All language subtitles for Secrets of the Solar System BBC Horizon 2015 1080p HDTV EN Sub_Subtitles01.ENG

There are some mysteries when we
look around the solar system,

where the theories
really don't match what we see.

Science fact can be a lot weirder
than science fiction.

We started finding planets

in places we'd never thought
could possibly form a planet.

We had to go back
to the drawing board.

How do you make solar systems?
How do you make planets?

It's as if somebody took
the solar system,

picked it up and shook it real hard.

Our planets might have moved.

They might have moved a lot.

All of a sudden, everything changed.

It's changed the way we look at

almost every process
in the solar system.

Sometimes the blood
splattered on the wall

can tell you more
about what happened

than the body lying on the floor.

The Royal Observatory in Greenwich

is the historical home
of British astronomy.

Discoveries have been made here
and mysteries unravelled.

It is also home to some
unique astronomical treasures.

This is an orrery,

a clockwork model
of the solar system,

and for most of the last
four centuries

this has been the way we think about
the planets in the solar system.

Of course, the scale is all wrong.

But it clearly shows the traditional
view of the planets

and their fixed orbits.

In the centre we have the sun

and then, around it,
we have the four rocky planets,

tiny Mercury rushing
around in the middle,

Venus, the earth with the moon
going around it,

and then Mars.

And outside of the inner planets,

we have the gas giants,

Jupiter, the largest planet of all,

and then Saturn
with its beautiful ring system.

And then the two outermost planets,
Uranus and Neptune.

Astronomers always thought
that the planets have been fixed

in these orbits since they formed,
more than 4 billion years ago.

Long enough for the earth to develop
into the haven it is today

for life to evolve.

A mechanical model like this
embodies an idea

of the solar system in which
the planets all keep

to these very neat, orderly orbits,

moving essentially in circles
and at fixed distances from the sun.

And the natural assumption to make
is that everything we see now

formed where it is
and has stayed there ever since.

The idea that the planets
are fixed in their orbits

has been the bedrock of our
understanding for hundreds of years.

But there are some mysteries
about our solar system

that mean we may have to rethink
everything we thought we knew.

It's time for a brand-new model.

DRAMATIC DRUMBEAT

Recently, astronomers
have started to unravel

the mystery of how
the solar system came to be.

And to explore it

we first need a more accurate
picture of our planets.

We need to alter the scale to
reflect the huge difference in size.

For example, Jupiter,
the largest planet,

is 11 times the radius of the Earth,

and if you look at the masses,
the difference is even greater.

Jupiter has about 300 times
the mass of planet Earth.

The sun we've left alone.

If we scaled that up too
it would fill half the room.

And, of course,
the planets are not all bunched up.

MECHANICAL WHIRRING

We need to push the gas
and ice giants much further away.

To be truly accurate,
with the planets this size

we'd have to make the orbits
several thousand times bigger.

However, exactly how
we ended up with this neat

and stable arrangement of planets

is still one of the greatest
mysteries in astronomy.

In trying to solve this mystery,

we may discover how
the earth came to inhabit

the perfect position
for life to evolve.

Getting an earth
where we have our earth today

was not a given
when this whole solar system started.

We may be able to understand
the remarkable chain of events

that created the biggest game
of pinball in the galaxy...

The solar system could have done
a lot of different things,

it could have evolved
in a lot of different ways.

We could have ended up with
our Jupiter right next to the sun.

And it looks like it was Jupiter

that defined the fate
of the solar system.

The giant planets' story
IS the story of our solar system.

We like to think that
the earth is really important,

but the truth is that,
if you were looking from afar,

our solar system is mainly
four big planets and some debris.

Could our place in the universe

really be nothing more
than a lucky accident?

The question that really arises

is how common is
a solar system like ours?

The mystery of the birth of
the solar system is set to unravel.

As they try to work out how
our solar system formed,

astronomers have noticed
some baffling puzzles.

If we look at the solar system
as it is today,

it seems quite neat and simple.

We have four small, rocky planets
close to the sun

and then four enormous
giants further out.

But when we try to model
the formation of the solar system

on a computer,
something doesn't quite add up.

It's really hard to get the model

to make the planets in the places
where we see them today.

Take, for instance, the curious case

of the undersized planet Mars.

If we look at the rocky
innermost planets,

Venus and Earth
have about the same mass,

and we'd expect Mars
to have a similar mass too,

but it actually doesn't.

It's only about one tenth
the mass of the earth or Venus,

and that's a mystery
that's very hard to explain.

This is the first
of four key puzzles

about the birth of the solar system
that remain unsolved.

And then, at the edge
of the solar system,

the two outermost planets,
Uranus and Neptune,

are much further away
from the sun than we'd expect.

It's very hard to explain
how they could have formed

and become so large at that great
distance from the central star.

If we go in and look
at the asteroid belt,

there are thousands of small,
rocky objects there,

but there are two broad types -

some of them are very rocky and some
have more of an icy content.

And yet these two types
are actually found

relatively close together.

It seems as though they formed
under different circumstances

but they've all ended up
in roughly the same place.

And, again, it's a mystery
as to how that happened.

And, closer to home,

how to explain the rapid
and massive bombardment

that left the moon
covered in craters.

There are many mysteries
in the solar system,

but by unravelling these four -

the size of Mars,

the formation of the outer planets,

the composition of the asteroid belt

and the bombardment of the moon -

we may be able to explain
how our planet Earth

found itself in a perfect position
for life to evolve.

And it all starts a long,

long time ago.

DRAMATIC DRUMBEAT

Four and a half billion years ago,

our sun burst into life

from the collapse of a massive
cloud of gas and dust.

So, in the beginning,

this is what you have
in our early solar system.

You have the young star just born,

and the leftovers,
just a cloud of gas,

the nebula,
the protoplanetary nebula,

full of hydrogen and helium,

dust and gas,

and ice grains forming.

And from this, eventually,
you form the planets.

We know surprisingly little about
exactly how planets are formed.

Most mysterious of all
is the most important -

the largest of all planets,
Jupiter,

which seems to have been made first.

The first-born -
giant, massive Jupiter.

The meanest and largest
of all the planets.

It sucks up more than half
of the existing nebula

and becomes
the king of the solar system.

We know that Jupiter
is made up almost entirely

of the hydrogen and helium
left over in this primordial cloud.

Which means it must have formed
incredibly quickly

because, as the new sun heated up,
it would have blasted the gas away.

And so there's a time limit.

Jupiter must have formed in
the astronomical blink of an eye -

just five million years.

But exactly how it grew so fast
and why it grew where it did

remains shrouded in mystery.

It's a mystery that Scott Bolton
is hoping to shed some light on.

He's sending a spaceship to Jupiter.

At the Jet Propulsion Laboratory
in Pasadena

is NASA's deep space
operations centre -

control room for space flights
to the moon and beyond...

..including Scott's mission
to Jupiter, Juno.

Four...three...two...one...

Ignition and lift-off
of the Atlas V with Juno

on a trek to Jupiter.

Juno launched in 2011

and is currently more than
2 million miles away,

speeding its way to Jupiter
at about 150,000mph.

Even at that speed,
it's a five-year journey.

In 2016,
if all goes according to plan,

the probe will reach Jupiter
and go into orbit

around the king of the solar system.

Scott and his entire team will be
in this room, watching intensely.

On the day of the Jupiter
orbit insertion,

this room will be completely full.

The spacecraft
is approaching Jupiter.

It's moving at an incredible speed -
like, 150,000mph, or even faster.

And when it gets to Jupiter
we have to slow down enough

that Jupiter's gravity field
can grab us.

So, we have a rocket on board,

we point it forward
and we fire it,

and that rocket has to burn
at just the right time

for just the right amount of time

for us to slow down the perfect
amount for Jupiter to grab us,

because if it misses
we fly right past Jupiter.

There'll be a huge cheer.

Once we get the data down that shows
us we're in orbit around Jupiter

the room will explode.

It's the same room where all NASA's
critical events are controlled from,

including the recent landing
of the Curiosity rover on Mars.

And here you see
it's right on that screen.

We'll celebrate just like that.

I can't wait.

Once in orbit, Juno will spend
a year circling Jupiter,

gathering vital clues
about how it formed.

Some of the most important data

that we really want
and can't wait to get

is things that are tied to
understanding the early solar system

and how Jupiter formed.

So, we want to know whether there's
a core in the middle of Jupiter.

Is there a core of heavy elements,

a concentration of materials,
down in the centre?

Or is it the same hydrogen
and helium and mixture of gases,

just squeezed down?

Knowing what's at the centre

is a vital clue to understanding
how Jupiter was built.

Building planets,
whether rocky or gassy,

is a tricky business.

But it's something that Juno
will shed some light on.

What Juno's about is actually
trying to discover

the recipe for the solar system.

How do you make solar systems?
How do you make planets?

And the stage that we're at is
we're collecting the ingredient list

and that's really an important part
of any recipe -

first you gather up the ingredients,
figure out what they are,

then there's some process
that you have to do

in order to bake your cake.

But the exact nature of that process
is not entirely clear.

The recipe for a rocky planet, like
the earth or Mars, is a slow one.

It can take up to 100 million years.

But the ingredient list is simple -

dust.

Dust starts out
in the early solar system

in a very fine grain, like this -
even finer.

Then, eventually, they start to stick
together through electrostatic forces

and they build bigger
and bigger pieces.

Eventually, the rocks got big enough,
they started to stick together

to the point where they started
to form their own gravity.

But there's a big leap
from dust grains

to rocks that are large enough
to clump together

through their own gravity.

These rocks,
even being so large as they are

that none of us could lift them,

they still don't have
important gravity.

Even larger rocks are needed
to really start to get enough gravity

to start to attract the rest
of the material for it to collapse

and start to form a planet
as large as Earth.

It's a slow process,

but there's no rush when it comes
to building a rocky planet.

Gas giants, on the other hand,
are trickier.

You have to make them fast.

Because Jupiter
and the other gas giants

are mostly hydrogen and helium,

and the sun is mostly
hydrogen and helium,

that tells us right away
that those planets had to have formed

while that nebula of hydrogen
and helium was still around.

There are two ways to build a gas
giant like Jupiter that fast.

We don't know exactly
how Jupiter formed.

The two main theories

are either it has a direct
gravitational collapse,

like we think the sun had,
from the nebula

and sort of builds, um,
from the outside in

and formed Jupiter pretty quickly,

or it starts to build
from the inside out.

If it collapsed
from the cloud of gas,

then it will be gas all the way
through to the centre.

But if the second theory is right,
then it first built a rocky core

up to ten times
the mass of the Earth,

which then drew in a blanket of gas.

Either way, it had to happen fast.

But if Jupiter was going to build
a heavy core that quickly,

it couldn't be done with dust alone.

There was another crucial
ingredient.

Ice.

Kevin Walsh is a planet builder.

His job is to create
theoretical models

of how the planets
in the solar system formed -

models that can best explain
the evidence and the clues.

I think that the most likely way
that Jupiter formed

was by building a solid core
of material

and then hauling the gas down
on top of it.

Either way that you form Jupiter,

either from accreting
straight from the gas

or building up a rocky core,
it has to be done

in four or five million years,

before all of the gas is gone
from the disc around the sun.

That's a lot quicker
than the time it took

to build a rocky planet
from dust alone.

But Jupiter had the help
of that extra icy ingredient.

So, we think that the key ingredient

that allowed Jupiter and Saturn
to form so fast,

compared to the rocky planets,

is that they formed
far enough from the sun

that water could condense
from the gas around the sun

and form ice,
and increase the density of material

and give you more material to build
a larger, rockier core faster.

That could explain how Jupiter
built a rocky core so quickly.

But it doesn't explain
why it grew where it did.

It's not unreasonable to think

it would form at the place
with the most ice.

That's a place called the ice line.

But it's not where Jupiter is today.

So, right now when we look at
our solar system, we look at Jupiter

and it's beyond the ice line
by a fair bit,

whereas we think it was really
advantageous to form Jupiter

right at the ice line.

So already that's suspicious.

If Jupiter was built
from a collapsing cloud,

we'd expect it to be further out.

If, on the other hand,
it was built from a rocky core,

we'd expect it to be closer
to the sun.

But it's not in either
of these two places.

So the big question is,

is Jupiter in the wrong place?

To even ask that question has,
until recently, been a heresy.

At the historic Chamberlin Telescope
in Denver, Colorado,

Kevin Walsh is following
in the footsteps

of some famous astronomers.

He's taking a closer look
at Jupiter.

You can see Jupiter
with the naked eye,

but looking at it through
a telescope like this

makes it a lot more fun.

The bands of colour
are really clear and crisp

and the moons are real bright.

It comes alive.
It becomes a real planet when you
look at it through a telescope.

Galileo was the first astronomer
to point a telescope at Jupiter,

more than 400 years ago,

and no-one ever questioned
that Jupiter will always be,

and has always been,
in that same orbit.

Jupiter, right now...

..looks the same as it would have
looked for Galileo.

It's a little bigger and brighter
through this great telescope,

but it's the same Jupiter,

so if I came back tomorrow night,
it'd look the same.

So that's the view -

the planets that we look at now
seem like they never change.

And why would they change?

This was the bedrock
of our understanding -

that the planets' orbits are fixed.

The first hint of something odd
came 35 years ago

from astronomers trying to calculate
how the solar system formed.

They kept getting a strange result.

Some of those calculations were
suggesting that it was possible

that a planet like Jupiter
could have been moved around.

It was a result so crazy
that it was totally ignored.

So if you built a model
to try to understand

some of the events
of the early solar system

and your model is telling you that
planets could have migrated or moved,

that Jupiter could have moved,

then it was telling you
that you probably made a mistake.

So the idea of planet migration,
it was just never possible.

Just didn't seem possible.

But in 1995, astronomers were forced
to face up to the impossible.

The mystery began to unravel
when dramatic evidence was uncovered

from somewhere
completely unexpected.

Astronomer Chris Watson

is searching for the weirdest
places in the galaxy.

He's a planet hunter,
one of a growing band of astronomers

involved in the hunt
for exoplanets -

alien worlds
circling around other stars.

It's one of the hottest fields
in astronomy.

Up until now, or until recently,
we've only had one planetary system

that we could study,
and that was the solar system,

the planets around the sun.

And there are about 100 billion stars
in our galaxy

and there are about 100 billion
galaxies in the universe,

and we could only study one.

But the study of planets
and planetary systems exploded

with the amazing discovery in 1995

of a planet orbiting
around another star.

Less than 20 years ago,
we first found another planet

around another star
that's like our sun

and that was a dramatic breakthrough.

And we now know
of over 1,000 planets.

And they're very strange -

these are nothing like
our solar system

and, in some cases,
I think really science fact

could be a lot weirder
than science fiction.

The planets they have been finding
look stranger than anyone imagined.

We've found planets
around binary stars,

where you actually have two stars
orbiting each other,

a bit like Tatooine off of Star Wars.

That would be a magical world
if you had a habitable planet there,

because you would imagine

you'd actually have two shadows
on everything

and two stars in the sky, as well.

None of the planets found
have been remotely like home.

Recently, a planet was discovered
a little bit bigger than Earth,

but incredibly close to its star.

It's probably a rocky world,
but it's so hot

it would actually be molten,

so...it'd be this,
but the actual lava on it.

And most puzzling of all
are the largest planets,

so weird as to seem impossible.

What was strange when we first
discovered these planets

is they were massive worlds,

they were gas giants
much like Jupiter.

But these were much, much closer
to their parent stars.

Jupiter lives out in the cold
outer reaches of our solar system,

taking 12 years to orbit the sun.

But these alien giants
were found in the fiery heat,

right next to their star,

hurtling around in crazy orbits
of just a few days.

They were nicknamed "hot Jupiters".

They're right up against
the host star,

and it's amazing, they really...

At the time, we thought,
"How did they get there?

"They really shouldn't be there."

Other scientists were thinking,
"Well, you're a bit crazy,

"these Jupiters should not be
that close to the star."

Everyone was baffled
by the existence of hot Jupiters.

They were planets that,
quite simply, shouldn't exist.

In theory, the only place
you could build a gas giant

would be out in the cold,
far away from a star,

because that's the only place

you can find
the necessary ingredients.

What I'm holding in my hand
is a lump of dry ice,

and this represents
the building blocks

of planets like Jupiter.

And this is fine -
it's quite happy out here,

far away from the fire
that represents our sun

or any other star that one of these
gas giants might be forming round.

But look at what happens
when I bring it closer to the fire.

Too close to the star
and the ice just turns to gas.

And without ice
you can't build a gas giant.

So there's very little left
after just a few minutes.

And what this means

is that gas giants
can't form close to the star.

The building blocks
just cannot exist that close.

They have to have formed
further away

where the raw materials can exist.

If these hot Jupiters couldn't
have formed where we find them,

it could only mean one thing.

So we think that, in actual fact,

these gas giants form further out,

then they actually
move towards the star,

they actually migrate inwards.

The planets are on the move.

The discovery that planets
could change orbit

was a shocking revelation.

It turned the world
of planetary science on its head.

The implications of a planet
the size of Jupiter

roaming freely around
a planetary system

could be devastating.

Over recent years, the search
for exoplanets has exploded.

So here we are,
nearly 2,400 metres up

on the volcanic island of La Palma.

What you can see before you
are suites of professional telescopes

and what we're going to do
is we're going to use one of these

telescopes to actually look at
planets orbiting another star.

So, this cloud
can be a bit of a problem.

Normally it's not so cloudy,
but we are in the depths of winter

and this is actually
quite local cloud.

20 years ago, all these telescopes
were busy looking at stars.

Now, increasingly,
many are focusing on planets.

There's quite a few telescopes here,
and probably every night

there's some project related
to extrasolar planets going on.

It is a really rich,
blossoming field of astronomy.

And, provided the clouds clear,

tonight Chris will be pointing
his telescope

at an exoplanet called WASP-84 b.

These clouds will clear.

But, even with clear skies,

spotting alien planets
is no easy matter.

To see other planets in our solar
system from Earth is pretty easy.

So this candle represents our sun

and if I pop down this little rock,
representing a planet,

you can clearly see
the reflected sunlight.

But even the nearest stars
are so far away

that the reflected light from
the planets gets completely lost.

So, now we have our star,
much further away,

and if I put my planet down,

while it's still reflecting
the starlight,

because you're so far away,

the reflected light
is actually drowned out

in the glare of the star itself.

Because the planets
are so hard to see,

astronomers have found other ways
to detect them.

One of the best ways
is actually to watch and see

if the planet actually crosses
in front of the star.

So, if we were an alien civilisation
looking back at our solar system,

we happen to catch Jupiter
transiting the face of our sun,

we would see a 1% dip
in the sunlight.

For a planet a lot smaller,
like the Earth,

that dip is much, much smaller -
it's minuscule.

And that's why it's so, so difficult
to detect these.

But techniques have improved
dramatically

and now, for astronomers
like Chris Watson,

planet-hunting is all part
of a night's work.

This is our telescope,
Telescopio Nazionale Galileo,

and this will be our baby
for the night.

The skies are clearing beautifully,

so I think we're in for
a really nice night ahead.

Thanks to ground-based
telescopes like this,

as well as space telescopes
like NASA's Kepler mission,

thousands of planets
have now been found.

And not just planets,
but entire planetary systems.

So this is the Kepler Orrery,

which shows the orbits and the sizes
of planets.

So these are candidates that
the Kepler space mission has found.

So these are transiting planets.

They don't, however,
look much like we'd expect.

And up there, on the top left,

you can see the orbits of the four
innermost planets of our solar system

from Mercury out to Mars.

What you can see
is the huge diversity

of all the different
planetary systems.

Each set of rings
shows a different planetary system

and each blob, a different planet,
with its size and orbit.

They break every rule in the book

and make us look like
the odd one out.

So we have large gas giant planets
in there,

and then you can see
the really short period,

really weird solar systems.

They really don't look anything
like our own solar system.

Some of these planets actually
have orbits of just a few hours.

There's even systems spiralling
around multiple planets in here.

That one's weird.
What's going on here?

Who knows what we might discover
in this rich smorgasbord of planets?

It is ridiculous, actually.

HE CHUCKLES

What is going on with that?

Extraordinary worlds.

Some may host life.

Our exploration
of these alien worlds

is only just beginning,

but already they're revealing
some incredible secrets.

Tonight, Chris and his team
are training the telescope

on a star they known
has a hot Jupiter orbiting it.

INDISTINCT CONVERSATION

They hope to reveal
just how devastating

a migrating gas giant could be.

So this star that we're looking at,
WASP-84,

was actually discovered to have
a transiting planet around it.

That transiting planet,
we know at the moment,

is about a little bit less massive
than Jupiter.

And we know its orbital period,

so its year is about
eight-and-a-half days,

and we're going to follow it
as it transits the star.

A planet the size of Jupiter

orbiting its star
once every eight days

is already pretty weird.

But some of these alien worlds
have even weirder orbits than that.

What's the air mass with that, then?
About 1.25?

'We would expect
the planet and the star'

to be spinning in the same way.

But we see quite a few systems
where that is just not the case.

Some of these planets are going
completely the wrong way.

If the star is spinning clockwise,

the planet is spinning
anti-clockwise.

169, we're talking about.
Yeah, it's about...

A planet orbiting
in the wrong direction

is a sign of some truly
cataclysmic event.

And tonight,
as it passes in front of its star,

Chris will be able to analyse
the orbit of WASP-84 b.

'The purpose of these observations
is actually'

to see whether we have
a nicely aligned system -

a bit like the planets we have
in our solar system,

where the star spins in the same
direction as the planet orbits.

Or do we have something
that would be the smoking gun

of a really violent interaction

which has maybe scattered that planet
into one of these weird orbits?

So, perhaps over the poles,

or actually spinning in the opposite
direction to that of the star.

But the big question is

what could be the cause
of such planetary upheaval?

Whoa!

After following the transit
through the night,

Chris has the verdict
on Planet WASP-84 b.

So, the transit's finished.
We've had a quick look at the data

and what we've found has actually
taken us a bit by surprise.

We thought that this planet system
would be misaligned.

Now that we've had a look
at the data,

it looks as though
it's actually aligned.

WASP-84 b turns out
to be orbiting the right way.

But Chris has found
many of these hot Jupiters

that are travelling in completely
the wrong direction.

It's evidence of how, in migrating,
they must have caused havoc.

With these very strange orbits,

it looks as though it's been
a very violent process.

To actually take one of these planets

and just chuck it
into a different orbit,

that's very violent.

One of the easiest ways
to do that is to have a collision.

Take two planets,
interaction between them,

and you can eject one planet

and fling the other planet
really close in to the star.

These giant gas planets
are the bully of the playground.

They have the power to throw
other planets around

like a game of cosmic pinball.

Beasts the size of Jupiter
are so vast

they can eject entire planets
from the system.

They can launch them
into crazy polar orbits.

They even have the power
to destroy entire worlds.

A planet like Jupiter,
the mass of Jupiter, the size of it,

just dominates planetary systems,

and it's got the power
to really decide

the fate of the other planets.

I think we'd be quite glad there's
not a hot Jupiter in our system.

We wouldn't be seeing this.

We've discovered other systems
where planets migrate

and hot Jupiters cause havoc.

But what about our own solar system?

Our planets certainly seem fixed
in their rigid, clockwork orbits.

Our earth has been the same distance
from the sun for 4.5 billion years.

Long enough to create
an atmosphere,

build mountains,

and for life to evolve.

But the evidence from other
planetary systems now means

a complete rethink on how
and where our planets formed.

When we started discovering planets
around other stars,

we started finding planets
in completely unexpected places,

places we never thought
could possibly form a planet.

We had to go back to
the drawing board and say,

"Wow, planets can move.

"Planets can really move.
Maybe that happened here."

It's a big leap,
and to make that leap

and say things might have been
completely unstable,

totally chaotic for a time period,
that's really hard to imagine.

But that's the leap
that we need to take.

The crazy results that suggest
Jupiter might have changed orbit

might not be mistakes after all.

Instead, migration could be
the key that unlocks

many of the mysteries of how
our solar system came to be.

Now that we've taken this tool of
planetary migration that we started

to understand by looking
at planets around other stars,

we've realised that it's absolutely
critical to understand

how our solar system formed
and evolved.

And central to it all
is mighty Jupiter.

Certainly in our planetary system,
Jupiter is the key.

It's over three hundred times
more massive than the Earth,

so Jupiter wins.

Jupiter decides what happens.

The inescapable truth
seems to be that planets move.

And, if it can happen
in exoplanetary systems,

it can happen in ours.

If we want to make a model
that explains

how our solar system came to be,

we have to break the brass rods
and set the planets free.

Once we accept the idea that the
planets can move,

we can begin to explain
some of the unsolved mysteries
of the solar system.

In particular, why Mars is so small

and the curious composition
of the asteroid belt.

Kevin Walsh has developed a model
of the early solar system

that involves a wild dance
of the planets.

It's an intricate and chaotic dance,

and if it had gone slightly
differently

it could have stopped our developing
solar system in its tracks.

In his model, Jupiter takes a wild
ride through the solar system.

It takes us right back
to the moment of birth,

when Jupiter had just formed
from the cloud of gas.

The key is that,
though Jupiter is really big,

it's 300 times the mass of the earth,

the gas disc around the sun
was much more massive,

so the gas can actually push
Jupiter in towards the sun.

As soon as it was born,
Jupiter began to migrate inwards.

Over the course
of half a million years,

it spiralled in towards the sun.

It was on its way to becoming
a hot Jupiter.

So, the idea that you could form
something as big as Jupiter

and have it pushed inward
by the gas disc

actually makes
a fair amount of sense,

because we see it,
we see it all over.

But something stopped Jupiter
from crashing into the sun

or ending up as a hot Jupiter.

So, if it formed
and started migrating inwards,

there must have been
a mechanism to stop it

and bring it back out to the outer
part of the solar system.

We think the key to stop its inward
migration, to keep it

from going all the way in towards
the sun is the presence of Saturn.

While Jupiter was on its wild ride,
Saturn was born.

Saturn is also growing. It's going
through the same process Jupiter did.

It's building a big core
and it's getting really massive,

and once it gets really massive as
well it can move in the disc also.

And it too began spiralling in
towards the sun.

So as Saturn is racing inwards,
it gets very close to Jupiter

and they actually get close enough
that they get locked in a resonance

where their orbital periods
are closely aligned

and they interact
very closely gravitationally.

Now, when these two get really close

it actually stops
Jupiter's inward migration.

The two planets were involved
in a kind of gravitational dance.

And, as they came close,
Jupiter changed direction

and was flung back
to the outer solar system...

..just like a sailing ship
changing course in a grand tack.

So this theory called
the grand tack is called that

because our planets
are moving inwards

and they get really close and they
stop and they turn and they go back

outwards, and it's kind of like
a sailboat tacking across the wind.

Jupiter's wild ride could explain
two key mysteries -

first, why Mars is so small.

So, when Jupiter migrates inwards

it kind of snowploughs
all the rocky material it sees,

it snowploughs it
and pushes it inwards.

Much of the dust and rocky debris

that would have gone on to build
Mars got pushed out of the way.

So, by Jupiter coming in
and clearing out all of this

material on its way,
it kind of reduces the total

amount of material
that Mars can feed on to grow,

and so Mars ends up kind of being
starved of rocky material

and only grows to be
a tenth the mass of the earth.

And this explains why Mars is
the planetary runt we see today.

The theory also explains why
the asteroid belt has an icy ring

and a rocky ring so close together.

During its travels, Jupiter
scattered everything in its path.

It threw rocks from the inner part
of the solar system outwards...

..and ice from the outer
reaches inwards...

..leaving the two distinct bands
we see today.

That's how we end up with
two different types of material

sitting on top of each other
in the middle of the asteroid belt

in a very small region.

So Jupiter's wild ride could
explain two key mysteries -

the size of Mars and the composition
of the asteroid belt.

And if it had travelled
any further in

the earth itself may have become
a very different type of planet.

But the birth of the solar system

wasn't the only turbulent time
in its history.

About 500 million years later,
4 billion years ago,

the solar system
entered its teenage years -

an intense period of trouble,
chaos and uncertainty.

It's a period of turbulence
that could explain

two further mysteries...

..the craters on the moon

and the birth of Uranus
and Neptune.

There are some mysteries when
we look around the solar system,

where the theories
really don't match what we see.

If we just take a bunch of small
icy objects

from which Uranus
and Neptune were made,

put them out in the outer part
of the solar system in our computer

models and watch how they grow,
it turns out they can't grow at all.

A couple of billion miles away
from these ice giants, on the moon,

there's a clue that
Hal Levison believes

could help solve the puzzle.

In fact, the moon
is covered in clues.

So, when you look at the moon,

some of these biggest crater
impact basins, like here and here,

all formed in a very short
period of time

that we call
the Late Heavy Bombardment.

It came roughly 500 million years
after the birth of the solar system,

when all the planets
had long formed.

And all of a sudden, out of the blue,

the moon got clobbered by big objects
coming in and hitting it.

And that indicates that you had
this very violent upheaval

and the only way that we can form
an influx like this, stuff raining

down in onto the moon, is through
changing the orbits of the planets.

To account for this
violent upheaval,

Hal and some colleagues
devised a new model.

It explains why
we now see Uranus and Neptune

in places they can't
possibly have formed.

We think what happened is
they formed closer to the sun

and got delivered
to where we see them today.

Uranus and Neptune must have
formed much closer in,

where there was plenty
of icy material,

and beyond them was a neat disc
of icy, comet-like objects.

But this was not a stable system,

and a series of small changes
led to a period of utter chaos.

These objects leak out of this disc,

get gravitationally scattered by all
these planets,

like billiard balls going around,

and get eventually ejected
to interstellar space by Jupiter.

That causes the planets' orbits
to slightly spread over time

and what we think happened
is that Jupiter

and Saturn got to the point
where Jupiter goes around the sun

exactly twice for every time Saturn
moves around the sun.

And that allows their tugs on
one another to become much stronger

and as a result, Jupiter
and Saturn get a little excited,

their orbits become less circular
and more inclined

and they start getting...
sort of tugging on one another.

Uranus and Neptune, which are much
smaller than Jupiter and Saturn, feel

that fight, feel that tension and as
a result, their orbits just go nuts.

In a sudden period of chaos,
Uranus and Neptune

were flung out into
the orbits we see today.

The ice giants, Uranus and Neptune,
get scattered into this disc

that existed outside their orbits,
and that thing went kaplooie.

Vast lumps of ice were scattered
everywhere,

raining into the inner solar system

and bombarding
the earth and the moon.

Every square inch of the earth
at one time got hit

due to this instability, so it was
not a very safe place to be.

We had this view that the solar
system was this nice clock

and things just moved around
in nice regular ways.

What this new model shows
is a real paradigm shift.

It says that the solar system is not
this nice, safe, quiescent place,

but can go through periods
of intense violence.

This new model of the solar system
is now dynamic and turbulent.

The prime mover in all this upheaval
is our playground bully, Jupiter.

Such a bully, in fact, that David
Nesvorny believes that Jupiter

may have been responsible
for the ultimate planetary crime.

He ran the new model
over and over again

with slightly different
starting conditions.

I ran about 3,000,
4,000 models like this,

just playing with the initial state

and at a time, I considered
the standard theory,

which was that the outer solar system
had four planets.

His results were alarming -

change the starting
conditions even slightly

and the solar system
looks very different.

Frequently, what happened
in my simulation was that Jupiter

just slingshots Uranus
and Neptune from the solar system

and they ended somewhere
in interstellar space.

So that wasn't right.

Obviously not right.

Then David had a radical idea.

If Neptune and Uranus didn't get
flung out of the solar system,

maybe something else did.

I couldn't quite fit the solar
system, how it looks like today.

So I was thinking and thinking
and thinking, and then I thought,

"How about if the solar system
had an extra planet?"

He started investigating
the possibility

that an entire planet
might have gone missing.

As ever,
the prime suspect was Jupiter.

So, now I am pointing at Jupiter,

so I can see the disc of Jupiter,

and then, nicely aligned,
four giant moons.

It has a huge influence...

..and could have had an even bigger
influence in the past.

It may even have been able
to eject an entire planet

from our solar system.

This is the solar system.

The sun is in the middle...

..then we have the
terrestrial planets.

Then there's the asteroid belt
and the outer planets.

To get the arrangement of planets
we see today, David thinks

we once had an extra ice giant
but it was thrown out by Jupiter.

I start playing with the possibility
that we had an additional planet.

So the best case I have found
was when I placed

this third ice giant between
Saturn and Uranus initially,

somewhere here.

What happens in this case is
that during the instability,

this planet evolves,
has close encounters with Jupiter

and Saturn and gets ejected
from the solar system.

The ejected planet may have been
a sacrificial lamb

that saved us from Jupiter's
destructive powers

and allowed our planets to settle
in the pattern we see today.

So, what became of our
missing lonely planet?

In the simulations I have, the planet
is ejected from the solar system

with a speed of about
1km per second.

But this happened about four billion
years ago, so do your math.

It will end up very far
from the solar system,

so today it can be
almost anywhere in the galaxy.

20 years ago, the mystery of the
solar system began to unravel.

Evidence from alien worlds
shattered the long-held view

that our planets have fixed orbits.

It led to a whole new understanding
of a turbulent and dynamic past...

..which makes us wonder, might
things have turned out differently?

The solar system could have done
a lot of different things,

it could have evolved in a lot
of different ways.

What we see in our own
solar system

is the result of a lot of
unlikely or random events,

and so our solar system is unique.

Ending up with a stable system
of planets was just a fluke,

a lucky roll of the dice.

It's amazing we survived at all.

Getting an earth where we have
our earth today was not a given

when this whole solar system started.

It took all these series of events
to get a rocky planet

of this size at this distance
with this amount of water

to build the earth
that we live on today.

The fate of the entire solar system,
including the earth,

was defined above all by the
movements of our gas giant, Jupiter.

If Jupiter's orbit moved
differently,

if Jupiter moved
into the inner solar system,

then it's unlikely
that the earth would be here.

Of all the planetary systems
so far discovered,

it seems we are the only one
with the lucky roll of the dice.

You might think that maybe
the solar system that we have here

is actually the oddball
and that the natural order

of things are these other systems
that we think of as weird.

And, if we are so unusual,

will we ever find anywhere else in
the universe so welcoming to life?

Even though our solar system might
be...let's say one in a million -

that may seem like
a really small number -

there are 100 billion stars
in the galaxy.

So even something as unlikely
as our solar system,

there may be lots of them around.