Original subtitles

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."

♪♪

♪ They'll say no one can see us ♪

♪ That we're estranged and all alone ♪

♪ They believe nothing can reach us ♪

♪ And pull us out of the boundless gloom ♪

♪ They're wrong ♪

♪ They're wrong ♪

♪♪

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,

we are trying to change everything

to fit those observations.

Messenger's evidence of the volatiles on Mercury's surface...

And the unusual size of its core...

Suggest an interesting new theory.

♪♪

It's possible that Mercury's life began

as much as 100 million miles further away from the sun

than it lies today...

♪♪

In the region of space where the young Earth is also forming,

a region with scores of planetary embryos

all fighting for position.

In the chaos,

it's possible that something large pushes Mercury off orbit

and in towards the sun.

♪♪

Mercury brushes against another planetary embryo.

♪♪

Just this glancing blow could be enough

to pulverize much of its crust and mantle...

♪♪

Leaving the material behind...

Perhaps to become part of the early Venus.

Mercury, now little more than a metallic planetary core,

continues towards the sun...

but escapes total solar annihilation

when it drops into the peculiar elongated orbit

we see to this day.

♪♪

With its astonishing revelations,

Messenger is helping to completely rewrite the theories

of Mercury's formation.

Before we sent the Messenger mission to Mercury,

I think we had a very simplistic idea

of what it was going to be like.

And Mercury turns out to be a more complex place

with a more interesting and complex history

than we had previously imagined.

But, of course, all things have to come to an end,

and once we were out of fuel,

we could no longer burn our engines

to keep from crashing into Mercury.

End sequencers, chief launch detector on.

Confirming your...

♪♪

After four years of observation,

the fuel Messenger uses to correct its orbit

finally runs out.

And Messenger adds yet another crater to this tiny world,

where any prospects for life were scorched away

when it was thrown too close to the sun.

But distance doesn't necessarily protect a planet

from the sun's power.

♪♪

30 million miles beyond Mercury

lies a world that looks to be far more Earth-like,

shrouded by an unbroken blanket of cloud,

an invitation to the imagination.

Looks so close,

and so well-known even to the ancients,

but so mysterious

because we couldn't see through to the surface.

Venus has been mysterious to human beings

for a long time.

Early astronomers figured out that Venus has an atmosphere,

and they figured out

that the reason it's so bright in our sky

is because it's completely covered with clouds.

Science-fiction writers were very captivated with the idea

that it was, you know, this swappy, swampy tropical jungle.

Is it another Earth?

Is it an Earthly paradise, even,

just sort of hidden beneath that, that envelope?

♪♪

We were so sure of Venus's habitability...

That the first missions were designed

for a splash landing.

♪♪

Throughout the '60s and '70s,

the Soviet Venera program sends multiple missions to Venus.

Many fail.

But with each attempt, they learn a little more

of the extreme conditions on the planet.

♪♪

♪♪

After 20 years of trying,

Venera 13 begins its perilous descent.

♪♪

♪♪

The craft is built to withstand pressures

that would crush a car in seconds

and temperatures that would melt lead.

♪♪

On the first of March 1982,

the Soviets take the first color photograph

of the Venusian surface.

For 127 minutes, the probe collects data.

♪♪

Far from being a benign ocean world,

Venus is a vision of hell...

Where no life can survive.

But when scientists analyze the atmosphere,

they find Venus was once far more welcoming.

Although there's very little water

in the atmosphere of Venus today,

and certainly none on the, on the surface,

we have evidence that it was once wetter than it is today.

The reason we can tell that Venus has lost a lot of water

is from the isotopic composition of hydrogen

measured in its atmosphere.

That is a clue that most of the water escaped long ago.

So Venus, when it was young,

would have probably had enough water

so that it most likely had liquid-water oceans.

♪♪

Far from the hell it is today,

the young Venus is in fact very similar to Earth.

♪♪

At just the right distance from the sun

for something wonderful to happen.

The heavens open.

♪♪

Liquid water floods the surface.

♪♪

Rivers run into shallow seas.

♪♪

Venus becomes an ocean world,

rich with the potential for life.

♪♪

As far as we know,

when Venus was a young planet,

it was very much like Earth,

with liquid-water oceans and a temperate climate,

the kind of place where life could have formed and thrived.

♪♪

Say life did arise on Venus,

which is at least plausible,

because it did on Earth during that time

in similar conditions.

What happened to it?

♪♪

Long ago, something happens

to cause Venus's hope for life to die:

a change in its parent star.

The fate of a planet's habitability is tied up

in the history of the sun...

Where the planet is in relation to the sun is key,

but also the, the age of that star.

♪♪

It's very well understood

that a star like the sun becomes brighter as it ages.

As the young sun warmed up,

the radiation balance started to shift.

Venus once may have had global oceans,

but as the sun became warmer and warmer,

the surface temperatures on Venus heated up.

♪♪

Gradually, over two billion years,

as the nuclear reactions at the sun's heart

heat it up,

the sun grows brighter.

Its increased energy output

causes temperatures on Venus to rise,

evaporating more and more water,

turning it into vapor in the air.

You've got the surface heating up,

you've got water going into steam.

Steam's a greenhouse gas,

the temperature goes up, more water turns into steam.

You can see where this is going...

The hotter it gets, the more that water evaporates,

the more water vapor in the air,

the more that radiation is blocked,

and the more it heats up.

It's a positive feedback, it runs away.

Rain evaporates long before reaching the ground.

♪♪

Venus's clouds grow thicker by the day,

until its face is forever lost to space.

What was happening beneath them is revealed in 1990...

When NASA's Magellan probe begins to explore the planet

from above.

♪♪

Equipped with cloud-penetrating radar,

Magellan is able to peer through the thick atmosphere.

And what it discovers

is a world covered in vast volcanic lava plains,

fields of small lava domes,

and in places, some of the largest shield volcanoes

seen anywhere in the solar system.

♪♪

Venus is the volcanic capital of the solar system,

with more extinct volcanoes scattered across its surface

than any other planet.

And we think many of these volcanoes were active

at the time Venus's atmosphere was being warmed by the sun.

Early on, the atmosphere of Venus

was being fed volcanic gases,

and a lot of those volcanic gases are greenhouse gases.

What's coming out of volcanoes today on Earth?

It's CO2, it's water vapor, it's methane.

Those are all really strong greenhouse gases.

So that early atmosphere of Venus was presumably

also being pumped full of these volcanic gases.

♪♪

Venus reaches a tipping point...

And a runaway greenhouse effect takes hold.

♪♪

Its oceans and all hope for life

vaporize.

Once it lost those oceans,

the CO2 built up and built up over time

and the oceans were no longer present

to suck out the CO2 out of the atmosphere.

So at that point, the runaway greenhouse just took over,

causing Venus to be the, you know,

sulfur hell that it is today.

Venus's moment in the sun sets,

its cracked surface now even hotter than Mercury,

the hottest of all the planets.

♪♪

As the sun's brightness increases,

the effects are felt

across all the terrestrial planets.

♪♪

Mars, much smaller and further out than Venus,

has its moment in the sun, too.

♪♪

With an atmosphere rich in greenhouse gases,

its rivers and seas flow freely across the surface

for hundreds of millions of years.

♪♪

But Mars, being much smaller than Venus

and with weaker gravity,

has a harder time holding on to its original atmosphere.

The molten iron core in its center

is too small to retain its heat.

♪♪

It cools and solidifies,

shutting off the smaller world's protective magnetic field.

Its intense aurora fades,

leaving it exposed to a bombardment

of high-energy particles from the warming sun...

The solar wind...

Stripping away its atmosphere,

leaving little to protect its water

from simply evaporating into space...

♪♪

The tiny traces left behind...

Frozen in patches across the planet...

Mars's chance for life to develop

extinguished by its parent star.

♪♪

♪♪

The fate of the planets

is intricately tied with that of the star that they orbit.

So for our own solar system, the sun dictates our fate.

♪♪

As our sun has changed, so has the potential for life

on our neighboring planets.

Their history shows

that habitability is a delicate balance...

♪♪

That doesn't always last.

♪♪

There is only one planet so far

that has retained its water and habitability...

And that's Earth.

What's remarkable about Earth

is the stability of those conditions,

that Earth has been able to maintain oceanic conditions

at its surface

throughout its entire history, through billions of years,

and that's what's facilitated the very rich

biological evolution of Earth.

Earth is a very special place.

It's the only place in the universe

where we know definitively that there is life.

For that to happen, you need

not just liquid water appearing on a planet,

but liquid water staying on a planet.

And that's the magic of Earth.

♪♪

Thanks to the size and geology of our planet,

the atmosphere has remained stable enough

for billions of years,

protecting the precious water

that has enabled complex life to evolve.

Life has woven itself into the fabric of the planet...

♪♪

To shape the continents and the oceans.

♪♪

So that now life itself maintains the very atmosphere

that protects our fragile ecosystems.

♪♪

But as the sun ages,

this delicate balance cannot last.

All of the planets are changing,

including our own Earth,

and as it evolves, life will have to change with it.

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.

Right now, we're at the period where Earth is getting

just the right amount of radiation.

But, over time, that radiation from the sun

is going to increase.

There will come a time that it will be so hot

that we will have a situation like poor Venus had,

where we could well have a runaway greenhouse.

♪♪

The aging sun continues to grow hotter.

♪♪

Temperatures on Earth rise...

Upsetting weather patterns,

raising great storms across the planet,

and devastating droughts.

♪♪

As plants around the world die out,

oxygen levels plummet.

Around a billion years from now,

the age of complex life on Earth

will finally draw to a close.

Earth will ultimately be like Venus and Mars, just mostly CO2.

There'll be nothing else, really, in the atmosphere.

And it will remain in that state as a sort of hot oven planet

until, ultimately, the sun,

when it uses up its hydrogen and goes into a different phase,

will become what we call a red giant.

You know, as we've seen in other stars throughout our galaxy,

throughout the universe,

our sun will continue to get hotter and hotter

over time.

At that point,

it will expand greatly to the point

where it will nearly engulf the Earth entirely.

And at that point,

the planets will, will lose their atmospheres

and they'll be just sort of swept away.

♪♪

As the sun exhausts its hydrogen fuel,

the sun's outer edge inflates.

It enters a red giant phase,

expanding millions of miles out into space.

♪♪

Mercury is the first to be engulfed.

♪♪

Then Venus's fate is sealed.

♪♪

Some models predict that Earth may barely escape

the fiery fate of its neighbors...

♪♪

Hanging on beyond the edge of the dying star with Mars.

♪♪

But the long era of the four terrestrial planets

will be over...

The lives lived on one of them

nothing more than a distant memory.

As the sun approaches its end,

the habitable zone will move outwards,

and any hope for life will move with it

into the outer edges of the solar system.

♪♪

There are places farther out from the sun

where life could exist, and, in fact,

where the potential for life could increase...

Places that may wake up to their biological potential

late in the sun's lifetime.

Out here is the realm of the gas giants.

So the places where life may find a home

would not be on the planets themselves,

but on the terrestrial moons that orbit them.

♪♪

For example, Enceladus,

which is a little icy moon around Saturn.

Or Europa, which is an icy moon around Jupiter,

or even Titan, which is a moon of Saturn.

♪♪

Saturn's moon Titan, in particular,

interesting possibility, an exciting possibility

for a... for a different kind of life

in a different kind of habitable zone.

In 1997, we set off to investigate

this potential future home for life.

T minus five, four. three, two, one...

And liftoff of the Cassini spacecraft

on a billion-mile trek to Saturn.

We have cleared the tower, T plus 20 seconds.

All systems are go.

♪♪

Cassini heads to the cold, distant reaches

of the solar system,

past Jupiter...

♪♪

Its destination nearly a billion miles away.

Its mission: to orbit Saturn and investigate its icy rings.

♪♪

♪♪

From here, it deploys a tiny probe

to explore Titan, a planet-sized moon

even bigger than Mercury.

♪♪

Surrounded by a thick, hazy atmosphere,

its surface has long remained a mystery.

The Cassini orbiter contained the Huygens probe,

designed to descend through Titan's atmosphere

and land on the surface,

the first landing of anything in the outer solar system.

So this is just ground-breaking history.

♪♪

During its descent, the Huygens camera sends back

these first astonishing glimpses of this distant moon.

♪♪

We saw bright highlands, but they had dark drainage channels.

♪♪

When you're looking at it,

so breathtaking to see this active surface.

♪♪

It opened up our eyes

to this magnificent and amazing very Earth-like world.

♪♪

Incredibly, the probe makes a soft landing

and sends back our first look

at the surface of Titan.

♪♪

It's surprisingly similar

to the landscapes on Earth.

Soon after Huygens landed,

Carrie Anderson began analyzing the data sent back

from this distant world.

When I look at this image that the Huygens probe took

when it was on Titan's surface,

I'm struck by the incredible similarities

that we have on Earth in the formation of the rocks

in flood plains and river plains on Earth.

Just like here, liquid water is flowing,

we know that similar processes are going on

and making pebbles just like this,

but the difference is the composition

is water ice so hard, frozen so solid

because of the low temperatures.

But we have the similar processes

forming rocks like this on Earth

and also forming rock-solid ice pebbles on Titan.

♪♪

The discovery of smooth, rock-like ice pebbles

suggests flowing liquid.

But at minus-300 degrees Fahrenheit,

there can be no liquid water on Titan.

So what is flowing out here in the cold?

The instruments pick up significant amounts of methane,

a flammable gas on Earth.

But the relatively high atmospheric pressure

and cold temperatures at the surface of Titan

mean that methane can exist as a liquid.

Titan could be wet

not with water, but with liquid methane.

So what we think is that the methane

probably rained out in some storms

very similar to storms on Earth.

♪♪

The shape of these very rounded-looking stones

most likely was due to probably liquid methane

pushing these ice-like rocks, raining down these channels,

and emptying into this open floodplain.

♪♪

Huygens survives for just a few hours,

but doesn't detect anything like enough liquid methane

to carve out riverbeds.

♪♪

But the probe's mother ship, Cassini,

remains active in orbit around Saturn.

♪♪

A year after Huygens landed,

Cassini passes high above Titan's poles

and sees something truly spectacular...

♪♪

Our first glimpse of liquid pooling

on another world.

Ontario Lacus,

a lake whipped by winds that erode the shoreline.

Ligeia Mare, where we have seen bubbles rising from the depths.

♪♪

And Kraken Mare, almost five times larger

than our own Lake Superior.

♪♪

In all, Cassini discovers hundreds of methane lakes.

♪♪

Few scientists think life can exist

on the surface of Titan today;

it is too cold.

But because of the presence of crucial elements,

it might be a very different story

if Titan were to warm up.

Titan today has a surface that's full

of these really juicy organic compounds,

but it may simply be too cold and therefore too dry

for anything biologically interesting

to be happening with them.

However, in the far future,

when Earth and the inner solar system

become uninhabitable,

we can imagine what might happen to Titan then.

♪♪

In the light of the aging, expanding sun,

when the far reaches of the solar system

receive more solar energy,

Titan will begin to warm.

♪♪

Its mountains of ice may shrink and melt.

And the frozen water they contain

may replace the liquid methane

as it evaporates away.

♪♪

The mountains could become oceans.

So you have this mixing, then,

of the hydrocarbons and liquid water,

and so you have this wonderful sort of nirvana

for some period of time,

where you have everything that you need for life.

♪♪

So even if Titan doesn't have life today,

and I think that's something

that we still have to search for,

there's every reason to imagine

that at some point in the future,

it could be a great place for biology.

♪♪

In a strange twist of fate,

at the end of the life of our star,

one of the last water worlds in the solar system

will be born.

♪♪

Titan will have a final brief moment in the sun.

♪♪

The story of our solar system shows us that habitability

isn't a permanent feature.

Our solar system is a dynamic place.

And our star has its own life cycle...

♪♪

Creating zones of habitability

that ebb and flow throughout its lifetime.

♪♪

Far from being unique to Earth,

there is hope for life throughout our solar system...

And, many believe, beyond.

One of the most amazing discoveries

of the past couple of decades

is that when you look up at the stars at night,

almost all of those stars host their own solar systems,

and so we know that within our own galaxy,

there are hundreds of billions of planets.

And with so many planets out there,

you can't help but think

that some of them have the right conditions for life.

♪♪

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 this universe

is full of a great number of habitable worlds.

It's a really remarkable time to be alive

and to be in this business.

And when I say "be in this business,"

everyone's in this business,

because I have yet to meet anyone who isn't interested

in that question, "Are we alone?"