All language subtitles for The Planets (2019) - S01E01 - A Moment in the Sun - The Terrestrial Planets

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 4 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 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...

...choked by a thick atmosphere.

Then Venus's neighbour - Earth.

And farthest of all - Mars.

A cold desert world.

Together, they form 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 4 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
are made out of the same stuff -

the carbon, nitrogen, oxygen, iron -

all those atoms were present
in the cloud that collapsed

to form the solar system
4.5 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 exist,

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's 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 kilometres

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
a 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,

and 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 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.

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.

Ground-lit solids have jettisoned.

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'll 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 kilometres
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 for ever.

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 kilometres
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 the 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 have had a different story to tell.

50 million kilometres 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.

You 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.

Now, 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 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...

...and temperatures that would melt lead.

On 1st March 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,

and 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,

3.5 to 4 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 millions 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.

Gases like carbon dioxide
and water vapour

in a planetary atmosphere
are transparent to visible light.

That is 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
a planet up and frying it.

Gradually, over 2 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 escaped into space...

...leaving only small traces behind,

frozen in patches across the planet,

where missions continue to search for the
first signs of extra-terrestrial life.

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

which is 1,500 kilometres across
and 9 kilometres 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 2 billion years
before it became hotter than Mercury.

And Mars lost its oceans and rivers
perhaps 3.5 billion years ago.

But unique amongst those worlds is Earth,

because it's remained pretty much
like this - liquid water on the surface -

for 4 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 of all the chance events
that had to happen over 4 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
6 and 8 billion years old,

perhaps 3 billion years older
than the Sun,

and it is now a red giant star.

It's exhausted the hydrogen fuel
in its core

and it's swollen up and cooled.

And that is what we think will happen
to the Sun

in about 5 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 a moon

orbiting around a planet
over a billion kilometres away.

So we got a camera down
onto the surface of a world

in the frozen far reaches
of the solar system.

What we 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.

And 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 would 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 warmed 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 of 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 worlds,

a 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 4 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,

you would've melted the instruments,
literally.

The other major challenge
is you had to protect the 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, 10,000 kilometres
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 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 hydrogens.

So we thought,
"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 Mercury is rotating

almost perfectly straight up,

so that there are craters
near the pole 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
of the closest planet to the Sun.

During Messenger's final orbit,
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.