Original subtitles

(planet explodes) (dramatic music)

- [Narrator] In a newly formed solar system,

worlds spawn from specks of dust.

Two kinds of planets are born.

Those made of rock and iron,

forged in the flaming heat of the sun,

and giants made from gas and ice,

in the cold, dark depths of space.

But their birth is far from peaceful.

Jealous titans starve their siblings, stunting their growth.

Planets collide,

(planets crashing)

inflicting scars still detectable 4.5 billion years later.

Young worlds stray from their orbits,

wreaking havoc on their neighbors,

annihilating some, and flinging others into space.

And when the inner and outer planets

square off in a cosmic conflict

destined to last tens of millions of years,

no world escapes untouched.

And the fate of Earth hangs in the balance.

(dramatic music continues)

(dramatic music fades)

(air whooshing)

(air whooshing)

(pulsing music)

From a swirling cloud of dust and gas,

our solar system is born 4.6 billion years ago.

Today, eight planets circle our sun.

Four in snug sunlit orbits:

Mercury, Venus, Earth and Mars.

And four giants looming in the distant darkness:

Jupiter, Saturn, Uranus, and Neptune.

Dividing these two groups of worlds,

like the border on a map,

is a ring of small, rocky objects called the asteroid belt.

Today, the planets appear to exist in perfect harmony.

Their relationships, as fixed and constant as their orbits.

- If you look at the solar system today,

it might look like an orderly system.

- [Narrator] But across the solar system,

curious anomalies hint at a distant past,

scarred by almost unimaginable chaos and upheaval.

- There are various indications of major events

in the history of the solar system.

- [Narrator] Indications which present many questions.

Why are the inner planets so small

compared to the giant planets detected around other stars?

One planet in our solar system, Jupiter,

is twice as massive as all the other planets combined.

How did it get so big?

Are there perhaps other planets

in the far reaches of our solar system,

in the region of icy rocks called the Kuiper belt?

- There's a few clues that points toward a ninth planet.

Maybe one day we'll find it.

But it's a little bit like looking

for a needle in a haystack.

- [Narrator] Follow the clues

and the illusion of peaceful balance falls away.

Instead, a different picture emerges,

one in which planets battle for supremacy

and entire worlds are obliterated and remade.

From this celestial conflict

no planet, not even Earth, will emerge unchanged.

(pulsing music continues)

The first sign of our solar system's tumultuous past

is the most glaringly obvious.

In the outer reaches, a single titan holds court

over three smaller but still massive planets.

Meanwhile, inside the asteroid belt,

four worlds huddle together

each only a fraction of the size of their larger neighbors.

- When you look at other exoplanetary systems,

of which we know of about 6,000 now, none of them,

none of them look like our solar system.

- In the last 20 years,

we've started to find thousands of extrasolar planets

orbiting around other stars in the Milky Way galaxies.

One of the most exciting systems is called TRAPPIST-1.

- TRAPPIST-1 is an extremely compact planetary system

in that it has seven rocky planets

about the size of the Earth,

about the same mass as the Earth,

but all packed into orbits

that are smaller than the orbit of Mercury.

TRAPPIST-1 itself, the star itself,

it's a teeny tiny red dwarf about the size of Jupiter,

and a pretty wimpy one at that.

This is actually fairly common

when we look at other planetary systems,

especially around these low mass red dwarfs

which overwhelmingly dominate

the galactic inventory of stars.

- Now that we have discovered

many, many, many extrasolar planetary systems,

we're starting to think that the solar system

might be the outlier

and that normal looks quite different.

- [Narrator] If you were to look at our solar system

from the perspective of an alien astronomer,

one of the first questions you might ask is:

Why is one planet so big compared to the others?

The planet is Jupiter,

named for the king of the Roman gods.

It lords its massive size over the other worlds,

containing twice the combined mass

of every other object in the solar system

except the sun.

(majestic music continues)

- You could fit 1,300 Earths inside of Jupiter.

That's how large this planet is.

- You can imagine the Earth being say a grape

and Jupiter being like a basketball.

(pensive music)

- [Narrator] Jupiter's tremendous size and mass,

and the enormous influence it exerts

over the rest of the solar system

results from when, where, and how it forms.

(star explodes)

4.6 billion years ago,

in the fiery heat of a young star,

planetary cores form from pebbles made of solid rock

and heat resistant metals like iron and nickel.

But further out in a disk of gas and dust

revolving around our sun,

called the protoplanetary disk,

extreme cold allows ice to mix with rock.

- If you wanna form a Jupiter,

you go to the outer parts of the protoplanetary disk.

You go so far out that water goes from a vapor form

close to the star, where it's very hot,

to a solid form far away from the star,

where it's much, much colder.

That location, that orbital distance

is referred to as the snow Line.

- [Narrator] In our present day solar system,

the water snow line lies about

four astronomical units, or AU, away from the sun.

In other words, four times the distance

between the sun and Earth.

This puts it close to where Jupiter is.

From the very beginning,

this invisible line decides how planets will form.

Those, like Jupiter, that are taking shape on the ice side

are destined to grow much faster and much, much bigger.

- When you have two solid objects colliding together,

whether or not they stick together,

or whether or not they fragment

depends on the composition of those materials.

- In any collision between particles,

snow can act like glue

and help grains to attach to each other,

favoring the fabrication and accretion

that eventually leads to the fabrication of protoplanets.

- [Narrator] Not only do icy planetary cores form faster,

there is much more building material

for the growing planets to eat.

- If you're right beyond the snow line,

you're right at this sweet spot

where there's lots of material,

and now you have all these ices to help you grow quickly.

- [Narrator] At first pebbles, then rocks,

then planetesimals begin to smash together

in the gaseous disk surrounding the young sun.

Jupiter is the first planet to develop a planetary embryo,

a ball of rock and ice, roughly the size of Mars.

- You have to build up a core of solids.

You have to build it up to 10, 20,

maybe 30 times the mass of the Earth

before that solid core can become massive enough

to start accreting gas.

- [Narrator] This is when Jupiter starts to gain

its signature feature, its thick, gaseous atmosphere.

- When Jupiter is first forming,

like all of the planets, it's really hot.

When you have really hot material,

it's hard for a gas envelope to stick to you.

You will boil away.

- The gas has to be able to cool

as it falls into the gravitational well of that solid core.

As it falls into that gravitational well, it heats up,

which prevents more gas from being accreted

and slows the growth of your would-be gas giant planet.

- [Narrator] But Jupiter is in a race against time.

Less than 10 million years following the birth of the sun,

the protoplanetary disk is already starting to thin

as gas is pulled into the sun

or dissipates due to solar radiation.

Within a few million years,

the cosmological blink of an eye,

the disk will be completely gone.

Fortunately, for Jupiter, once it cools enough

to hold on to its gaseous atmosphere,

it grows exponentially,

pulling hydrogen and helium towards it

like water down a drain.

- This slow accretion process continues

until the amount of gas that you've accumulated

becomes comparable to the mass of the core.

At that point, the gas can cool quite efficiently,

and instead of this slow gas accretion phase,

you enter what's called a runaway gas accretion phase,

and suddenly the growth rate of your planet

goes from this to this.

(dramatic music)

- [Narrator] During Jupiter's rapid expansion,

its massive gravity consumes everything within its reach.

But maybe everything went too fast.

Hidden deep within the planet are signs

of a 4.5 billion year old planet-sized wound.

- The Juno spacecraft has gone and visited Jupiter,

orbited around it,

and measured things like gravitational anomalies

that tell us about the actual interior structure of Jupiter.

- We found, surprisingly, that the core of Jupiter

is not as dense and not as condensed as we might expect.

It appears to be somewhat fuzzy.

By that we just mean it's a bit more diffuse

or a bit less concentrated.

So that is certainly a mystery

that people are trying to understand.

Why does it look different than some of the other planets?

(dramatic music continues)

- [Narrator] What caused Jupiter's fuzzy core is a mystery,

but one possible explanation

is a high velocity, head-on collision

between Jupiter and another giant planet.

(dramatic music continues)

- A giant collision of this nature

would've had to have been a Neptune-sized planet

that collided with planet Jupiter in the early stages.

- Would've gone plowing right through its atmosphere,

would've run into the core,

shattered the core,

and dispersed material from the core over a range of radii.

(dramatic music continues)

- [Narrator] The two planets' cores mingle

and combine in Jupiter's gaseous mantle,

adding as much as 5% to Jupiter's overall size and mass.

- Usually you would expect that heavier material

would settle down to the center of a planet.

But it seems like the whole system has been mixed up

and is still recovering from that.

- [Narrator] Jupiter survives its near-death experience

and develops into the planet we know today.

But what kind of world is this titan of our solar system?

- Jupiter is so different from Earth

that it's really difficult to compare them.

Its distance from the sun make it relatively cold.

Although, if you dive into

the different layers of its atmosphere,

you can reach extremely high temperature

all the way up to its core.

- [Narrator] Jupiter's upper clouds are made of ammonia ice,

which form the visible bands

that give the planet its distinctive look.

- On Earth, in our atmosphere, we have one layer of clouds.

On Jupiter, that process is more complicated

because there are three different chemicals.

And as a result, you get three different cloud layers.

So that's part of what adds to the beauty of the planet,

you get these three different colors

overlapping with one another.

(ominous music)

- [Narrator] In the upper layer,

storm winds whip around the planet

at speeds of over 500 kilometers per hour.

- Because of this turbulent nature

that's going on in this atmosphere, clouds on Jupiter,

we see these massive storms that brew across the planet.

- [Narrator] Below this,

things start to become very strange.

- If you just dive 500 kilometers

into the cloud tops of Jupiter,

the temperatures increase by a thousand degrees Celsius,

and the pressure is a thousand times greater

than what we see it on the surface of the Earth.

This is enough to crush a spacecraft flat into a pancake.

- If you go another 500 kilometers down,

the pressure is 10,000 times greater than on Earth.

That's enough to squeeze things like hydrogen or helium

into liquid or a solid form.

So we think at those depths there's a rain of helium

that is falling into an ocean of hydrogen.

And there would be an ocean 10,000 kilometers deep.

- This would be the deepest ocean in the solar system,

and it's over 8,000 degrees Celsius,

so hotter than even the surface of the sun.

- [Narrator] 30,000 kilometers below that

lies the planet's damaged core,

diffused into the liquid metallic mantle

like a shattered egg yolk 25 times bigger than Earth.

- The core of Jupiter is almost like the core of a star.

We think the temperature is around 35,000 degrees.

So that's much, much hotter than the surface of the sun.

On the other hand, it's still well below

the core temperature of a star.

- [Narrator] As enormous as Jupiter's heat and pressure are,

they fall short of making the planet a failed star,

as some once believed.

- It's important to think about

what differentiates planets from stars.

And the main thing is their mass.

You have to be massive enough

to be hot enough in your center

to allow nuclear fusion to become a star.

And it turns out that Jupiter

falls very far short of this threshold.

- If Jupiter were 10 to 15 times more massive,

then it would enter into this category of brown dwarfs.

If it were 75 times more massive,

then it would actually enter into the range

of main sequence stars that burn hydrogen to helium

in their core.

- [Narrator] Although not big enough to become a star,

the Jupiter of 4.6 billion years is due to unleash

a wave of chaotic change throughout the young solar system,

second only to the formation of the sun itself.

Like a jealous bully that will tolerate no rivals,

it dominates its region of space,

its gravity trapping primordial pebbles and planetesimals

just outside of its orbit.

(dramatic music)

Continually tugged and knocked about by Jupiter's gravity,

these rocks are never able to form a planet of their own,

so they remain in a ring of raw planetary material.

This ring still exists in the form of the asteroid belt

between Jupiter and Mars.

- You often picture flying through this dense asteroid field

trying to dodge these rocks.

But in reality, you could probably just close your eyes

and just travel a straight line through this asteroid belt

without ever interacting with a rock.

- We think, in the past, there was enough material there

that you could have formed a planet

out of all of that material.

So why didn't we? It's because of Jupiter.

The giant Jupiter, with all of its gravity,

is constantly interacting with all of these rocks

in the asteroid belt, scattering them

and preventing them from collapsing and making a planet.

- [Narrator] Although Jupiter prevents the formation

of a planet in the nearby asteroid belt,

it can't stop Saturn from developing,

but it can stunt its growth.

- Jupiter grew really quickly

and it swept up a lot of the nearby material,

which would not leave much behind for other nearby objects.

- [Narrator] Even starved of building material,

Saturn grows into a giant.

- Saturn is about 80% the size of Jupiter in diameter,

but it contains only one third of its mass.

- Saturn is incredibly low density.

If you were somehow able to put Saturn

in a bathtub of water, it would actually float.

- [Narrator] This early in its development,

Saturn lacks its most recognizable characteristic,

its signature icy ring.

In fact, Saturn's visible rings appear to be the result

of a relatively recent cataclysm.

- The current theory for the formation of Saturn's rings

is that two of its existing icy moons

had a catastrophic collision

that basically disintegrated both of those bodies,

ejected all of this icy particles into space

that eventually coalesced into a very thin disk

that we see today.

- The reason why we know they are fairly recent,

it's because they are made out of very clear ice.

If they had been formed really early

in the formation of the solar system,

they'd be dusty and darker.

And so they are recent and most likely, they will fade out

in the next couple of hundred millions of years.

(ominous music)

- [Narrator] If Jupiter starved Saturn,

the next planets to form make do with even less.

Uranus and Neptune develop into planetary embryos

shortly after the gas giants.

They make use of the same rich building material

from which Jupiter and Saturn are constructed.

However, given their late start,

the new additions to the planetary family

simply can't keep up with Jupiter and Saturn's accumulation

of solid rock and gas.

This leaves Uranus and Neptune much smaller

and less massive,

with thin hydrogen and helium atmospheres

instead of thick clouds.

- Uranus and Neptune,

the so-called ice giants in our own solar system,

orbit in the outer parts of the solar system,

beyond the orbit of Saturn, where it's much, much colder.

It's actually so far away from the sun

that not only is it beyond the water snow line,

it's beyond the snow lines of other volatile materials

like methane, carbon monoxide, ammonia.

These materials, like water, will freeze onto dust grains

and become available to be accreted into the solid cores

of a would-be giant planet.

And that's where this ice giant nomer comes from.

- [Narrator] 2.9 billion kilometers from the sun,

Uranus carves a lonely orbit in freezing darkness,

a single revolution taking nearly 84 years to complete.

Out here, the sun is small and dim,

only 5% as big as it appears from Earth.

Though stunted by Jupiter and Saturn's greedy collection

of icy dust and gas,

Uranus is still huge compared to Earth,

measuring four times wider and 14.5 times more massive.

(dramatic music continues)

The planet's most unique feature

is its highly unusual orbit.

Instead of spinning like a top,

Uranus rolls like a beach ball.

- Uranus is actually spinning sideways

as it orbits around the sun.

It's tilted at about 98 degrees,

so almost exactly edge on.

For a long period of its orbit,

one pole is pointing at the sun

and would have daylight all the time.

And then many years later,

as Uranus goes to the other side of its orbit,

the opposite pole would be facing the sun,

and the far side would be pointing to interstellar space

without any sunlight.

- [Narrator] Unlike most other planets,

Uranus also spins clockwise.

(dramatic music continues)

The cause of Uranus's strange rotation

is likely a grazing impact

with an object twice the size of Earth

3 to 4 billion years ago.

(dramatic music continues)

- It's hard to imagine a planet

forming in a protoplanetary disk

without having its spin aligned with the spin of the disk.

If it's now tilted on its side,

there must have been a huge collision

that produced that rearrangement.

(planets crashing)

- [Narrator] This planetary sideswipe

could also explain other mysteries about Uranus.

- Uranus is actually colder than Neptune,

which is further away from the sun.

So why is that? Why is Uranus colder?

In fact, it can be -220 degrees Celsius on Uranus.

- [Narrator] The cause for its frigid temperatures

likely stems from this ancient collision.

- Another effect of this impactor

is believed that it disrupted the interior of Uranus

and it caused the sort of hot material in Uranus's interior

to become dredged up and merge close to its surface,

where it can more easily be radiated away.

- [Narrator] Beyond Uranus, at such an extreme distance

the sun appears little more than a bright star,

lies Neptune.

Like its sister planet, Neptune is an ice giant.

- Uranus and Neptune are both ice giants

in that we believe both of them formed

from a large collection of different types of ices.

So compositionally, they're extremely similar.

They're also extremely similar in their size and their mass,

which gives rise to this twin analogy.

(pulsing music)

- [Narrator] Neptune is a strange and exotic world,

completely unlike our own.

- Neptune's atmosphere is actually incredibly violent,

with supersonic winds

that are the fastest in the solar system.

- When you dive 3,000 kilometers

into the cloud tops on Neptune,

you get this very blue color,

it almost reminds you of the oceans of the Earth.

But when you actually dive into

what is making up these oceans,

it's a lot of methane gas mixed with hydrogen and helium.

The pressures are so great

that any carbon atoms that might exist

can actually be pressurized into diamonds

up to a meter across.

It's essentially raining diamonds in this ocean.

- [Narrator] Beneath this 8,000 kilometer deep ocean,

Neptune is believed to have a solid core,

slightly bigger than the Earth.

The temperature at the center of this ice giant

is over 5,100 degrees Celsius,

and the pressure is 7 million times that of Earth.

(ominous music continues)

(star explodes)

Returning to 4.6 billion years ago,

the great gas and ice giants aren't the only worlds

starting to grow in our young solar system.

Inside the asteroid belt,

much closer to the heat and light of the sun,

other planets are forming.

But even here, they feel the effects of Jupiter's gravity.

The titan's pull prevents planetary building rocks

from passing through the asteroid belt.

This means the emerging worlds of the inner solar system

are fated to be much smaller

than their outer world system kin.

And because they form inside the water snow line,

they will be forged from fire instead of ice.

- When the inner planets form,

it's thought that the protoplanetary disk

was at a really high temperature of about 1,700 degree.

So for any collision to be successful in accreting,

they have to be close to the melting point

of elements or different material

that is located in the disk at that time.

So we're talking about metals,

iron, nickel, aluminum, calcium.

All of these have the right melting point,

so that collision at those temperature

would successfully accrete material

and become eventually a planet.

Another thing that favors this

is the magnetic field of the sun.

Magnetic fields tend to attract

those larger particles made out of metals.

And all of this together created an environment

where the chemical composition of inner planet

would typically be denser,

composed of these heavier elements.

(pensive music)

- [Narrator] The dry, rocky dust

is less likely to stick together

than the icy rock found in the outer solar system.

And so planetesimals are slower to form.

And because the circumference of the planetary disk

shrinks as you approach the sun,

there is less material for the growing planets

to add to their mass.

- The big planets form

in somewhere between 4 and 10 million years, we think,

whereas the inner rocky planets take longer to form,

they might take 10 to 20 million years to form.

- [Narrator] This slow growth means the inner planets

never become gas or ice giants

like their cousins in the outer solar system.

- They were forming under the shadow of Jupiter,

where Jupiter, so massive,

is dominating the dynamics of the material

that's going in to forming the terrestrial planets.

It's inevitable that would have influenced

exactly what types of terrestrial planets

formed in the solar system.

- [Narrator] Starved of the abundant building supplies

of the outer solar system,

the inner worlds seem inevitably destined to be smaller

than their gas and ice giant cousins.

But recently discovered planets around distant stars

suggest the planets closer to our sun, including Earth,

are unusually small.

1,800 light years from Earth,

in the constellation of Cygnus

lies a sun-like star named Kepler-452.

Orbiting this star is a planet named Kepler-452b.

- Kepler-452b is orbiting its star

at about the same distance as the Earth

in roughly 385 days.

It is often referred as a twin sister of the Earth.

- [Narrator] But there is one striking difference;

Kepler-452b is 60% larger in diameter than Earth.

- Super-Earths are a type of exoplanet that you can think of

as basically being a scaled-up version of the Earth,

something that has the same bulk composition as the Earth,

but is two, three, five, 10 times more massive.

- The gravity that you would feel

if you would be standing on their surface

is likely similar to what we have on Earth.

They tend to lie at distances

that are close to the habitable zone.

And, therefore, if we would look for planets

that have similar characteristics to Earth,

our best chance is to study those super-Earths,

which are really abundant.

- [Narrator] Super-Earths

are the most abundant type of planet

discovered in our galaxy.

In fact, there may be tens of billions

of these intermediate sized worlds in the Milky Way.

Half of all sun-like stars may have one.

- Super-Earths are one of the most common types of planets

in our galaxy,

so it's strange that we don't have one

in our own solar system,

and an active area of research is trying to answer why.

- [Narrator] Some evidence points to the possibility

of an ancient cataclysm,

a literal world-ending event

that changed the destiny of our solar system.

- Our dynamic early solar system in terms of planets

is dominated by the planet Jupiter.

And we know where Jupiter lives today in the solar system,

but there's some evidence that perhaps

it has not always been at that location.

- [Narrator] Planets, it has been discovered, can migrate,

changing the radius and shape of their orbits.

- There's a popular theory called the Grand Tack theory,

tack coming from the sailing term of moving in and out,

that suggests that Jupiter moved into a much smaller orbit

than it's at today,

very much disrupting the inner planet formation,

and then at a later time moved back out

close to where we see it today.

- [Narrator] There's one thing

that can move a giant planet like Jupiter,

and that is the protoplanetary disk itself.

- When a giant planet forms,

it carves out a gap in the gaseous disk.

And so that gap that is carved out by the planet,

as gas is flowing through this gap,

it exerts a torque on the planet.

And that torque results in an exchange of angular momentum

between the planet and the surrounding gas

that can cause the planet to migrate

either inward or outward.

And this is something that we think

Jupiter would have experienced in the early solar system.

- [Narrator] As the gaseous disk is pulled into the sun,

it is believed this

is what caused Jupiter's momentous migration

into our own solar system,

one that would have a profound and devastating impact

on the inner planets.

(ominous music continues)

Rolling back the clock

to 5 million years after the birth of the sun

provides us with front row seats to the coming catastrophe.

- Within the Grand Tack scenario,

Jupiter begins by migrating in through the disk,

and as it migrates inward toward the central sun,

it's encountering planetesimals,

the ingredients that would go into forming

more and more super-Earths.

The bulk of this solid material

is getting swept up, dynamically stirred,

and eventually gonna form a collisional cascade

that goes into destroying itself

and potentially destroying a super-Earth

that might have already existed there.

- [Narrator] Jupiter's immense gravitational influence

pulls the young super-Earths

into chaotic, overlapping orbits.

Inevitably, they smash into one another,

colliding like cars in a planetary-scale demolition derby.

In the cosmic blink of an eye,

entire worlds are pulverized or pummeled to pieces

in a firestorm of molten hot debris

that lasts hundreds of thousands of years.

(ominous music continues)

Some of the wreckage is absorbed by Jupiter.

Most of the rest is pulled into the sun.

What remains swirls around the star

like debris from a shipwreck caught in a current.

- If we had super-Earths

and they were destroyed through some process,

there's still a lot of material in the inner solar system

and so another generation of planets

could grow out of the debris or from the leftover material

that's still there in the protoplanetary disk.

However, that disk would have been depleted,

so you grow smaller, second-generation planets.

(ominous music continues)

- [Narrator] On its inbound migration,

Jupiter appears to be destined to fall

into a close orbit around the sun,

becoming an incandescent gas giant,

a hot Jupiter ruling over a barren stretch of space.

- Hot Jupiter is a case where you have a planet

with the mass and composition of a gas giant,

but in a very, very tight orbit around its parent star.

In systems with hot Jupiters,

we think that migration inwards has probably cleared out

most of the material in the inner solar system.

- If our solar system ended up with a hot Jupiter,

we essentially wouldn't exist today,

and Jupiter would be a loner.

(tense music)

- [Narrator] But then,

another planet comes to our solar system's rescue.

- In the Grand Tack scenario,

Jupiter did not become a hot Jupiter.

Something must have stopped its inward migration.

We believe that thing to be Saturn.

As Jupiter was orbiting through the protoplanetary disk,

so too, was Saturn behind it.

And as their orbital distances were shrinking,

the ratio of their periods changed

until it formed a nice orbital resonance,

one that's believed to be about three to two.

- [Narrator] Orbital resonance is like a grand waltz

with one planet spinning off

and then crossing paths with its partner

to a specific and predictable rhythm.

This celestial dance can have a profound effect

on the stability of both planets' orbits.

- As Saturn's doing three for every time Jupiter does two,

Saturn's exerting these little pulls on Jupiter

that are changing its orbit.

- When Jupiter and Saturn

locked into this three-to-two resonance,

the inner migration stopped

and that dynamical locking actually caused them

to back out again in the Grand Tack.

- [Narrator] Relatively quickly,

a few hundred thousand years perhaps,

Jupiter and Saturn migrate back to the outer solar system.

- So essentially, Jupiter was on a collision course

into the inner solar system to disrupt everything.

But Saturn stopped it, pulled it back,

and allowed the formation

of the inner solar system to occur,

which ultimately allowed for our existence today.

(pensive music)

- [Narrator] The inner solar system

Jupiter and Saturn leave behind

is very different from the one they found.

The super-Earths are gone

and with them most of their matter.

As rocky debris is pulled into the sun

or flung out into the asteroid belt,

from the meager remains,

the small rocky worlds of our current solar system

start to form:

Mercury, Venus, Earth, and Mars.

- The terrestrial planets have cores made of mostly iron,

which make them extremely dense

compared to the gas giants in the outer solar system.

Earth, for example,

is the fifth most massive planet in the solar system,

but it's the most dense.

Even tiny Mercury is more dense than the mass of Jupiter.

- [Narrator] But Jupiter hasn't finished sculpting

the other planets just yet.

Approximately 55 million years

after its first sojourn into the inner solar system,

it embarks on a second migration

that will change the outer solar system

as profoundly as it transformed the inner system,

including hurling an entire mysterious planet

into deep space.

- After Jupiter and Saturn have reversed their course

and started to migrate back

into the outer parts of the solar system,

they don't park there.

Other forces start to become at play

that continue to sculpt the dynamics of the solar system.

- There's a new phase of drama

that occurs in the outer solar system,

which comes from the interaction of the outer planets

with objects in what's called the Kuiper Belt.

- The asteroid belt is this belt of rocky material

between Mars and Jupiter,

but it's not the only belt in our solar system.

We have a whole other system of small solar system bodies

that start just beyond the orbit of Neptune

and go out to larger radii.

We call that the Kuiper Belt.

Kuiper Belt objects are small, icy bodies

that didn't end up going into making the planets.

- [Narrator] A handful of the objects inside the Kuiper Belt

are big enough to be considered dwarf planets.

- Dwarf planets are spherical objects

that are more massive than comets,

but not quite as massive

as the terrestrial planets in our solar system.

- The reason why we don't classify it as a planet is

although it looks like what we picture a planet to be,

it hasn't cleared the debris out of its path.

- The most famous dwarf planet, of course, is Pluto,

but there are a handful

of other dwarf planets that are known

that are actually comparable to Pluto in size.

- [Narrator] Although the Kuiper Belt occupies

a very large region of space,

it's relatively empty,

containing only about one-tenth of the mass of Earth.

However, much earlier in the solar system's life,

the belt is believed to contain significantly more matter,

perhaps as much as 20 to 30 Earth masses,

enough for a Neptune-sized planet.

- The Kuiper Belt never formed into a planet

because Neptune formed and it disrupted the gravity

that any of those objects experience,

enough so that there was never the ability

for those objects to accumulate into a more massive planet.

- [Narrator] As Jupiter and Saturn return

from the inner solar system,

their gravity pushes Uranus and Neptune

further away from the sun.

(ominous music continues)

When Neptune moves closer to the Kuiper Belt,

its gravity starts stirring up the rocks floating there.

- Many of those trans-Neptunian objects

start to flow to the inner solar system.

And although they might seem small,

millions of these objects

start to have little, tiny gravitational interactions

with the gas giants

and indeed, really start to dynamically stir the planets.

Comets are actually believed

to originate from the Kuiper Belt.

One of those small rocky objects

whose orbit actually take it through the Kuiper Belt,

past all the planets

and into the inner parts of the solar system

where they're superheated by the sun,

causing the material on the surface

to actually become vaporized

and blown off the comet in two tails,

a gas tail and a dust tail.

(dramatic music)

- [Narrator] For every action

there is an equal and opposite reaction.

Each comet Neptune pushes inwards,

causes the ice giant to slowly drift outwards

further from the sun.

- As the planet Neptune starts to pull

on some of these Kuiper Belt objects

and bring them more inward in the solar system,

it's transferring some of its angular momentum.

And so over time, what that's going to do

is lead to a drop in its own angular momentum.

And that means it's going to move further outward

in the solar system.

- [Narrator] The planetesimals

flung by Neptune towards the sun

are caught by Uranus's gravity

and slingshot deeper into the solar system.

This causes Uranus to drift outwards, just like Neptune.

When Saturn's gravity interacts with the Kuiper Belt,

the same process repeats.

But once the Kuiper Belt objects reach Jupiter,

the game changes.

- When these trans-Neptunian objects from the Kuiper Belt

interact with Jupiter,

Jupiter's immense gravity

will actually sling them out of the solar system.

That exchange of angular momentum

between the trans-Neptunian objects and Jupiter

actually causes Jupiter to migrate inward ever so slightly

toward the asteroid belt.

- [Narrator] After millions of years of gradual migration,

Jupiter moving inwards and Saturn drifting outwards,

the two planets fall into a new orbital resonance.

For every orbit around the sun that Jupiter completes,

Saturn makes two.

As the planets pass each other, gravitational kicks

cause their orbits to stretch out and elongate,

becoming elliptical rather than circular.

- As Jupiter migrates inward ever so slightly,

that has drastic effects on the orbits

of the other giant planets.

- [Narrator] Jupiter and Saturn's elliptical orbits

start to destabilize Uranus and Neptune,

causing their orbits to become increasingly eccentric.

Just like Jupiter and Saturn,

soon, the entire solar system is thrown

into a whirling, chaotic dance.

- Dynamical simulations have actually shown

that as Jupiter undergoes this slight inward migration,

the orbits of Uranus and Neptune actually flip.

- [Narrator] An even more dramatic transformation

may have occurred in the outer solar system

during this time.

- Not only have Uranus and Neptune flipped,

but their orbits actually appear to be too widely separated.

One of the ways that modelers try and get around this

is actually adding a fifth gas giant planet

to the simulation.

(pensive music)

- [Narrator] The missing planet

is sometimes called the fifth giant.

Believed to be an ice giant similar in size to Neptune,

its birth orbit likely lies between Saturn and Uranus.

- If there had been a fifth planet,

then I think the first thing I'd want to know is

where did it go?

- Did it get ejected out of the solar system,

or did it just get sling-shotted

into the outer part of the solar system

where it still persists today,

but just beyond our detection capabilities?

- [Narrator] Ejected from the cradle of its birth,

the fifth giant might have become a rogue planet.

- These are objects that no longer orbit a star,

they just wander off into space.

Some of them could even retain some of the warmth

they had from their assembly,

and therefore they could potentially

still be a place where one found life.

- [Narrator] Hundreds of rogue planets have been detected

and it is estimated there could be tens of billions

or even trillions of them wandering our galaxy.

- We may also be able to detect that fifth giant planet

if it existed out there.

The problem is, we don't really know where to look.

(dramatic music)

- [Narrator] There is another possible fate

for the fifth giant.

Astronomers have long suspected

that a mysterious ninth planet,

sometimes called Planet Nine,

may be hiding out beyond the Kuiper Belt.

Past the trans-Neptunian objects,

another group of objects piles their lonely orbits

at distances of up to 250 AU.

These extreme trans-Neptunian objects

are called the sednoids.

Ranging in size between 300 and 1,000 kilometers,

they can take over 11,000 years

to complete a single orbit of the sun.

- One of the dynamical properties of the sednoids

is that they tend to have highly elliptical orbits.

- It is extremely hard to explain these orbits

without having another object

being involved into the equation.

And that's where come Planet Nine,

or a planet that we have not yet discovered.

- In order to explain the orbits of these sednoid objects,

we would need to have a planet

that is 10 times the mass of Earth,

and it would need to be 20 times the distance

as Neptune is from the sun.

At this distance it would take up to 20,000 years

for that object to make one revolution around the sun.

- [Narrator] It's possible this planet, Planet Nine,

could be the missing fifth giant.

- If Planet Nine actually does exist

beyond the orbit of Neptune,

it's gonna be extremely difficult to find.

It's pretty low mass.

It's extremely far away from us,

and we don't know what it's made of,

so it's very difficult to predict how bright it's gonna be.

- At such far distances from the sun,

there's such little amount of light,

so we'd have to rely heavily on the orbits of other objects

that are perturbed from the presence of a giant

in the outer regions of the solar system.

- For instance, the detailed study of the orbit of Uranus

led ultimately to the discovery of Neptune.

And further study of the orbit of Uranus

eventually led to the discovery of Pluto.

- As we continue to find more exoplanetary systems,

we continue to improve our picture

of what the diversity of planetary systems look like

throughout the Milky Way galaxy.

As we add more and more pieces to this puzzle,

it helps to put our solar system in a more galactic context.

And even though today our solar system looks quite unique,

in the decades to come, we're gonna continue to find

more and more exotic exoplanets

unlike anything that we've detected so far.

And that is gonna give us the best possible picture

of understanding how our solar system fits in the cosmos.

(dramatic music)

- [Narrator] With the outer planets in turmoil,

closer to the sun, across the asteroid belt,

the newly formed inner planets

are also victims of the cosmic upheaval.

It's believed during this time

there was also an extra planet in the inner solar system.

That means there are five rocky planets circling the sun

instead of the four we know today.

Jupiter's gravitational force sets two of these planets

on a deadly collision course.

The catastrophic impact

will forever alter the destiny of our home planet.

(dramatic music continues)

(dramatic music)