The Planets and Beyond

Imagine an alien world...

A scorched, barren landscape

hot enough to boil water,

where mysterious holes in the battle-scarred surface

reveal a labyrinth of winding tunnels,

each wide enough to house a city.

This world is dead.

Yet it breathes life into its nearest planetary neighbor.

This extraterrestrial place is closer than you might imagine.

This is our moon.

Have you ever dreamed about traveling out of this world?

My name is Mike Massimino,

and I turned that fantasy into a reality

when I space-walked to repair the Hubble space telescope.

Nice job, Mike.

Now, as we expand our knowledge of our solar system

from Mercury to the cosmic unknowns,

I want to take you on a personal journey to...

captions paid for by discovery communications

welcome to "the planets." I'm Mike Massimino.

My love of space began in 1969,

when I saw Neil Armstrong walk on the Moon.

Since then, I've been obsessed with outer space,

especially the Moon.

But where did this gigantic piece of rock

that orbits the Earth come from?

My childhood heroes, the Apollo astronauts,

provided essential clues.

Look at our solar system from an alien visitor's perspective.

What would surprise you?

What would make you look twice?

The surprising answer?

Our oversized moon.

If you look at our moon and compare it

to all of the other moons in our solar system,

it's actually quite odd.

It's a giant moon. It's almost like we're

a binary planet... two objects orbiting around each other

to some degree.

Our planet-sized moon is unique in the solar system,

and this battle-scarred giant serves as our guardian angel.

Its vast gravity raises the tides that breathe life

into the Earth's oceans.

The Moon stabilizes the tilt of our planet,

regulating the climate and seasons.

Without the Moon, humans may never have evolved.

We owe everything to our moon,

yet its formation is one of the greatest

unsolved mysteries in planetary science.

There are many ideas for the origin of our supersized moon,

but they all start the same way...

4.6 billion years ago.

When the solar system first formed, it would've looked

very different from the way it looks now.

Instead of having a few planets and mostly empty space

surrounding the sun, you would've had a... a disk,

and this disk would have been thick.

It would have been composed of gas and dust and rocky bodies.

The infant sun sparks into life,

blowing away the clouds of gas that are closest to it.

The rocky fragments that are left behind clump together

over time to form planets like Earth.

But there are many more of these planets than we see today.

Giant rocks jostle for position,

crossing paths as they struggle to find stable orbits.

Is it possible that one of these planets was destined

to become our moon?

There have been a lot of ideas about

where our oversized moon came from.

One idea was just that it was wandering around

in our solar system, and we captured it.

According to the capture theory,

a wayward planet passes a little too close

to the early Earth, and our planet's immense gravity

captures hold of it.

The planet settles into orbit around the Earth,

and it becomes the Moon we see today.

This theory seemed to check all the boxes,

but scientists needed proof it was right.

If it was really true that the Moon was a captured planet,

you would expect that its constituents

would be different than the Earth.

It was something that formed in a different place than the Earth

and then got captured, so you would expect them

to be made of different stuff.

To prove the theory,

scientists needed to compare the Moon's earliest rocks

with similar examples found on Earth.

The best rock for comparison is a mineral that could've only

have formed when the newly-born moon was still molten.

On Earth, this type of rock is formed

in highly-volcanic places like Iceland.

This is what we want.

You see this anorthosite, how... how white it is.

Anorthosite forms in a different way than normal basalt.

Um, you can almost think of it, uh, like the white foam

on the head of a dark beer, sort of floating up to the top.

So in a magma environment,

this would form and then just float up to the top of a...

Of a magma sea.

Getting a sample of anorthosite from the Moon

is important because darker rocks

could've had their chemistry altered by asteroid impacts.

We really had to have a piece of... of this anorthosite rock

because we just couldn't learn about the origin of the Moon

from the dark materials.

In the 1970s, Apollo astronauts collected samples

and brought them back to Earth for comparison.

The results shocked the scientific world.

What they found was that the composition of the Moon

was almost exactly the same

as the composition of the crust of the Earth.

So this idea that the Moon was a captured planet from elsewhere

in the solar system was out.

There was no way that was true.

With the captured planet theory

blown out of the water,

planetary scientists took action.

Scientists said, "look, we've got to get together"

and figure this out."

And they went to a conference together in Kona, Hawaii,

all the top scientists in planetary science,

and they hammered out all of the leading ideas.

They locked themselves into a room together

and emerged from that meeting and said, "yes, this can work."

The new theory focused on the water contents

of the Moon rocks.

They were surprisingly dry.

Something must have heated up the Moon to unimaginable

temperatures, and we were led to think that it must be

some sort of collision.

The new theory was built around the idea

of a catastrophic collision 4.5 billion years ago.

But it wasn't an impact into the Moon.

It was an impact into the Earth.

There was a crazy idea that the Earth formed,

and, while it was still young,

another planet-sized object,

something about the size of Mars,

came in and hit the Earth...

...blew off a huge amount of material,

which then coalesced and formed the Moon.

This was a pretty cool idea.

It was groundbreaking, I guess, literally.

Scientists name the Mars-sized planet Theia,

and they modeled how the impact would have played out.

4.5 billion years ago,

Theia catches the Earth with a glancing blow.

The impact throws molten debris far into space,

forming a ring of burning rock around the Earth.

If we could travel back in time and...

And somehow stand on the surface of the Earth when the Moon

was forming, it would have been amazing.

A bright ring of fire was stretching across the sky,

and here in the sky would've been a bright ball

of magma glowing,

with all kinds of shrapnel and small rocks being attracted

to that point because of the immense gravity.

This was the proto-moon, the thing that would eventually

become the Moon we have in our sky today.

Our moon forms in under a year.

Its crust is almost chemically identical to the Earth's

because they share a common origin.

This impact idea, as weird as it sounds,

actually does the best job explaining everything

that we see about the Moon.

The impact hypothesis becomes the leading theory

for the origin of our oversized moon.

But when space probes journey to the far side of the Moon,

they find something that throws the theory into chaos.

The Moon has a dark secret...

It was born with a twin.

For centuries, scientists have wanted to know more

about the glowing sphere in our night sky.

In 1959, the first photos were taken

of the far side of the Moon.

Those early photos revealed a remarkable past

and led to a radical theory...

Our moon has a secret twin.

Take a look at the Moon tonight,

and you might see a familiar face

or perhaps a rabbit,

or a tree, depending on your culture.

Myths and legends surround the patterns etched

on the surface of the Moon,

but what can these marks tell us about the riddle

of our oversized guardian?

These dark patches appear to be unique to the Moon.

Centuries ago, astronomers thought they were seeing

enormous oceans of liquid water.

We now know the dark stains are actually ancient floods

of volcanic lava.

Planetary scientist Jani Radebaugh

takes a bird's-eye view of similar features on Hawaii,

using her research kite cam.

These lava flows we're standing on are identical

to the things we would see if we were standing

on the dark patches of the Moon.

They are dark in color.

They're made of basalt, and they flowed out