Secrets of the Solar System Horizon

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.