NOVA

Nearest to the young sun,

four worlds form,

each endowed with ingredients for life.

There are a lot of places in the solar system

where environments could be habitable.

Four stories...

With surprising beginnings.

This was quite a surprise and immediately told us

that some of the older ideas of how Mercury formed

could not be correct.

Venus may have once had global oceans,

but as the sun became warmer and warmer,

the surface temperatures on Venus heated up.

The orbits of the planets and their current locations

have changed over the history of the solar system.

So where things are today

doesn't mean that's necessarily where they formed.

They could have and probably did move

from different locations to where they are now.

Their destinies diverging,

until only one remains hospitable.

Life as we know it depends on liquid water.

Only Earth has had that ability to retain liquid water

over its long history.

That is what's remarkable about our planet.

But for how long?

We have to realize

that as stable as we think the sun is

and as permanent as we think

the habitability of the Earth is,

it's not that way.

As our sun destroys our planet,

a mission to an outer world reveals a new hope.

I believe that we live in a very fertile universe.

And of course we won't know until we make that discovery.

But if I had to bet right now, based on what we do know,

I would place money on the idea

that, um, this universe is full

of a great number of habitable worlds.

"The Planets: Inner Worlds."

Right now, on "NOVA."

Major funding for "NOVA" is provided by the following:

For the first few million years after the sun's birth,

there are no worlds to see it rise.

Just a vast cloud of dust and gas...

Left over after the sun's formation.

Then, over tens of millions of years,

gravity draws this debris together...

To form the first planetary embryos...

That become the four rocky worlds

closest to the sun,

their dramatic stories dominated

by the changing fortunes of their parent star...

As each in its turn has become more or less hospitable.

Today,

scorched Mercury orbits too close to a pitiless sun.

Further out, overheated Venus...

Choked by a thick atmosphere.

And farthest: Mars...

A frozen, desert world.

Of the four, only on one...

Has something extraordinary emerged.

Earth is a tremendously special place,

with all of the just-right,

finely tuned ingredients for life.

Life as we know depends on liquid water.

Only Earth has had that ability to retain liquid water

over its long history.

That is what's remarkable about our planet.

Earth is the only planet that we know of that has life.

It appears to be unique in the solar system today,

but we only are seeing a small snapshot

of time and even space.

There are a lot of places in the solar system

where environments either have been,

or in the future could be, habitable.

And yet, today the sun shines on only one habitable world.

Why ours and no others?

As we've left the blue planet

and explored our sister worlds,

we've discovered something surprising.

Our rocky neighbors were not always so hostile.

Somehow, they too may have once supported

near Earth-like conditions.

So what happened?

Most surprising is Mercury.

A world of mystery.

The least explored of the rocky planets...

Until recently out of reach

because of the enormous difficulty

of navigating into orbit around a planet

so close to the sun.

Five, four, three...

Main engines start... two, one, and zero,

and liftoff of Messenger on NASA's mission to Mercury.

A planetary enigma in our inner solar system.

Now going through the sound barrier.

Following a direct route to Mercury

would be impractical.

A spacecraft would arrive with so much velocity

that it would need to haul

a prohibitive amount of heavy fuel

in order to slow down enough to enter orbit.

We just had spacecraft separation.

Ground-lit solids have jettisoned.

So Messenger controls its trajectory

by stepping from one planet to the next,

using gravity to slow itself,

spiraling inwards toward its target.

Even so, Messenger still approaches Mercury

moving so fast

that it is forced to fly past the planet three times...

Slowing on each pass...

Until, after nearly seven years of flawless navigation,

it arrives safely in orbit...

At last able to begin its true mission:

to map Mercury's surface.

That first image ever acquired from orbit around Mercury

was amazing.

When it finally came in, it looked perfect,

it looked exactly like we expected it to be.

For me, that was the real moment when I realized

that we had successfully made it into orbit,

and everything was working.

This pioneering voyage has led to a completely new idea

of how Mercury may have formed.

Mercury is the most cratered planet

in the solar system...

With many puzzling features that hint at a violent past,

like its eccentric orbit and bizarrely slow rotation.

Mercury has a very unusual orbit.

If you were on the surface of the planet,

you would actually have to go around the sun twice

to get one full Mercury solar day.

Also highly unusual

is the planet's disproportionately large core.

Mercury is basically

a big ball of metal

surrounded, like, a tiny bit of rock.

How do you end up with a planet that has so much metal inside

and then almost none on its surface?

That was a huge mystery.

And the major question is,

"Why?

"Why is it like this,

when the rest of the planets aren't?"

Mercury's odd core and strange orbit

make it unlike any other planet.

But most perplexing is its chemistry.

So very soon after we got into orbit,

we started to get chemical data back from the surface.

And we immediately got some surprises.

Zero-six forward...

The tiny probe detects volatile chemical elements

in concentrations that no one had thought possible

this close to the sun.

These are elements that go into rocks,

but that evaporate at relatively low temperatures.

The volatile elements on Mercury are surprising.

A lot of the formation theories predicted

that you shouldn't have that

on the planet closest to the sun.

Immediately told us

that some of the older ideas of how Mercury formed

could not be correct.

What Messenger discovered about Mercury

could suggest a new twist

in the story of the solar system.

Just a few million years after its formation,

the young Mercury is still seething

with the heat of its violent birth.

Slowly, a crust forms,

rich with volatile elements.

If it had been close to the sun,

these elements would have evaporated

before the rock hardened.

So what could explain

why they are so abundant on Mercury today?

The orbits of the planets and their current locations

have changed over the history of the solar system.

So where things are today

doesn't mean that's necessarily where they formed.

They could have and probably did move

from different locations to where they are now.

One possible thing

is that Mercury didn't form where it is today,

it formed much closer to the other planets,

maybe even outside of Venus or Earth,

or somewhere in between there.

If Mercury had formed near Earth

and remained

at a more comfortable distance from the sun...

Its destiny could have been very different.

But it wasn't to be.

So what turned it

into the strange, small, sun-scorched world

we see today?

We don't know the full story.

What we have right now are theories,

and as we have new observations,