All language subtitles for How the Universe Works s06e03 Dark History of the Solar System.en

Our solar system is a strange thing.

It's radically different from
all the other planetary systems

we see across the galaxy.

We are an exception
rather than the rule.

To understand why,
scientists peer deep

into our solar system's
secret history

and find a dark and violent past
of planetary homicide

on a mass scale.

The solar system is a ghost
of what it used to be.

You end up with
the last survivors

being a bunch of freaks.

Our home could be
one of those freaks.

The rock beneath
our feet could have been

from long-dead planets.

And those long-dead
planets could help explain

why there's life on Earth.

In a spiral arm of a huge galaxy
called the milky way

spins an extraordinary
planetary system,

our solar system.

For millennia, it was
the only one we knew,

but all that has changed.

Astronomers have discovered more
than 2,600 planetary systems

to date, but none of them
are quite like our own.

The incredible thing
about astronomy

is when you look out
into the universe,

and you realize you have
completely misinterpreted

your own home.

So one important thing
we've learned

in discovering planets
around other stars

is that our system
isn't the normal system.

It's not what we see everywhere.

In fact, as we discover
more planets

orbiting other stars,

we see that ours is an oddball.

Most other solar systems look
completely different than ours.

February 2017, NASA makes
a huge announcement

about a system
in the Aquarius constellation,

Trappist-1.

Trappist-1 is a little unusual.

It's a little bit smaller
and cooler than the Sun,

but it has seven planets
orbiting it.

And you think, "well, that's not
that peculiar. We have eight."

But these are seven
roughly Earth-sized planets.

There's not a lot
of variety there,

and they also orbit
the star very close in.

All seven planets
somehow orbit closer

to their star than Mercury,

our innermost planet,
does to our sun.

Perhaps one of
the greatest puzzles

that have come out of finding
planets around other stars

is that they typically
have orbits

well inside
the orbit of Mercury.

It's really odd, in my view,

that the solar system
is hollowed out.

There's nothing inside
of Mercury's orbit.

Why is that?

The mystery
of the missing inner planets

is like a cosmic whodunit,

turning scientists
into detectives asking,

"do we really know how the story
of our solar system unfolds?"

The whole process is,
like, cosmic CSI.

You're trying to put together
the clues to find out something

that happened when nobody else

was there to watch it happen.

Like detectives,
scientists start

with the simplest explanation.

So in some ways,
the early solar system

is like a pool game.

All the billiard balls represent
the pieces, the building blocks,

the planetesimals
or the planetary embryos

that are going to come together

eventually to build
the final system of planets.

In planetary formation,

the simplest theory is that
the planets all formed

where we find them now.

It's called the classical model.

How does this play out?

We start with
a few planetesimals

that collide with each other,

and they grow
a little bit larger

in this region
of the solar system.

This process continues, and you
grow all the way up to planets,

with each planet in each of
the zone of the solar system

creating material just
from its neighborhood

and no one's really
moving around very far.

But this classical model
can't explain why

our inner solar system

is missing all kinds
of material.

The classical model has
no natural explanation

for why Mercury is the last
thing that we know of,

inward towards the Sun,

that there are no planets,
no asteroids,

nothing inside Mercury,
is still a mystery

that the classical model
can't easily explain.

The area close to our
sun isn't just missing asteroids

and small planets.

It's also missing
really big ones.

The very first exoplanets,

the very first alien
worlds we discovered,

were Jupiter-mass
or bigger planets

orbiting their starts
very closely,

even closer than
Mercury orbits the Sun.

Astronomers
have so far discovered

around 300 gas giants

scorchingly close to their suns.

They call them hot Jupiters...

But how they form is a mystery.

Gas giants like Jupiter
should be born out of the cold,

far from their suns.

It's very hard to imagine
hot Jupiters forming

where we see them today.

The temperatures,
the distances from the star,

where we find hot Jupiters
are so hot,

it's hard to imagine
any material

condensing out
of the solar nebula.

This kick-started the idea

that maybe these hot Jupiters,
as they were called,

may have actually formed
farther out,

like near where our
Jupiter is now,

and in the early solar system

they started migrating inward
toward their star.

So what happens when
a planet the size of Jupiter

moves inward?

Can this help explain the inner
solar system's missing mass

and answer why
we don't have a hot Jupiter?

To find out, Kevin Walsh
and colleagues simulate

the first 10 million years
of the solar system.

They call this model
the grand tack.

The grand tack model
is a scenario

designed to help understand

how the terrestrial planets
could have formed,

thinking about
what the giant planets

might have been doing
in the early solar system.

The planets form within
a thick disk of gas and debris

that surrounds
the newly-formed sun.

The grand tack model simulates
what happens if Jupiter moves

in towards the Sun
through this disk.

It's pushing all of
the asteroids in its path

into the inner solar system.

All of that material is what is
going to come together

to form the rocky planets.

Jupiter's immense
gravity pulls in more

and more material,

forming a dense wave of debris
bulging out behind it.

The pressure of this bulge
pushes Jupiter further inwards.

Like a wrecking ball,
Jupiter should clear out

all the planet-building material

from the entire
inner solar system

and become a sun-hugging
hot Jupiter,

but something checks
Jupiter's path of destruction.

If Jupiter had hung around
much longer

in the inner solar system,
we wouldn't be here,

so something must have
drawn it out very rapidly.

And what could possibly move
a big, massive planet rapidly?

And the answer is another big,
massive planet.

Saturn...

It forms just after Jupiter,

and is hot on Jupiter's heels,

as it, too,
migrates towards the Sun.

Saturn is pretty big itself.

The combined effect of
the two giant planets

migrating is that once

Saturn is large enough,
it can actually change the way

that the gas disk is interacting
with both the planets.

And it can stop Jupiter's inward
migration and help to turn

Jupiter around

and almost pulls it back
to the outer solar system.

Like a sailboat
switching direction,

Jupiter tacks away from the Sun.

The behavior of the giant
outer planets

leaves our solar system
with no hot Jupiter

and has dramatic effect
on the small inner planets, too.

As it's coming back outwards,

what has Jupiter done to
the inner solar system?

It has removed all of the
material in its path,

all the way down to where
we find the Earth today.

And all of that material pushed
into essentially a narrow band

in the inner solar system is
what is going to come together

to form the rocky planets
as we find them today.

According to
the grand tack model,

without Jupiter,
the rocky terrestrial planets

of the inner solar system
might have never formed.

One of those planets is Earth.

So as much as we owe
our existence to Jupiter,

we also owe it to Saturn

because if Jupiter had kept
moving in closer to the Sun,

we almost certainly
wouldn't be here now.

The grand tack
may provide a vital chapter

in the story
of Earth's formation.

That answers why
we're missing a hot Jupiter

but doesn't explain
why there's nothing

between Mercury and the Sun,

nor why we're missing one of
the most common types of planet

in the whole galaxy,

a giant rocky world
up to 10 times

the size of Earth...
A super-Earth.

The Kepler space telescope

leads the charge in the hunt

for exoplanets
around other stars.

It's confirmed
over 2,000 new worlds.

The single-largest
finding to date,

over one-third of those planets
are super-Earths.

A super-Earth is a type
of rocky planet

that has a mass a few times
the mass of the Earth,

and as we look around the
galaxy, we find them all over.

Our solar system
doesn't have one,

and you have to ask
the question... why not?

What's different about us?

In our solar system,
planets range in mass,

with Jupiter being the largest

and Mercury the smallest.

But weirdly, we have nothing
in the super-Earth-size range,

which is between
Earth and Uranus.

Why is there such
a big gap in masses

between the Earth and Uranus,

which is roughly a dozen
times the Earth's mass?

It's a big jump
from one to a dozen.

Why?

In 2015,

Konstantin Batygin
tries to find the answer

to why we have no super-Earths,

and why there is nothing
within the orbit of Mercury.

He reconstructs the grand tack
model with one key difference...

The simulations start
with six super-Earths

in our solar system center

in a tight orbit around the Sun,

typical of other systems
we've observed.

He calls this new model
the grand attack.

One of the realizations
that has come out

of studying
the grand tack scenario

is that Jupiter's migration
would have really unleashed

a veritable grand attack

upon the inner solar system.

The grand attack model
ramps up Jupiter's action

so it sends swarms
of giant asteroids and planetary

embryos into the inner
solar system on tight-knit,

overlapping orbits.

The result?

Carnage.

Each big body
will experience a collision

with another big body
once every 20 to 200 orbits.

This is exceptionally fast
on cosmic-time scales.

What this means is that
you take the entirety

of that overlapped
population of bodies,

and you smash them up
into smaller debris.

Jupiter is like a little kid
with a hammer, you know?

It just comes in and is
whacking around at everything,

and it's making a mess
of the inner solar system.

It's a game
of cosmic pool on steroids.

Let's start with our Jupiter,

for our model here
of the solar system.

It's causing a bunch of
very violent collisions

between all of this debris
that it's sweeping up.

These huge collisions are
making an enormous amount

of really small material,

which can drift really fast
inward in the solar system

due to the drag
from the gas around the Sun.

These planetesimals
collide over and over,

pulverized to
the size of gravel.

For the smaller debris,
hitting this dense gas cloud

around the Sun is like
plowing into a headwind.

The swarm of rubble loses
the momentum that keeps it

in orbit around the Sun
and starts spiraling in,

but it hits roadblocks.

So as all this debris rushes
inward in a big wave,

it gets dragged in until it gets
stuck behind the super-Earths.

Debris builds up until the
super-Earths finally give way.

The super-Earths are like a dam

that can't quite resist
the flow of water

and begins to recede
and eventually gets

kind of pushed onto
the surface of the Sun,

together with the flux
of collisional debris.

It's a remarkably swift process.

In just 20,000 years, all the
super-Earths crash into the Sun.

After the dramatic evolution

of the inner solar system,
there's only a fraction

of the original mass left.

The solar system is a ghost
of what it used to be.

There's nothing
in the first 39 million miles

from the Sun.

But slightly farther out,

there's a narrow ring
of rocky debris,

about 10% of the original
material swept in by Jupiter,

just enough to rebuild
the inner solar system.

A few survivors, small
planetesimals start to regroup.

Over millions of years,

four small, rocky planets form.

Mercury, Venus, Earth, and Mars

form from this leftover debris.

The planet that we're
standing on may not be

an original generation
solar system planet.

It's kind of like building
a house with cinder blocks

from a house that sat on
that spot but was demolished.

Not only are we breathing the
atmospheres of long-dead stars,

the rock beneath our feet

could have been
from long-dead planets.

So Earth could be
second-generation planet

formed from the wreckage
of the grand attack.,

But there was one thing
the super-Earths

took with them
to their fiery grave...

The supply of hydrogen
and helium

in the inner solar system.

When you look at
the Earth's atmosphere now,

we don't have any hydrogen
or helium in it.

There was hydrogen
and helium in the disk

where the inner planets formed,

but that became part
of the super-Earths,

the first generation planets.

When Jupiter came in
and dropped them into the Sun,

they took their hydrogen
and helium with them.

So the composition of
the the air around you right now

may be due to the fact

that we're
a second-generation planet.

Coming in second
might not sound great,

but maybe second place
is the reason we're here.

So, the Earth we now see
is Earth 2.0.

Would Earth 1.0 have been
conducive to life?

That's an interesting question.

Earth's atmosphere is
a fertile blend of gases

that allows life as
we know it to flourish,

an atmosphere that might have
been completely different

if Earth was
a first-generation planet.

It's entirely possible
that life, like us,

needs to have
a second-generation planet

to arise in the first place.

Could our planet be more unusual

than we'd ever thought?

How special is the
Earth in a cosmic setting?

We don't really know the final
answer to this question,

but evidence is beginning
to point to the fact

that the Earth is actually
kind of rare,

and we should really
appreciate our planet.

Finding out what
happened to our solar system

is like studying
a cosmic crime scene.

To reveal the solar system's
secret history,

we need to look
in unusual places,

the last surviving pieces
of the violence

from which our home was born.

Our solar system is
a celestial cold case,

and it's hiding the traces
of its violent past.

What you have now is
a crime scene that has dried up,

and you're trying
to find little clues as to

what happened
4 1/2 billion years ago.

It's a really, really
difficult problem to solve.

Solid evidence
could be hard to find,

but sometimes we get lucky.

We don't have a time machine,
so it's hard to go back

in time 4 1/2 billion years

and look at the solar system and
see what it was doing back then.

However, sometimes
nature provides,

and if you don't have
a time machine,

sometimes a time capsule
will do just as well.

And in fact,
we have time capsules

of the early solar system,

and we call them meteorites.

Most meteorites
are chunks of asteroids

that fall to Earth.

Depending on their origin,

they come in different
shapes and sizes.

Asteroids really are
like space fossils

because they were formed
4 1/2 billion years ago,

but they've basically
remained dead.

They are the leftovers, the
remnants of planet formation.

They're the last little bits
that haven't become planets yet.

To understand why meteorites
are such useful clues,

we first need to know
how planets form.

It's a process called accretion.

The cloud of hot gas swirling
around the Sun condenses

and clumps into larger
and larger bodies.

We find traces of this process
inside meteorites,

in tiny mineral beads
called chondrules.

Chondrules are literally
the seeds

of all of the structure
in our solar system.

Most chondrules condense

out of the cloud of hot gas

around the Sun
as the solar system forms.

Chondrules have been
described poetically

as droplets of fiery
rain that have solidified.

They are little globules
of silicate melt

that were produced in
the very earliest history

of our solar system.

These globules of melt
solidified to form

these little spheres.

It really tells us about the
process of agglomeration

of smaller objects
to form larger bodies.

But some chondrules tell us

not only about a planet's birth,

but also its death.

A meteorite called gujba

contains two very different
kinds of globules.

So a gujba is a type
of meteorite

that's made up
of little spherials

of silicate material

as well as spherials
of iron-nickel metal,

and it's very unusual
in that regard.

These metal spherials that
we find in gujba are formed,

we think,
around 5 or 6 million years

after the solar system forms.

At that point,
there was not enough hot gas

lingering in the disk

to form the chondrules
we see in the gujba.

So how did this globules form?

The only way to really produce
these globules

is another process,

and we think in this case
it was some kind of process

like collisions.

Collisions so violent
they vaporized

the silicates and metals.

Solid turns to gas, and
then gas to liquid.

But what planetary body
contains enough metal to be able

to produce the droplets
we see in gujba?

Only something big enough
to have an iron core.

When an object grows
large enough,

its gravity become strong enough
that it differentiates,

and what we mean by that is,

heavy stuff is pulled down
and sinks into the center,

and lighter stuff floats
to the top,

so you have a differentiation
of material.

Earth is a classic example of
a body that is differentiated.

while heavy metals remain in the crust

The crust and the core
are separated

by a molten silicate layer
known as the mantle.

Gujba is a perfect example
of the fact

that you had large
planetary bodies

that were differentiated
into irons and silicates,

and they were colliding
at velocities

great enough
to scatter their pieces

out into the nebula again.

That is just amazing to me,

that this really,
really violent process

in history that's captured
in these tiny little fragments.

Gujba reveals
the differentiated planets

were commonplace
as early as 5 million years

after the formation
of the solar system.

There were so many
of these planets,

they often smashed together.

If you go down this
path of planet formation

by giant impacts,

you end up with
the last survivors

being a bunch of freaks.

Freaks built
from the dead embers

of past generations.

And this violent scenario
raises the question,

just how many planets
did it take

to build the inner solar system?

Simulations say at least 30,

but just four survived.

The secret history
of the solar system

is hard to interpret,

but astronomers devise
radical new solutions

to connect the dots.

The grand tack model helps
explain why we see

no hot Jupiter planet.

The grand attack model
provides an answer

to the lack of material
within Mercury's orbit

and why our solar system
has no super-Earths.

These are theories designed
to crack the cosmic cold case,

but to understand
our red neighbor, Mars,

scientists need another
simulation, a 30-planet pileup.

When you look at the action
in a solar system,

you essentially have our
smallest planet on the inside

and then it gets
larger and larger

as you go from Venus
to the Earth,

so you would naturally expect
Mars to be larger than it is.

It should be 10 times bigger
than it is, but it's not.

Mars' size isn't
its only mystery.

It's also much older
than we expected.

Scientists have refined
the age of Mars' mantle

based on the chemical
composition

of a piece of martian meteorite.

The sample blew off from the
planet during a violent impact

and made its way to Earth.

It revealed that Mars
formed rapidly,

within the first 2 million years
of the solar system's birth,

well before the Earth.

Mars is small, and Mars
formed really,

really fast compared
to what it should have.

The Earth is 10 times
more massive.

It formed in 100 million years.

Mars formed in 2 million years.

This doesn't make sense.
It's our neighbor.

It should look just like us.

Everything about Mars
feels wrong.

These two mysteries
might help explain one another.

Scientists think it's possible

that around 30 other
similar planets

formed alongside Mars

within the first 2 million
years of the solar system.

So what happened
to this 30-planet pileup?

Time for another game
of cosmic pool.

So in this model,
we very quickly form

20 or 30 Mars-sized planets.

This is a pretty
jam-packed system.

The planets are pretty
close to each other,

and it's just on
the hairy edge of stability.

This colony of Mars-sized
planets builds rapidly.

In the early days
of the solar system,

there's enough gas around
to keep their orbits

from crossing each other,

but after 20 million years,
the gas has gone,

and their orbits
start to intersect.

When it goes unstable,

it's then a pretty loud
and chaotic place.

As Mars-sized bodies collide
with each other

to build the Earth and Venus,

we get a series of huge,
violent collisions.

Over the next 100 million years,

the Mars-sized protoplanets
annihilate each other

to eventually form
second-generation planets,

Venus and Earth.

Yet one planet stood back
and watched from the sidelines.

That planet was Mars, and that
is the secret to its old age,

compared to Earth.

If Mars is indeed older
than the Earth,

that would imply that it's one
of the original planetary

embryos of the solar system.

Mars is essentially done early.

It is on the outside
of this whole process,

sitting out not accreting
any more mass

and watching while
the Earth and Venus

form out of the other
big bodies that have been built.

So what prevented Mars
from colliding

with the rest
of the planetary embryos?

The answer... Jupiter.

During the grand tack, Jupiter
moved to the same distance

from the Sun
that we find Mars today,

and in the process ate
the red planet's lunch.

Jupiter
removes all of the material

that Mars otherwise would
have been building on

for the next tens
of millions of years,

essentially clears out a big
chunk of the solar system

and starves Mars.

If Jupiter were not there,

than we would have expected Mars

to have formed a fully-fledged
super-Earth planet.

Earth formed from
the wreckage of this pileup,

but a reminder
of this population lives on,

every time we look to the night sky...
The Moon.

We were convinced we knew
how the Moon formed.

Turns out, we were
completely wrong.

A distance observer
studying our solar system

would notice something
strange right away...

The size of Earth's moon.

Most planet's moons
are tiny by comparison.

How did we get a moon so big?

For a while we've realized
it couldn't have formed

at the same time as the Earth.

It just doesn't make sense.

The standard idea of
the Moon's formation

is that an object
about the size of Mars

collided with the early Earth.

A lot of the debris was thrown
into orbit around the Earth,

and it coalesced
to form the Moon.

We call
this Mars-sized object Thea,

but exactly when
and how the Moon formed

remains a mystery.

Ever since the Apollo missions,

we've been searching
for a piece of lunar rock

that can unlock this secret.

Melanie Barboni's team at UCLA
is one of the few groups

authorized to analyze these
precious lunar samples.

But there's a problem...

Most moon rock is contaminated

and damaged by violent events
in the more recent past.

Asteroids hit the Moon,
and there are geological

processes
that do a lot of mixing,

and it's very difficult
to find a pristine sample

from the Moon's very formation.

Melanie and her team
have come up with a novel

answer to that problem.

Rather than date
the entire rock,

they isolate a tiny, pristine
crystal within a lunar sample,

known as a zircon.

Now we don't want
the whole rock,

we want only tiny zircon
that are inside those rocks.

These zircons formed

just after Thea's collision
with Earth.

Once the molten crust
of the Moon,

it cooled and solidified.

This is much smaller
than the grain of sand

you find on the beach.

Zircon is the most perfect clock

that nature gave us
to date the Moon

because it's very resistant.

Here you can see
its surface is very smooth.

There is no fractures on it.

Zircons tick off time
like clocks.

They contain large
radioactive elements

that decay into smaller ones.

Scientists can tell
how old the crystal

is by measuring
the radioactive decay.

The zircons Melanie found
rewrite the history of the Moon.

The Moon is around
140 million years older

than what we thought.

This means the Moon formed

no later than 60 million years

after the birth of the Sun.

This places the formation
of the Moon right in the middle

of the destruction of
the 30-planet pileup.

It's entirely possible
that Thea was once a member

of this colony
of Mars-sized objects.

It wasn't just some Mars object

that was out in
the outer solar system

and came careening in
and smashed into us.

It was one of these
no-longer-existing planetesimals

that slammed into us
and formed the Moon.

But scientists looking
for traces of Thea

on the Moon draw blanks.

One of the intriguing things
about moon rocks

is how similar they are,
chemically, to rocks on Earth.

It has the same
geochemical fingerprints,

the oxygen isotopes,
of the Earth,

and all the other chemical
isotopes of the Earth.

It looks just like Earth rock.

If the Moon really is the
product of one giant collision,

well, whatever hit the Earth,

there should be different
proportions of that on the Earth

as opposed to the Moon,
but we don't find that.

The Moon is identical
to material from the Earth,

except it's missing heavy
elements, iron and nickel,

found in the Earth's core.

Instead, it mainly contains
lighter rocky elements

found in the Earth's
crust and mantle.

Why?

It wasn't a head-on collision.

It was a grazing collision.

Now, that's important because
the heavy material

was starting to sink
into the center of the Earth,

and the lighter stuff
was floating to the top.

And if this were
a grazing collision,

then that lighter material
would have been splashed out,

and that's what would
have formed the Moon.

And the Moon is, in fact,
less dense than the Earth,

which makes sense if it formed
from this lighter material

that was near the top.

It looks like you took a blob
of the Earth's mantle

and just put it into space
around the Earth.

A single head-on
collision would leave traces

of both Thea and Earth's core
on the Moon,

but a glancing blow wouldn't
knock off enough material

to form a moon as big as ours.

One way we can end up
with the Earth-moon system

that we see today
and solve all these problems,

is that instead of having
one big collision,

there were a series of
several smaller collisions.

Each impact grazes off
a section of Earth's crust

that forms a ring
around our planet.

With each small collision,

material would have been thrown
into orbit around the Earth.

Eventually this
collisional debris merges

to form a small,
new moon, a moonlet.

Now after several of
these collisions,

you'll have debris from each
collision circling the Earth.

Some of it is still in
the form of debris,

some of it is in
the form of moonlets.

Eventually they coalesce
to form our current moon.

It seems our moon may well be

the product of a series

of cosmic collisions
in the early solar system.

What we see when we look up in
the sky now isn't the moon,

but it's basically
the last moon that survived.

It was just the one
that happened to be there

when all of these
impacts stopped.

The closer we come to
understanding our violent past,

the more we appreciate
the calm of the present,

but as we try
to predict our future,

it seems we are destined
for chaos once again,

as a distant mystery planet
in the outer solar system

moves in from the cold.

The birth of our solar system...

Violent, chaotic, catastrophic.

When we look at the solar
system when it was very young,

all of our models pretty much
say the same thing.

It was not nice and orderly.

It was a disaster.

And then things settled down.

Life had a chance to take hold
and evolve under very stable,

very friendly conditions.

So when you look around
right now,

you're seeing the story of
an ancient, violent past

that has smoothed out
into the wonderful environment

we know today.

Our solar system
might seem stable,

but there is still something
very strange about it.

There is still one
enduring mystery,

and that is why
the solar system tilted?

The eight planets orbit

in roughly the same flat plane,

but compared to the spin axis
of the Sun,

that plane is tilted, making
the Sun look lopsided.

And it turns out, when
you look at the Sun's tilt,

it's actually
tipped by 6 degrees,

the plane of the solar system.

And that may not sound
like a lot,

but it's actually quite a bit

compared to the tilts of all
the planets of the solar system,

and this is an outlier.

It's strange.

What could have done that?

The tilt contradicts
what we know about

how the solar system formed,

a spinning cloud
collapses into a disk.

The spinning disk then becomes
the Sun and all the planets.

It should all be spinning
on the same axis.

So one possible way that
you could change the orientation

of the pull of the Sun relative
to the plane the planets are in,

is if there was
something out there

tugging on the planets
for a very long time.

2016, Cal tech astronomers

Konstantin Batygin
and Mike Brown

claim they've found
the missing something...

Planet 9, a theoretical
giant orbiting off-kilter

in the far reaches
of the solar system.

Planet 9 resides on a long
and substantial orbit,

and it itself is pretty massive,
about 10 Earth masses or so.

If it's orbiting the Sun on a
highly elliptical tilted orbit,

every time
it gets close to the Sun,

it's going to tug on the planets
just a little bit.

But over hundreds and thousands
of orbits, it can actually tip

the orbits of all the planets
in the solar system,

but it won't tip the Sun.

Over billions of
years, the planetary system

slowly twists out of alignment
with its original plane.

Planet 9's distant reach

may solve the mystery
of the solar system's tilt,

but it might also have
a disastrous effect

on the outer planets as
the Sun starts to die.

the Sun, just like every other
star in the universe,

has a life cycle.

It was born.

It is currently living its life,
and it will die.

As it dies, it will bloat up
into a red giant star,

and then the outer layers
will begin to drift away.

Now what that means is that

the Sun will be losing
mass very quickly.

The thing that holds us
in orbit around the Sun

is the gravitational
pull of the Sun.

Jupiter, Saturn,
Uranus, and Neptune

will move away from the Sun

as its outer layers expand,

but that's not the case
for distant Planet 9.

Scientists think
that Planet 9 orbits

so far out that as the Sun dies,

it will loosen its gravitational
grip on the planet.

Planet 9 starts to feel
the influence of other objects

more than the Sun.

It turns out a passing
star, for example,

could affect its orbit,

or even tides
from the galaxy itself,

our galaxy's gravitational field
can affect this planet,

and drop it into
the inner solar system.

This change in Planet 9's orbits

could be disastrous
for the solar system.

And if that happens,

it could actually
wreak havoc on the gas giants

Jupiter, Saturn, Uranus,
and Neptune

and distort their orbits,

maybe dropping them into the Sun

or flinging them out of
the solar system, as well.

Planet 9 could make
the death of the solar system

just as violent as its birth.

Right now we're in this
wonderful sweet spot

where life can evolve
and take hold between

two eras of almost
unimaginable violence.

If there is a Planet 9,
then it's kind of a rehash

of what happened
in the early solar system,

when everything
was really chaotic

because of a giant planet
moving inward.

The same thing
could happen again.

Born in chaos.

Perhaps ending in worse.

One thing is clear...

What we thought we knew
of our cosmic home

grows more intriguing

with each new clue
to this once cold case,

now a very hot one.

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