your friends start to spin the merry-go-round faster

and faster and faster, eventually, it goes so fast

that you can't hang on anymore and you kind of fly off.

So when that happens,

you're experiencing centrifugal force.

NARRATOR: And it turns out, Haumea is spinning incredibly quickly.

This world is spinning so fast,

it experiences an entire day and night in under four hours.

It's the fastest known rotating object in the Kuiper Belt.

So if this is our model of Haumea,

gravity is acting to try and make it

into a sphere,

but because Haumea is spinning so quickly,

it actually means that centrifugal forces

can make it propel away from itself.

And you'll notice that it starts to become more egg-shaped

as it spins.

Oh, cool, it's really egg-shaped now.

I'm gonna turn it off before it kills us.

[laughs]

The immensely fast rotation of Haumea spinning around is

what explains the shape that we see as a stretched out oval

as opposed to a perfectly round sphere.

MLINAR: It's just been

forced to deform

into this completely football, egg shape.

It has no choice, it has to be that shape.

NARRATOR: And Haumea's spin may also be responsible

for the formation of the two icy moons

orbiting this strange world.

PROUDFOOT: So most people generally think

that Haumea was formed in a giant collision.

That impact probably got Haumea spinning really, really fast.

When something spins too fast, centrifugal force beats gravity

and things can actually become detached from the body.

So if we go back to the analogy

of kids riding on a merry-go-round,

these would be the kids that fell off the merry-go-round

when it was going too fast.

So that's one way of making tiny little icy moons around Haumea.

NARRATOR: Haumea and its moons formed

in an ongoing battle

with gravity pulling the world together...

...and its spin pushing it apart.

The battle between these two forces--

gravity and spin-- creates a truly strange world.

♪

But it isn't the only oddball in our solar system.

Travel in from the Kuiper Belt...

...past the ice giants...

...and past Saturn...

...and we discover what happens

if we dial up a planet's gravity.

This is a world so enormous

you could fit all the other planets inside it

with room to spare.

And such a gargantuan planet has massive moons

that also feel the effects of Jupiter's pull.

Since 2016,

NASA's Juno spacecraft has been exploring Jupiter

and its many moons...

...including one unlike any other in the solar system.

SCHENK: Ganymede is really big.

It's about 3,000 miles across.

It's almost as big as the planet Mars.

It's really big.

NARRATOR: But it's not just the size of the moon

that makes Ganymede unusual.

DOUGHERTY: The surface of Ganymede looks

weird in that it's got lots of

craters on the surface.

It's got grooves and cracks

on the surface.

COLLINS: The surface is mostly ice,

in some places there's a thin layer

of rocky dust on top of the ice.

And you might see some icy mountains in the background.

NARRATOR: And above this icy surface,

Juno witnessed strange ribbons of light,

An aurora

dancing above the poles of the moon.

A spectacular light show

that has helped reveal something even stranger

about this world.

We think that Ganymede might have a secret

hiding beneath the surface.

NARRATOR: The aurora above the surface

helps us peer beneath it.

O'DONOGHUE: The aurora of Ganymede are produced when

electrically charged particles are

flowing down magnetic field lines

and they're hitting the atmosphere,

which is made of oxygen

and they're causing it to glow

in green and red lights.

SCHENK: Ganymede's magnetic field is a lot like Earth's.

It's generated by a liquid magnetic

iron core.

If you stood on the surface of Ganymede

with a compass and you looked at it,

the needle of the compass would point to the north pole

of the magnetic field, just like it does on the Earth,

it is the only moon that you can do it on

because it's the only moon in our solar system

that has an internal magnetic field.

NARRATOR: And Ganymede's aurora

rocked back and forth across the moon.

QUICK: Because Jupiter also has a magnetic field

and Ganymede sits within Jupiter's magnetic field,

it should cause Ganymede's aurora

to rock back and forth.

NARRATOR: But when scientists used

the Hubble Space Telescope to look

at Ganymede's aurora,

something didn't add up.

DOUGHERTY: The images of the aurora at Ganymede

showed that they weren't rocking back and forth

as much as we expected them to.

Because the motion of Ganymede's aurora

don't match scientific predictions,

we think there must be something else there

that's affecting them.

NARRATOR: If there were a second magnetic field being generated

within Ganymede,

that would interfere with the aurora,

reducing the rocking.

But the only way to generate that extra field

would be if another layer,

besides the molten core, were conducting electricity.

There has to be something else.

SCHENK: That something else

turns out to be a liquid layer,

an ocean underneath the surface.

Ganymede's internal ocean is damping down

the oscillation that we see.

QUICK: It's extremely cool

that we can tell that there's an ocean

beneath Ganymede's surface,

despite never having a lander there.

NARRATOR: Scientists estimate

Ganymede has a global ocean.

60 miles deep,

hidden beneath around 95 miles of rock-hard ice.

DOUGHERTY: It's pretty mind-blowing,

if you think about it.

This moon out in the outer solar system,

which is much smaller than the Earth,

could potentially have more water within it

than we have in our own oceans on the Earth.

NARRATOR: Of all the water worlds in the solar system,

Ganymede's ocean is the largest.

DOUGHERTY: One of the questions I always ask myself

is how does an icy moon like Ganymede get this huge ocean.

NARRATOR: Strange gouges on the surface of Ganymede

hint at a fascinating theory.

These are impact craters,

not individual ones like we see on most other worlds

but a long chain of them.

To understand how these form,

we have to look back to the monster living next door...

...Jupiter.

Veronica Bray Durfey

is a planetary scientist who studies impact craters...

...on the surface of Ganymede.

DURFEY: A lot of planetary science these days is,

you know, I-I wait for the pictures to come back

from spacecraft that have been to the planets.

But there's something a lot more personal

about getting it through a telescope.

And to-to see all of the Galilean moons out tonight,

that's always extra special.

This pinprick of light just on the edge of Jupiter,

that's Ganymede.

It's the biggest of the Galilean moons.

It's the biggest moon in the solar system.

It's bigger than the planet Mercury.

NARRATOR: And Ganymede's location,

orbiting Jupiter,

may play a part in how the moon got its hidden ocean.

Any objects that have mass

will have a force of attraction between them,

and that's gravity.

The larger the mass,

the larger the gravitational attraction

between the two objects.

Because Jupiter's so massive,

it has a really massive gravitational pull.

So this means that it attracts

a lot of bodies of the solar system towards it.

If an asteroid or a comet gets close enough,

it can feel the pull of Jupiter.

COLLINS: So if you're drawn in toward Jupiter

by its gravity and you don't

quite hit Jupiter, but you get very close...

Jupiter's gravity is so strong

that it will start to pull

bodies apart.

SCHENK: We've actually seen the process.

This was back in 1993

when astronomers observed a comet

broken up after a close passage of Jupiter.

And it was called Shoemaker-Levy 9

after the astronomers who discovered it.

COLLINS: It had been disrupted

by Jupiter's gravity into...

a string of objects.

And looking at its orbit, they realized

that it was going to come back

a year later and actually hit Jupiter.

NARRATOR: Watching this series of comet fragments explode

as they hit the dark side of Jupiter

provided scientists with clues

as to how these strange crater chains

were formed on Ganymede.

SCHENK: So on Ganymede we observed these chains of craters,

all nicely lined up in a row,

evenly spaced, very peculiar.

So when we saw the chain of cometary fragments

that make up Shoemaker-Levy 9

and we saw that in 1993,

it suddenly occurred to us,

that same set of cometary fragments,

if it the moon on the way out,

would form a crater chain just like this.

NARRATOR: Ferocious, high energy impacts

create these incredible chains of craters.

But Jupiter's gravity means that so much more

has hit Ganymede than just these torn apart objects.

And it is these violent collisions

that may help explain the moon's vast hidden ocean.

COLLINS: The early history of the solar system was a very chaotic place.

There were more asteroids and comets

flying around, impacts were just

a much more common occurrence.

Nothing was spared the chaos

of the early solar system.

NARRATOR: As Jupiter drew in countless asteroids and comets

with its immense gravity...

...Ganymede was caught in the crossfire.

Each impact delivers huge amounts of heat

and energy to the early moon.

DURFEY: And this allowed Ganymede to heat up

and some of its components to become molten.

And once you have that molten mix,

you're going to get differentiation.

NARRATOR: Differentiation

is where gravity organizes material

based on its density.

DURFEY: We can visualize this differentiation.

So if this oil is our low density material...

...we can add

a higher density material.

This sand.

NARRATOR: Shaking the jar lets us imagine

what Ganymede would've been like at the beginning.

A mixture of high and low-density materials.

DURFEY: And then over time

the gravitational pull

will help this separate out.

And so in Ganymede's case,

that is the high-density metals

falling towards its core

and the low-density ices

remaining at its surface.

Differentiation takes millions and millions of years.

But this will not take that long.

NARRATOR: But gravity had one more trick to play.

DURFEY: As the dense material

heads towards the core of Ganymede,

it flows past the less dense material,

and this creates heat through friction,

keeping Ganymede molten for longer

and making its differentiation

almost a self-sustaining system.

NARRATOR: And this continued until Ganymede's interior

separated out into different layers.

MLINAR: So we know that Ganymede got hot enough to melt completely.

And not just to separate the ice

from the rock,

but actually to separate

the metal from the rock

inside of Ganymede, as well.

NARRATOR: And over hundreds of millions of years

the moon started to cool.

MLINAR: What will happen is that the ice

deep inside Ganymede starts

to freeze from below, but it also freezes from above,

and then you get left with this layer

of salty water that just won't freeze.

NARRATOR: But this is just one theory about Ganymede's ocean

and how it got the heat to form.

When I get asked the question will we ever know

exactly what happened at Ganymede,

and the answer's probably no,

we will be able to come up with suggestions

as to what might've happened.

Um, but we'll probably never know it completely.

But to me that's part of what makes it

so interesting to study.

Because there are always new ideas.

There are always new things that we can measure.

Always new techniques that we can try.

NARRATOR: Leaving this hidden ocean world behind,

with its bizarre surface...

...we head out away from the sun...

...past Saturn...

...to see what strange things can happen

when you pair a massive world

with a tiny one.

Uranus is pretty odd to begin with.

The entire planet knocked onto its side,

likely by some massive impact in the past.

But that's nothing compared to how weird

one of its moons is.

GROUND CONTROLLER: Four, three, two,

one.

We have ignition and we have lift off.

NARRATOR: On August 20, 1977,

spacecraft Voyager 2

was launched to explore the outer planets

of the solar system.

The Voyager mission was really exciting, it was...

a rare mission of first exploration.

NARRATOR: And after more than eight years...

...Voyager 2 reached Uranus.

It was the first, and remains the only spacecraft

to visit this planet and its moons.

COLLINS: And we'll never have that

experience again of seeing those places

for the first time.

NARRATOR: As it flew past Miranda's south pole...

Voyager 2's cameras captured images of one

of the most astonishing surfaces

in the entire solar system.

MLINAR: I think the images of Miranda

were shocking when they came back.

SCHENK: Because we weren't really expecting much.

We were expecting it to be cold and dead.

Not much happening.

And yet there's signs of some kind of activity inside.

It looks like somebody

was making an art project.

Like somebody ripped up

pictures of two different moons

and glued them back together again.

It didn't look real to me the first time I saw it.

NARRATOR: Scientists spotted plunging canyons.

With cliffs taller than Mount Everest.

And ridged patches that punctuate the surface.

All on a moon only 293 miles in diameter.

Less than the width of Arizona.

MLINAR: It raised a lot of questions

as to what's going on, on those small icy moons?

SCHENK: It was a real resetting event.

It told us that, you know, small bodies can be

very interesting and dynamic, too.

And we had to sort of go back

and understand why that was the case.

NARRATOR: The moon's size offers a clue.

MLINAR: So Miranda is much, much smaller than the Earth.

So the surface gravity on Miranda is about...

1/100th that on the surface of the Earth.

NARRATOR: And that means the cliffs can soar

to unbelievable heights.

QUICK: Miranda's tallest cliff is pretty high

compared to its overall size.

The tallest cliff is six miles high.

That's equivalentto 2%

of Miranda's diameter.

NARRATOR: That would be like Earth having a cliff

160 miles high.

QUICK: The reason that Earth's tallest mountain

is only five and a half miles tall

is because Earth is more massive and because it's

more massive, it has much stronger gravity,

and gravity won't allow

mountains or cliffs to grow that tall on Earth.

NARRATOR: Like other worlds,

Miranda suffers the occasional meteorite strike.

But because of its weak gravity...

...the results play out in slow motion.

With the debris taking over eight minutes

to fall the height of its tallest cliffs.

Compare that to Earth,

where the same drop would take less than a minute.

COLLINS: Jumping on Miranda would be a lot of fun.

You could jump really high in the air

because gravity is so low.

In fact, just trying to walk normally would be difficult.

NARRATOR: There are countless other small, icy worlds

also with weak gravity,

but we've only seen strange patches like this on Miranda.

So where did this weird patchwork surface

come from in the first place?

All scientists have to go on are those images...

...grabbed in 1986 as Voyager 2 streaked past.

SCHENK: One of the keys to understanding Miranda

is to recognize that there is actually order

to this apparently chaotic picture.

COLLINS: You've got this ancient, cratered terrain over here

and then these patches like here, here and up here

that we call coronae.

NARRATOR: Where regions of Miranda's surface have been remade.

COLLINS: And inside that patch,

you see are these ridges and troughs

that are like stretch marks on the surfaces.

NARRATOR: It looks as if the surface has been ripped apart...

...suggesting these scars were formed by internal forces.

COLLINS: If you had some kind of

warm material inside Miranda,

it's less dense and it starts rising up

uh, in a big blob.

And as it comes up toward the surface,

it pushes the material out of its way...

...ripping the surface apart.

QUICK: It cracks and it fractures open

and we're left with a corona.

NARRATOR: This left hundreds of miles of canyons...

...where the surface cracked open along fault lines...

...creating this incredible landscape.

I think since the Voyager images came back

people have been wondering

how a body so small could be so active.

Small bodies lose their heat rapidly,

so we were expecting it to

not have any real geologic history.

COLLINS: So the big question is,

where did the heat come from

to drive the creation of this bizarre landscape?

NARRATOR: For that, we have to look to Miranda's history.

Our best theory involves the moon's giant parent planet,

Uranus...

...and another quirk of gravity.

QUICK: Billions of years ago,

Miranda had a more eccentric orbit.

And what that means is that when Miranda orbited Uranus,

it wasn't a perfect circle,

it was more in the shape of an ellipse.

So, when Miranda

is very, very close to Uranus,

the gravity from Uranus

sort of deforms it into more like an egg shape.

And then when Miranda's farther away, it's more round.

And that stretching and squeezing

causes a lot of friction on the inside

and that friction results in heat,

which we call tidal heating.

NARRATOR: And it's this tidal heating

that drives Miranda's geology

and forms the coronae.

SCHENK: Not only is the gravity responsible

for bringing the material together

that created Miranda in the first place,

but because of its gravitational interaction

with Uranus, it's also responsible for the energy

that remade Miranda later on.

NARRATOR: Today, Miranda has a nearly circular orbit.

And scientists think that heat is mostly gone.

QUICK: Because Miranda's so small,

it would've been difficult for it to hold onto

its heat for a long time.

But the Uranus system is not very well explored.

There are a lot of things we don't understand

about Uranus and its moons.

And we should prepare to be surprised

next time we go back.

NARRATOR: Though now frozen,

Miranda reveals how small worlds

can be shaped through gravity by larger ones.

But traveling inwards towards the sun, we see evidence

that small worlds can also leave a big mark of their own.

On one of the most spectacular structures in the solar system.

Glimmering rings of rock and ice...

...they are one of the hallmarks of our cosmic neighborhood.

And when examined more closely,

show signs of remarkable organization.

Lumps of ice and rock spread out in a thin disc,

split into hundreds of repeating tracks and gaps

that look like grooves on a record.

Looping for hundreds of thousands of miles

through space.

Saturn's rings are amazingly complex.

And the more we zoom into them,

the more complex seem to be.

It is one of the wonders of the solar system.

NARRATOR: Scientists think the rings may have first formed

when a moon strayed too close to Saturn...

...and was pulled apart by its gravity...

...creating a jumble of trillions

of individual fragments of rock and ice.

So what turns such chaos

into the ordered beauty we see today?

NASA's Cassini spacecraft

gave us the best view of the rings we've ever had.

OLUSEYI: The photographs from Cassini

gave us Saturn's rings at all angles.

We saw them reflecting light from the sun,

we saw Cassini look through them toward the sun.

Just the spectacular beauty of them.

Just mind-blowing.

NARRATOR: And lurking among the loops of rock and ice...

Cassini imaged one of the most startling moons

in the entire Saturn system.

EL MOUTAMID: So, Pan is this weird, tiny object.

It is only 17 miles across.

And it looks, for me, like a walnut.

And, uh, it looks like it has a dusting of material around it

that could easily break off if you were to touch it.

NARRATOR: Despite its small size, Pan has a big impact

on the structure of the rings.

BROOKS: Pan is a great example of how gravitational interactions

can shape Saturn's rings

and create the gaps that we see.

NARRATOR: Pan orbits inside a wide track within Saturn's rings

called the Encke Gap.

EL MOUTAMID: We think Pan created the Encke Gap

by cleaning the orbit and by accumulating

all this ring material, dust and ice, around it.

O'DONOGHUE: This ring material has settled

specifically on Pan's equator

again and again and again and that meant

there is this large, uh, ridge around Pan itself.

OLUSEYI: It looks like an empanada

because so much water and ice from the rings

have built up around its equator.

NARRATOR: For millions of years,

Pan has been nibbling away,

clearing particles out of its orbit...

...and creating this pathway...

...only that can't be the full story.

Pan is just 17 miles across,

yet it orbits within the Encke Gap

that's 200 miles wide...

...far broader than Pan could clear

through simply snacking alone.

So the big question is, how can a small moon like Pan

carve out such a huge gap in Saturn's rings?

In addition to sweeping up ring particles,

Pan also managed to open up the Encke Gap

by pushing away the particles on either side of the gap

through gravitational interactions.

NARRATOR: This turns out to be a quirk of orbital physics.

If a particle of ice gets close to Pan,

the moon's gravity gives it a tug,

speeding the particle up or slowing it down.

That moves it to a new orbit,

clearing a path through the rings.

And Pan is not alone.

The Cassini spacecraft also spotted tiny Daphnis...

...just five miles across,

clearing its own gap in the rings.

Even tiny, odd worlds can create structures of staggering scale.

But not all the gaps have moons embedded in them,

including one of the biggest,

the massive Cassini Division,

almost 3,000 miles wide.

So how did these gaps in the rings form

without a moon inside clearing the way?

James O'Donoghue is a planetary scientist

who studies Saturn and its rings.

O'DONOGHUE: Saturn's rings are an amazing example

of the most beautiful and complex patterns

being produced by a single event

occurring over time again and again.

We see these patterns occurring all across nature

in various forms, as we can see here,

with ripples running across these sand dunes.

The ripples form when the wind

is gliding over the surface of the sand dunes,

and it's lifting up small pieces of sand

and bouncing them along the surface.

NARRATOR: When the bouncing grains hit the surface,

they kick up more grains.

And as this process repeats,

the ripples form.

And just like wind creates structure

in these sandy dunes,

over 700 million miles away,

in the Saturn system,

regular, repeating gravitational interactions

form the structures of the rings.

O'DONOGHUE: This is Saturn and its rings--

it's not to scale-- and we also have

a selection of moons.

Here is Pan and here is Pandora,

and we also have the moon Mimas.

And we also have Titan,

which is 50% larger than our moon,

which is much further out.

Saturn has over 140 moons.

We only show four here because it would be too busy,

and they interact

with each other gravitationally,

in a really complex way,

and it weirdly leads to a lot of order.

NARRATOR: One of these moons, Mimas,

which has more than a passing resemblance

to the Death Star from "Star Wars,"

creates the Cassini Division,

despite being around 40,000 miles away from it.

And it does this all thanks to gravity.

O'DONOGHUE: The Cassini Division is the biggest gap in the rings,

and it's produced by a gravitational interaction

between Mimas and the ring particles

around about here.

NARRATOR: Mimas is in a two to one orbital resonance

with the ring particles of rock and ice

that would be found in the Cassini Division.

O'DONOGHUE: And what that means is

that Mimas, for every one orbit

that it makes around the planet,

the ring particle makes two.

And because these are in resonance,

every time Mimas

and the ring particle meet,

they actually meet at the same point in space around Saturn,

and Mimas implants a gravitational tug

onto the ring particle, which changes its orbit.

NARRATOR: And Mimas has this gravitational relationship,

not with just a single ring particle,

but all the ring particles in the same orbit.

Each time the moon and the ring particles align,

Mimas's gravity tugs at the fragments of ice and rock,

like an invisible hand...

...opening up the giant gap.

And there are more moons sitting outside the main rings

creating structures within them.

EL MOUTAMID: It is fascinating that,

even if the moons are far away from the ring,

they still have an impact on the ring,

and this is the magic of gravity.

Gravity is the main force,

that it is shaping everything in universe,

including the Saturnian system.

NARRATOR: The orbital dance of Saturn's moons

create the constantly changing

and dynamic pattern within the rings.

One we are lucky to see.

Saturn's rings allow us to see gravity at work,

constantly shaping our solar system.

But leave these beautiful patterns behind...

...and we see how a planet's size

and the influence of gravity

can have astonishing consequences for life.

More than half a billion miles closer to the sun...

...through the asteroid belt,

rubble left over

when gravity failed to pull a planet together,

and we reach the inner rocky worlds

where we find perhaps the most bizarre world of all.

A true outlier unlike anything else.

Our solar system's beautiful blue marble.

TRIPATHI: We're living in this

amazing period of the Earth's history

when we have liquid water in the form of oceans

on the surface of our planet,

and that is remarkably unique across,

not only the solar system,

but the thousands of other planets

we've discovered to date.

NARRATOR: The fact that Earth has oceans on the surface

turns out to be, again,

thanks in part to gravity,

which pulls down on the atmosphere.

ROWE-GURNEY: So our atmosphere is made up of lots of gasses,

and that gas exerts a pressure

on the surface of the Earth,

and that pressure stops water

from evaporating into space.

NARRATOR: But if Earth were smaller,

it might have been a different story.

TRIPATHI: If it was much smaller, it wouldn't have enough mass,

and, therefore, enough gravity to hold on to an atmosphere.

We're lucky to live on a rocky planet

that is large enough to keep its atmosphere in place.

NARRATOR: With little to no atmosphere

to press down on the oceans...

...water would boil at much lower temperatures,

and Earth would become a desolate, barren ball.

ROWE-GURNEY: So, without the atmosphere,

Earth wouldn't have life on it,

uh, and we wouldn't have the ability to breathe,

um, there wouldn't be oceans,

uh, and forests and trees.

Uh, we wouldn't have anything

like the Earth that we know today.

NARRATOR: Earth's size has helped shape its destiny.

Too small,

and Earth could've been a misshapen potato

with no atmosphere.

Too big,

about ten times its current mass,

it could've grown to become a gas giant...

...with little hope for life as we know it.

It turns out,

life can run riot across the surface of the planet...

...because Earth is the right size

with just the right amount of gravity.

The same force that has helped shape all the other

radically different,

wonderfully strange worlds out there.

The more we go out and visit our solar system in detail,

the more we discover things we've never seen before.

SCHENK: We're not entirely sure why we see so many

different sizes and shapes

and complexity of planetary bodies,

uh, but we think that gravity

has a very strong role to play in it.

Without gravity, the universe would be a pretty boring place.

It's gravity that assembles

the materials of the universe

into the large structures that we see.

TRIPATHI: Gravity is the backdrop

that's setting the stage for other forces to get to work.

ROWE-GURNEY: We need to study these strange worlds in the solar system

because, without them, we wouldn't understand

how all of these forces come together to create them.

NARRATOR: But our solar system only contains

a fraction of the strange worlds out there.

DOUGHERTY: We talk a lot about strange worlds in our solar system,

but there are certainly stranger worlds out there

that we haven't found yet.

NARRATOR: And scientists

will never stop looking for new, weird worlds.

I don't think I'm ever gonna get bored of strange worlds.

There's so much out there to explore and discover.

It's what gets me out of bed every day.

So, the strangeness is only just beginning.

♪