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.