Original subtitles

The night sky is ablaze with stars...

..hundreds of billions in our galaxy alone...

..many larger, brighter and more majestic than our sun.

On the scale of galaxies and stars, the planets of our solar system

are little more than grains of sand caught momentarily

in the light of the sun.

But on those motes of dust, for over four billion years,

great stories have played out unseen.

Stories of worlds born...

..and worlds lost.

Planets forged amongst the calm...

..and the chaos.

Their destinies more entwined than we ever imagined.

We know this...

..because in the last few decades...

..we've sent spacecraft to all seven...

..of the worlds beyond our own.

These are the stories that they return to Earth,

the stories of the planets.

For the first few million years after the sun formed,

there were no planets to see it rise...

..just clouds of dust and the gas...

..the leftovers from the birth of the sun.

Over tens of millions of years, the dust began to stick together...

..and form the first rocks.

Eventually, gravity assembled the rocks

to create planetary embryos...

..that in time formed the four closest planets to the sun.

Today, Mercury is the closest of all,

enduring the sun's full glare.

Further out...

..lies Venus...

..chocked by a thick atmosphere.

Then Venus's neighbour, Earth.

And farthest of all, Mars...

..a cold desert world.

Together, they formed the only rocky so-called terrestrial planets

in the solar system.

And of the four...

..one is unique.

Just look at this...

..and listen to it.

This is what a planet looks and sounds like

after four billion years of evolution by natural selection.

There is nowhere else in the solar system

that looks and sounds like this,

which is interesting, when you think about it,

because all the planets and moons and are made out of the same stuff -

they're carbon, nitrogen, oxygen, iron.

All those atoms were present in the cloud that collapsed

to form the solar system, four-and-a-half billion years ago.

And yet Earth appears to be exceptional -

a lone living planet in an otherwise desolate solar system.

So what is it that makes this place so special?

Is it fate? Is it chance?

These are important questions

because Earth is the only place we know of

where the most complex phenomena in the universe exists,

the thing that brings meaning to the universe - life.

Earth is a special world in our solar system

and perhaps even for thousands of light years beyond.

Our world certainly has unique properties.

It's the right size and distance from the sun

to have retained an atmosphere that's protected its oceans

of life-giving water for billions of years.

But as we've left the blue planet,

and explored our sister worlds...

..we've discovered that each appears to have had a moment

when it enjoyed almost Earth-like conditions.

Every one of our rocky neighbours has a story of what might have been.

Mercury is a small, tortured world.

More than any other planet,

it's endured the unflinching glare of the sun for billions of years.

Mercury is a world of mystery and apparent contradictions.

It's in quite an elliptical orbit,

which means it can be as far away from the sun as 70 million km,

but as close as 46 million.

That means that temperatures at midday

can rise to 430 degrees Celsius on the surface,

but, at night, because it's a small planet and it's got no atmosphere,

temperatures fall to minus 170 degrees.

It's also locked into what's called the spin-orbit resonance,

which means the planet spins precisely three times on its axis

for every two orbits

and that in turn means that its day is twice as long as its year.

That means that I could walk over the surface like this

about 2mph

and keep the sun at the same point in the sky.

I could stroll in eternal twilight.

Mercury is the least explored of the inner rocky worlds...

..because getting to a planet in such a strange oval-shaped orbit,

so close to the sun...

..is a tremendous challenge.

Five, four, three,

main engine start, two, one and zero.

And lift off of Messenger on Nasa's mission to Mercury...

..a planetary enigma in our inner solar system.

Now going through the sound barrier.

A direct route to Mercury is impractical.

Now going through the period of maximum dynamic pressure.

A spacecraft would arrive with so much speed

that it would need vast amounts of fuel to slow down

and enter orbit around Mercury.

We just had spacecraft separation.

So Messenger controlled its trajectory

by stepping from one planet to the next,

using gravity to slow itself,

spiralling inwards towards its target.

Even so, Messenger approached Mercury at such high speed

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

..slowing on each pass...

..until, after almost seven years of flawless navigation,

it arrived safely in orbit.

Messenger set about its mission to map Mercury's surface...

..and began revealing the secrets of the most cratered planet

in the solar system in exquisite new detail.

Messenger was able to do much more

than just take images of Mercury's surface.

By tracking radio signals emitted by the spacecraft,

we're able to see very slight changes in the orbital path

around Mercury, as seen from Earth,

and that allows us to map out Mercury's gravitational field.

There are also instruments that allow us

to see how the planet wobbles around

as it spins on its axis.

And putting all these measurements together

allows us to take a cross section through the planet,

to see what it's made of.

And when we do that, we find something very strange.

Mercury's core extends out about 85%

from the centre of the planet to the surface.

It's almost entirely an exposed planetary core.

It's as if the rocks of the surface were smashed away

and removed at some point in its past.

And there was more.

The tiny probe began detecting chemical elements in concentrations

that no-one had thought possible this close to the sun.

The discovery of relatively large concentrations of elements

like sulphur and potassium on Mercury's surface

was a huge surprise.

If you think back to the time when the planets were forming,

you don't expect high concentrations of those elements close to the sun,

where Mercury orbits today, because they're so-called volatile elements.

They boil away easily,

so you only find high concentrations further out,

in the colder reaches of the solar system.

So Mercury is an enigma

and discoveries like these have forced us to completely rethink

our theories about the formation of the planet.

Just a few million years after its formation,

Mercury was still seething with the heat of its violent birth.

Slowly, it cooled and a crust formed.

Over time, the crust became enriched

in the volatile elements that were escaping Mercury's interior.

But this could only happen if Mercury started out

not in the position we see it today...

..but much further out.

We now think Mercury was born perhaps 170 million km

further away, close to the orbit of Mars...

..a place where, if it had stayed,

its destiny could have been very different.

But it wasn't to be.

The young planetary embryo was ripped from its promising position

long before it could mature.

Today, it's hard to imagine the planets

in orbits other than the ones we see in the night sky.

They feel eternal, permanent.

It's natural to think of the solar system

as a piece of celestial clockwork, almost like a Swiss watch.

So if we knew where all the planets were at some point in time,

let's say today, then we could imagine calculating

exactly where they're going to be at any point in time.

Now, that is true if there's only one planet and one star.

So imagine that's the sun and this is Mercury.

Now, we know the gravitational force between Mercury and the sun.

And, indeed, if that's all there is,

then we can calculate its orbit around the sun

with essentially infinite precision.

But add in one more planet, let's say Jupiter over there.

Now there's a gravitational force between all three of these objects

and it turns out that, even in principle,

it is not possible to calculate exactly

where they're all going to be in the future

or where they were at some point in the past.

This means that any uncertainty, even of a few metres

in our knowledge of the position of the planets,

can lead to radically different predictions.

And that's because the system itself, the orbits of the planets,

are not stable over very long timescales.

So planets don't necessarily remain in the same orbits forever.

And the evidence we've gathered

from the volatiles on Mercury's surface,

and the unusual size of its core,

suggests that this may have been what happened.

If Mercury began its life

170 million km further away from the sun...

..then it would have been in a region of space

where the young Mars was also forming.

This region was full of scores of planetary embryos,

all fighting for position.

Amongst the chaos, something large kicked Mercury inwards

towards the sun.

Mercury collided with another embryo.

A glancing blow saw much of its crust

and mantle lost to space.

Much of this material remained behind,

perhaps helping to form the early Venus.

If this theory is correct, then Mercury,

now little more than a planetary core, continued towards the sun

and ended up in the peculiar elliptical orbit we see today.

The idea that Mercury's outer layers were stripped away

in some violent collision many billions of years ago

is a superficially attractive one,

but the theory does have problems.

Any collision violent enough to do that heats up the planet

and that boils away the volatiles.

So you have to think of a very specific kind of collision,

or perhaps even multiple more delicate collisions,

in order to fit the data.

So I think it's fair to say that the precise nature

of Mercury's formation is still one of the great unsolved mysteries

in planetary science.

After four years of observation,

and its discoveries that hint at Mercury's turbulent past,

Messenger finally ran out of fuel...

..and added yet another crater to this tiny world

that just perhaps could've had a different story to tell.

50 million km beyond Mercury,

shrouded by an unbroken blanket of cloud...

..lies a world which, at first sight,

has the potential to be far more Earth-like.

See that bright point of light out there in the evening sky?

That's Venus.

It's so bright because it's quite a large planet,

about the same size as the Earth. It's not too far away.

But, in particular,

because it's shrouded in highly reflective clouds.

That's the frustrating but also tantalising thing about Venus.

Even through a big telescope, when you look at it,

it is featureless. You never see the surface.

And that meant that, even until the 1950s,

astronomers speculated that it might be a living world,

with jungles and forests and rivers and oceans.

So much so, in fact, that when we first sent a spacecraft to land

on the surface of Venus, we prepared for a splash landing.

Throughout the 1960s and '70s,

the Soviet's Venera programme sent multiple missions to explore Venus.

Many failed...

..but, with each attempt, we learned a little more

of the extreme conditions on the planet.

After 20 years of trying, Venera 13 began its perilous descent.

The craft was prepared to withstand pressures

that could crush a car in seconds...

..in temperatures that would melt lead.

On March 1st 1982...

..the Soviets took the first full colour picture...

..of the Venusian surface.

Even under the most extreme of conditions,

the probe sent its precious data home to Earth...

..until, 127 minutes after touchdown, it finally succumbed.

Far from a benign ocean world...

..Venus is a vision of hell...

..where no life can survive.

So where did it all go wrong for Venus?

Well, that is a good question and it's an important one.

It's been said that we won't fully understand the Earth

until we understand Venus

and that's because the planets are so similar.

Venus is the same size as the Earth.

It's the same composition, as far as we know.

And although it's closer to the sun, it's not as close as Mercury.

So why is it that one world remained heaven

whilst the other became hell?

Counterintuitively, the surface temperatures today on Venus

are hotter than those on Mercury...

..and the story of Venus's climate is further complicated

by the fact that, over the lifetime of the planet,

the sun itself has been evolving.

As the sun gets older, the star burns hotter and hotter and hotter.

That means that, in the past, when the sun was younger,

it must have been cooler.

It's called the faint young sun

and that has a big impact on the planets.

At the time, when life was just about beginning on the Earth,

three-and-a-half to four billion years ago,

the sun was fainter and that means that Venus was cooler.

In fact, temperatures on Venus at that time

would have been like a pleasant spring day here on Earth.

Within a few million years of its formation,

the surface of Venus had cooled.

The planet now found itself at just the right distance

from the faint young sun for Venus to experience a sight

familiar to us here on Earth.

The heavens opened.

Great torrents flooded the surface.

Rivers of water flowed.

Venus became an ocean world.

The planet's atmosphere allowed it to hold on to the oceans...

..by acting as a blanket, keeping the surface temperate...

..thanks to the greenhouse effect.

The greenhouse effect is pretty simple physics.

The gases like carbon dioxide, water vapour and the planetary atmosphere

are transparent to visible light

and it's obvious because there's a source of visible light,

the sun, and I can see it.

So that radiation falls onto the surface of the planet

and it heats it up. The rocks then re-radiate that

out into the atmosphere again,

but, this time, not as visible light, but as infrared,

which my eyes can't see.

Now, carbon dioxide and water vapour absorb infrared,

and so they trap that energy and the planet heats up.

Now, that's not necessarily a bad thing.

The Earth would be at an average temperature

of around minus 18 degrees Celsius without the greenhouse effect,

but there is a thin line between heating the planet up and frying it.

Gradually, over two billion years, the young sun grew brighter.

Temperatures began to rise, lifting more and more water vapour

into the atmosphere.

The greenhouse effect grew more intense.

Rain evaporated long before reaching the ground.

Venus had reached a tipping point.

A runaway greenhouse effect had taken hold.

Venus's moment in the sun was over.

Its cracked surface today is even hotter than Mercury's,

making Venus the hottest of all the planets.

As the young sun's brightness continued to increase,

the effects were felt across all the terrestrial planets.

Mars, much further out than Venus, enjoyed its moment in the sun too.

With an atmosphere rich in greenhouse gases,

rivers flowed across its surface for hundreds of millions of years.

But Mars, being smaller than Venus, couldn't hold on to its atmosphere.

Much of its water evaporated and...

..and escaped into space...

..leaving only small traces behind,

frozen in patches across the planet...

..where missions continue to search

for the first signs of extraterrestrial life.

There's a crater on Mars called the Hellas Basin,

which is 1,500 km across and 9km deep.

That means you could put Everest on the floor

and the summit would not reach the rim.

The air pressure is so high down there

that liquid water can exist.

So I suppose it's not impossible to imagine microbes

coming up from deep below the surface to bask in the midday sun

before disappearing back down below again

to survive the cold of the Martian night.

But if life does exist out there,

it will certainly only be simple life.

There will be nothing anywhere near as complex as you or me,

or even this plant.

The story of the solar system is, in a sense,

a story of instability and constant change,

at least for the inner rocky worlds.

Mercury has changed its position radically.

Its orbit now takes it close to the searing heat of the sun.

Venus probably had water on its surface

for around two billion years before it became hotter than Mercury.

And Mars lost its oceans and rivers

perhaps three-and-a-half billion years ago.

But unique amongst those worlds is Earth

because it's remained pretty much like this,

liquid water on the surface, for four billion years,

and that has allowed complex carbon chemistry to develop.

Today, our planet is dominated by life.

It's in every nook and cranny. I mean, look at this place.

This is a volcano in the middle of the Atlantic Ocean

and it is literally teeming with life.

And think about all the chance events that had to happen

over four billion years

just to produce the little creatures in this rock pool.

Life has woven itself into the fabric of the planet.

It's an integral part of every continent and every ocean.

It plays a crucial role

in maintaining the balance of our atmosphere...

..that keeps our planet temperate.

Of all the terrestrial planets,

Earth has enjoyed the longest moment of them all...

..but it can't last.

Earth will ultimately follow the fate of the other rocky planets

because, even though we don't feel it day-to-day,

the sun's ageing process is relentless.

We can say with confidence what's going to happen to the sun

towards the end of its life,

partly because we understand physics

and the nuclear physics of what happens inside the cores of stars,

but also because the life cycle of stars

is written across the night sky.

Take that bright star there, for example.

It's called Arcturus.

It's around the mass of the sun, perhaps a little bit heavier,

but it's between six and eight billion years old,

perhaps three billion years older than the sun -

and it is now a red giant star.

It's exhausted the hydrogen fuelled in its core,

and it's swollen up and cooled.

And that is what we think will happen to the sun

in about five billion years' time.

As the sun exhausts its hydrogen fuel in the core,

its outer edge will inflate.

It will enter a red giant phase,

expanding millions of kilometres out into space.

Mercury will be the first to be engulfed.

Then Venus's fate will be sealed.

Earth may just escape the fiery fate of its neighbours...

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

The era of the four terrestrial planets will be over.

The lives lived on the surface of one of them

nothing more than a distant memory.

But that's not quite the end of the story.

Right at the end of the sun's life, something wonderful will happen.

A collection of icy worlds that have lain dormant

for the entire history of the solar system will awake.

These are the worlds that orbit the outer planets,

the moons of Jupiter and Saturn.

These distant worlds that circle the outer gas giants

will begin to warm...

..like Saturn's moon Enceladus...

..or Jupiter's moon Europa.

Amongst all these moons, there is one above all others

that we think perhaps has the best chance

of becoming a place that we'd recognise.

Way out in the cold, distant reaches of the solar system...

..past Jupiter...

..around the icy-ringed planet Saturn,

orbits a gem...

And the Cassini spacecraft is on its way to Saturn.

..Titan...

..a planet-sized moon bigger than Mercury...

..surrounded by a thick atmosphere of nitrogen and methane...

..with a surface that has long remained a mystery.

The Huygens probe was our first chance to explore

beneath the clouds...

..and its camera sent back these first glimpses of the distant moon.

Wonderfully, the craft made a soft landing

and continued to beam back what it saw.

This is a remarkable photograph and, as is always the case in science,

the more you know about it, the more wonderful it gets.

This is a photograph from the surface of the moon

orbiting around a planet over a billion kilometres away,

so we've got a camera down onto the surface of a world

in the frozen far reaches of the solar system.

What you see here is something that looks like a flood plain,

or a riverbed, very much like this, actually.

And we can say that this is a riverbed or a flood plain

because these rocks on the surface...

..look like this.

They've been smoothed and eroded by flowing liquid.

We know these are in fact boulders of frozen water.

They're frozen solid because the temperature on the surface

of this moon is minus 180 degrees Celsius.

That raises an interesting question.

If it's so cold...

..then what was the flowing liquid?

Huygens detected significant amounts of methane...

..a flammable gas on Earth.

But the relatively high atmospheric pressure

and cold temperatures at the surface of Titan

means that this methane exists as a liquid.

Titan could be wet...

..not with water, but with liquid methane...

..driving rock-like chunks of ice down mountain channels

and out into open flood plains.

Huygens survived for just a few hours,

but didn't detect any trace

of liquid methane at its landing site.

But the probe's mother ship Cassini remained in orbit around Saturn.

A year after Huygens landed,

Cassini again flew high above Titan's North Pole

and discovered something seen nowhere else in the solar system

beyond Earth.

Liquid pooling into not just one, but scores of great lakes.

Cassini discovered lakes...

..of liquid methane.

Earth has a strange cold twin.

In some ways, you could just imagine floating in a boat on those lakes

and it'll look something like this,

except this would be liquid methane gas

and those mountains there would be mountains of frozen water ice

as hard as rock.

What's also fascinating, and in fact tantalising,

is that Titan has a complex chemistry

and that chemistry is carbon chemistry - the chemistry of life.

So we found molecules like hydrogen cyanide,

which are the building blocks of amino acids.

We found molecules called vinyl cyanides,

which chemists and biologists speculate

could form some sort of cell membranes.

And so all the ingredients for life are present on Titan.

Now, very few scientists think there will be life on Titan today.

It is, after all, minus 180 degrees Celsius at the surface,

but, because of the presence of all those ingredients,

it might be a very different story if you warm Titan up.

In the light of the old expanding sun...

..the far reaches of the solar system

will receive more solar energy.

Titan's atmosphere will begin to warm.

Mountains of ice will shrink and melt as temperatures rise...

..the frozen water they contain replacing the liquid methane.

Mountains will become oceans of water.

In a strange twist of fate, at the end the life of the sun...

..the solar system's last ocean world

will wake up to its own biological possibilities.

This distant moon will enjoy its brief moment in the sun.

It's easy to think of habitability as a permanent feature

of a world's defining characteristic, if you like.

So the Earth is a living planet because it's in the Goldilocks zone

around the sun - not too close and not too far away.

But things are more complicated than that.

Solar systems are dynamic places.

Planetary orbits can change and stars can vary in brightness,

so planets that were once heaven can become hell.

We now understand that the Earth has been a fortunate world,

an oasis of calm in an ever-changing solar system,

that's maintained a stable climate, perhaps against the odds,

for the four billion years it took complex living things to evolve.

We don't know how many planets like Earth there are

out there amongst the stars,

places where the ingredients of solar systems

have assembled themselves into structures

that can dream of other worlds,

but we have to take the possibility very seriously

that there might be few.

And that would make Earth, and us, extremely rare and precious.

Mercury is the most enigmatic of all the planets and difficult to study.

The real problem is that it's really hot.

It's the planet closest to the sun

and obviously that makes it challenging.

So you have to protect the spacecraft

from the heat from the sun, and the way Messenger addressed this

was to put the whole spacecraft behind a giant ceramic sunshade.

It had this sunshade. If it didn't face the sun,

it would have melted the instruments, literally.

The other major challenge is, you have to protect a spacecraft

from the heat reflected from the planet

and the way we dealt with that

was to be in an extremely elliptical orbit,

where we flew in very close over the North Pole and took observations.

The instruments would heat up and then we'd fly like 10,000km

farther out from the planet...

..while we cooled off, and, in this way,

heat up, cool down and kept everything below

the danger temperatures, where instruments could be damaged.

I was in charge of the camera team.

I don't think I slept much of the night before.

I was really anxious to get that first image back.

When that image came back,

we just started pointing at all the features

that had never been seen before and saying, "Look at this, look at that."

Over time, the close-up flights of Mercury's North Pole

allowed the team to peer deep into the shadows

of one particular crater.

We realised that we could actually design a way

to take a picture using a very long exposure

to see inside these dark craters.

Messenger was designed with instruments

that could specifically look at their reflectance

and also measure hydrogen.

So we saw, "OK, there's a lot of hydrogen there,"

and there was like this fantastic case built for these ice deposits.

Incredibly, Messenger detected hundreds of billions

of tonnes of frozen water ice...

..scattered in the permanent shadows of the polar craters.

The fact that water ice can survive for a very long period of time

is a reflection of the fact that the Mercury is rotating

almost perfectly straight up, so that there are craters near the pool

that are so deep and completely shaded

from sunlight ever hitting them.

So ice could be stable in those polar regions

that are permanently shadowed for billions of years.

Messenger had confirmed the existence

of an essential ingredient for life...

..on the surface at the closest planet to the sun.

During Messenger's final orbits, all the fuel was depleted -

there was nothing that we could do -

and every orbit just brought it slightly closer to the planet.

One of our engineers realised

that we still had some helium on board the spacecraft

and, if you blew helium out the back it would work like fuel,

and that managed to extend us for a few extra weeks.

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

The last day, many of us watched the signal

as it went behind the planet and never came back

and we knew it had crashed

and it left us with a real bittersweet feeling,

because we were happy at the success,

but, of course, sad that it was over.

We all took so much pride in this amazing spacecraft

that lasted way longer than any of us had planned for it to last

and that had told us so much about Mercury

and really changed the way that we looked at this planet.

Touchdown on the red planet...

..to uncover...

..the story of Mars.

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