Original subtitles

(ominous music)

From the drama of our planet's origins

and the birth of our solar system

comes one of the most startling revelations

of modern science.

The solar system we see today, quiet, stable,

was once a battlefield.

Newborn planets blasted through space,

competing for stable, circularized orbits

inside a grunge-style mosh pit of gas and dust.

For those that find the right balance,

the prize is survival;

for the rest, world-shattering destruction.

A new look at the chaos of creation

and a frightening possibility in our distant future.

(dramatic music)

(ethereal music)

Mars.

The Curiosity rover is searching for clues

about the origins of the red planet.

It confirms the presence of oxygen and nitrogen isotopes

hidden in the rocks and soil.

We use isotopes to try to figure out

the history of planets,

partly because they are immune to many of the changes,

the chemical changes that occur

when you have things like collisions and so forth.

Isotopes, the relative abundances of these isotopes

are like fingerprints.

Curiosity confirms a unique mix

of isotope fingerprints.

The isotopes indicate that Mars formed elsewhere

in the solar system and moved into our neighborhood.

(dramatic music)

Our solar system is full of oddities

pointing to an imperfect birth

and a malformed evolution.

All of our planets go around the sun

in the same direction that the sun is spinning.

This is the same direction that the clouds

within our original nebula began to rotate.

Six planets spin around their poles in the same direction.

For them, the sun rises in the east

and sets in the west.

Yet two planets spin the opposite direction.

For Venus and Uranus, the sun rises in the west

and sets in the east.

Uranus not only has a retrograde spin,

it rolls on its side like a bowling ball.

At Neptune, the icy moon Triton orbits backwards,

opposite from the direction of Neptune's spin.

Do these planets spin backwards

because they were rocked by titanic collisions in the past?

So we see evidence in the architecture of the solar system

for not only collisions, like Earth's moon

and the fact that Venus is rotating in the wrong direction

and Uranus is on its side, and so forth,

all these things are attribute to collisions.

We also see in the Asteroid Belt and in the Kuiper Belt --

the outer asteroid belt, if you like --

we see that the orbits of these things

look like they've been disturbed.

(dramatic music)

Closer to home, our own Earth

has an inexplicable 23-and-a-half degree tilt.

Its spin axis is radically misaligned

from its magnetic pole.

And our moon is comparatively large for a planet our size.

Now, a new theory may be able to explain

many of these oddities.

It is called the grand tack hypothesis.

(intriguing music)

Four-and-a-half to five billion years ago,

a gas giant planet arose inside a primordial disc

of gas and dust.

Jupiter didn't just form where it is,

but formed then moved inward towards the sun.

As it spirals toward the sun,

Jupiter herds asteroids and rubble.

Jupiter's natural tendency is to drift in slowly

through this debris field that it's traveling around.

The inner solar system is also thick

with gas and dust.

The birth of our planetary system is well underway.

Numerous worlds are born in this region,

including the Earth.

Primordial skies are ruled by chaos.

Jupiter's approach destabilizes these planets.

Their orbits decay into wildly swinging ellipses.

Some are tossed out.

Others fall into the sun.

Their numbers are unknowable.

These are ghost worlds from a bygone age.

Jupiter just causes all heck to break out

in the solar system, and all that debris

in the outer solar system gets flung inward

towards the inner solar system.

And it was a busy time in the very early acts

of our system, and things were colliding

with each other all the time.

And then it stopped.

Jupiter's invasion of the inner solar system

is mysteriously halted.

The planet makes turn;

or, in sailor's parlance, a grand tack.

(dramatic music)

Lurking behind Jupiter is a second gas giant.

Saturn.

So as Jupiter was migrating inward,

Saturn was following it and growing.

And as Saturn was growing it came to have a size

that it had a gravitational impact on Jupiter,

which became more important than the gravitational

interaction between the gaseous disc and Jupiter.

They reversed direction.

In a sense you can think of Saturn and Jupiter

feeding off one another and moving back out.

Gravity tugs each passing planet.

This transfers orbital energy from one world to another.

All you have to do is exert a very subtle, little,

periodic force at the right time,

and you can have amazing changes in the motion

of the object you're pushing on.

Once enough energy has been transferred,

the planets synchronize their orbits.

They are said to be in a resonance.

For planets, resonances are achieved and maintained

through the mutual push and pull of gravity

through the fabric of space.

They're a key factor in the continuing evolution

of our solar system.

This effect can be duplicated in the lab.

We have 10 metronomes here,

set up on a swinging platform.

They have little weights on pendulums here,

but they've all been set at the same frequency.

And I'm gonna try to start them as best I can

completely out of phase, randomly,

so their oscillation is going to transfer energy

by the swinging of this plastic sheet

to the other metronomes that are out of phase

to get into phase with the majority.

(metronome clicking)

It's inevitable that there's going to be some majority group

which starts out swinging more or less together,

and they are eventually gonna win out.

Just doing this as randomly as I can.

(metronomes clicking)

Aha!

Maybe in this corner I see four

that are pretty closely synced.

Yes, now it's more like five.

Six.

Now you have one that's almost completely out of phase.

Every time the majority hits a beat,

it's going to give a little impulse,

a little push to the platform, and that push is transferred

to the metronome that's not in sync.

Because the platform, you can actually see this platform

vibrating back and forth in sync

with the majority of these metronomes.

Working on this one.

So we're pretty close to resonance right now.

As the metronomes achieve resonance,

it's important to notice how the platform shakes.

Now, it doesn't have to just be the force

that's being transmitted by the plastic platform.

It could be a force at a great distance,

for example the force of gravity.

Just the small, little, seemingly insignificant pushes

building up over multiple cycles

can have dramatic energy transfer.

That is how planets do it.

Like the metronomes,

the two gas giants form a resonance.

We don't know exactly how long that took.

But we have some fiducial marks for timing

in the solar system formation,

and that means that the grand tack

had to have occurred relatively quickly,

talking hundreds of thousands of year, perhaps,

to a million years.

The two planets retreat

until they reach their current positions.

The sequence of planets as we find them today

is based on Jupiter and Saturn's orbits.

The planets eventually achieve a sequencing

that many students learn through a mnemonic,

such as "my very educated mother

"just served us nine pizzas."

Was there a time when the mnemonic was scrambled?

Seems very likely the answer is yes,

not to mention a lot of additional letters

were probably in their as well.

Jupiter's menace of the inner solar system

is finally over.

If Saturn had not formed at the right time

and the sufficient size,

Jupiter would have continued migrating in,

throwing out objects, unfortunate objects

in the inner solar system, and ending up

very close to the sun, where it would stay.

Yes, the Earth would be gone.

Or the Earth would have had a terrifying encounter

with Jupiter, and would have had its orbit

changed dramatically to gosh knows what.

There is mounting evidence

that Earth had a terrifying visitor.

Not Jupiter, but another world:

Theia.

Theia was essentially another proto-Earth,

where there were a number of these object flying around,

and Theia took time to grow, just like the Earth did.

They probably had rather similar histories.

And for one reason or another,

the orbit of Theia was perturbed

such that it collided with the Earth.

(dramatic music)

Theia was this planetary body

that was roughly the size of Mars.

And through this collision,

a lot of particles were ejected.

Probably completely destroyed.

We don't really know how much of Theia was preserved.

Earth is rocked off its axis.

Its surface liquefied.

Chunks of Earth's mantle are shoved into space.

As a planetary body, Theia ceases to exist.

Its remains are absorbed by the Earth

and intermingled with the debris field.

A new planetary body is formed:

the moon.

Further evidence is found in rock samples

from the Apollo moon landings.

This is a sample from the moon, if you can see it.

This was collected by the Apollo 15 mission.

Moon rocks contain isotopes

identical to those found on Earth.

When the moon arose, it is first covered with a magma ocean.

People imagine the lunar magma ocean

to be like this magmatic chamber I am describing,

but at the surface and covering a whole planet.

I see it as this ocean, like the Pacific,

but this has to be completely magmatic orange

and probably floating around and probably moving.

Conditions in the lunar magma

are as hellish as we can imagine.

But a microscopic treasure forms in the magma:

crystal zircons.

These are the same gemstones used in jewelry,

but the zircons in the Apollo moon rocks

yield a different treasure.

So those zircon are very important

because we know they crystallized

in this lunar magma ocean.

We know roughly when they crystallized

in the lunar magma ocean.

So they are one of these old piece of the moon

that we are looking for, one of these old piece

that we can use to date the origin of the moon.

Zircons not only give the age of the moon,

they set a specific date for the collision.

The age of the moon is 4.51 billion years old.

4.51 billion years ago,

Theia becomes part of Earth and forms the moon.

But the story is not over.

Space probes measure the moon

slipping 3.8 centimeters further away each year.

One day, the moon will break free.

When that day comes, there will be no more tides,

no more romantic moonlit nights.

Could planetary orbits be inherently unstable?

Could the chaos of planetary migration return?

Haute-Provence Observatory, France.

It's here that a discovery from a faraway star

gives one of the biggest revelations

about our own solar system.

The story begins when Swiss astronomers, Didiet Queloz

and Michel Mayor, notice something unusual

about a star 50 light-years away,

in the constellation of Pegasus.

Everything about this star is ordinary,

a main sequence midlife yellow dwarf

just like our sun, but with one strange difference.

The star at Pegasus 51 is rocking back and forth.

It's a weird anomaly astronomers have never seen before.

They check their instruments.

Everything is working, including their new spectrograph,

a device that splits the starlight from Pegasus

into rainbow colors.

Hidden inside the colors are patterns of lines.

By tracking the day-to-day movement of these lines,

astronomers make a startling discovery:

Pegasus 51 has a planet.

But no one has ever seen a world like this.

It's half the mass of Jupiter,

yet it's extremely close to its star;

nine times closer than Mercury is to the sun.

The planet at 51 Pegasus must be inside the corona,

boiling at temperatures over a million degrees Fahrenheit.

Soon, another planet is found around another star.

And then another.

And another.

Astronomers have now confirmed over 3,700 exoplanets

beyond our solar system, nearly all of them

are Jupiter-class planets grazing their host star.

They're a new, previously unknown type

called hot Jupiters.

They're so numerous, hot Jupiters challenge theories

about the origins of planetary systems.

It's very difficult to make a planet close to the star,

because there isn't enough mass to build a giant planet

very close to the star, and there's gravitational

frustrations for trying to build a planet

very close to the star.

Astronomy is shaken with a new revelation.

Planets do not stay put where they're born.

When they're big enough, they migrate.

It's a process called planetary migration.

And yet, our own solar system has no hot Jupiter.

Astronomers realize the Jupiter in our solar system

was once on the move as well.

But its migration was halted by a resonance with Saturn.

Pegasus 51 shows where a planet lands

when its migration is not blocked.

The strongest evidence for the grand tack hypothesis

comes not from our solar system

but from exo-worlds charted around other stars.

Planetary migration is a universal concept.

We see it, evidence of it, out there

in extra-solar planets, other solar systems.

There's no reason why planet migration

shouldn't have operated in our own solar system.

(intense music)

The New Horizons probe finds evidence

for roving planets within our solar system.

While charting ancient craters on Pluto

and on the surfaces of its moons,

the science team discovered many craters are the same age.

This suggests that they were formed by a single event.

Even way out here, tiny Pluto was smashed

by a wandering planet.

The Pluto catastrophe may be related

to other planetary migrations in the outer solar system.

It was noticed that the exact orbits of the giant planets,

particularly the outer giant planets,

the icy planet, Uranus and Neptune,

can be explained by their migration outward.

There's a point in time about 3.8 billion years ago,

where Uranus and Neptune trade.

And it's because of what some of these mean motion resonance

interactions that we were talking about earlier.

So this mean motion resonance

involving Jupiter and Saturn and so forth

just causes all heck to break out in the solar system.

Uranus and Neptune all of a sudden

at 3.8 billion years, they literally swap places

and all that debris in the outer solar system

gets flung inward towards the inner solar system.

The disruption in the outer solar system

causes a new wave of violence.

Astronomers call this epic the late heavy bombardment.

(dramatic music)

Much of the cratering we see on our moon today

is from this period.

It may be possible that swapping orbits with Uranus

is how Neptune got its moon.

(ominous music)

Today, the epic of planetary migration appears to be over.

The solar system seems stable.

But what does the future hold?

Computer simulations reveal what may be

the greatest threat to the solar system

in over four billion years.

It comes from a very special relationship

between Jupiter and Mercury.

Mercury's orbit is slowly perturbed

thanks to a subtle but constant

gravitational nudge from Jupiter.

A new resonance like the one between Saturn and Jupiter

that saved the inner solar system

is forming between Jupiter and Mercury.

In 2001, computer models for the solar system

were run 2,500 times.

They plug in the positions and the orbits

of all the planets in the solar system in a computer.

And they just let it run through time,

through millions of years, hundreds of millions of year,

billions of years, to find out whether or not

these orbits are stable.

Change the location of one planet, say Mercury,

by one millimeter, and you find that that change

will give completely different predictions

about where everything is going to be

millions of years in the future.

To see how quickly and easily things can change

we have only to go back to our metronomes.

All I have to do is stop this platform

from moving so that it cannot swing freely.

Now there's no way for the metronomes

to influence each other.

They're good metronomes, but they're not perfect;

they can't be going at exactly the same frequency

and the same phase, and they're just going to fall apart,

because they have no way of forcing the others

to go to resonance.

The same applies to planets.

Any small change can disrupt their harmony

or restore it.

Now that the platform is free to swing again,

it can again transfer energy and sync them up

just like we saw the first time.

Scientists want to understand the consequences

of a destabilized planet Mercury.

In one case, Mercury leaves the solar system.

The loss of its gravitational pull

disrupts the balance of both Venus and the Earth.

Earth and Venus swap orbits.

The super-heated atmosphere of Venus cools.

Massive rains pour onto the face of the desert planet.

Oceans arise.

The land cools.

And the air thins.

Even the remains of an ancient visitor begin to cool off.

Venus becomes like Earth.

But the reverse happens to the Earth.

As Earth settles into Venus's orbit, temperatures rise.

The air becomes unbreathable.

Glaciers melt.

Oceans boil.

The sun looms larger in the sky,

only to be obscured by a thickening cloud cover.

Suffering will be great,

but brief.

The entire four-billion-year pageant of life

is cooked in a matter of days.

Complete and utter destruction

and elimination of all life on Earth.

I'm not just talking about higher life,

I'm not just talking about civilization,

but everything, a sterilization of the planet

is something that I would want to think about a bit.

Sterilized, uninhabitable, and quiet.

There is another possibility, equally dark and apocalyptic.

A runaway Mercury is deflected by the gravity of Venus

and barrels toward the Earth.

It may be a frightening encore

to the opening act of our solar system.

The equilibrium of the ages is over.

If this scenario is correct, Mercury crashes into the Earth

just as Theia did four billion years ago.

The question is: could it happen today?

(intense music)

Or is it a fate far away in the future,

at a time when mankind itself is but a distant memory?

The problem is that, even with a perfect computer

that understands all of the laws of motion perfectly,

not just gravity but all the other subtle forces

that go into it, you cannot give it accurate enough

initial information about the locations,

the masses, the sizes, the speeds

of all of the objects in the solar system.

Odds are we may never see such a calamity.

And yet, among the billions of stars in our galaxy,

how many worlds are on the move?

How many will share this fate?

(ominous music)