All language subtitles for The.Planets.2019.S01E05.Ice.Worlds.1080i.HDTV.DD2.0.H.264-CtrlHD

Lift off of Messenger on Nasa's
mission to Mercury.

Far, far away...

.. beyond Mars...

.. past the storms of Jupiter,
and the rings of Saturn...

.. conditions become very different.

Temperatures plummet,
and distances between worlds

are measured not in millions,
but in billions of kilometres.

Here lie the most mysterious
planets of them all.

Uranus.

A pale blue marble
hanging in the dark,

frozen depths of space.

And, further out...

.. the solar system's final
true planet...

.. Neptune.

Beyond, we thought we'd only ever
find tiny, lifeless worlds...

.. frozen to the core.

How wrong we were.

We live at a time when our
civilisation has unprecedented

reach, and we have a near-permanent
presence at Mars,

both in orbit and on the surface,
and we've had spacecraft orbiting

Mercury, Venus, Jupiter and Saturn
for extended periods.

But, once we get beyond Saturn,
things become much more difficult.

The distances are vast.

Travel times are measured
not in one months,

but in years, decades, even.
And that means that we've only

visited the outer planets once.

Each visit is a voyage
into the unknown, as we strive

to reach the most distant planetary
systems ever visited.

And, when we do get there, our
spacecraft are travelling so fast

that they fly straight through,

only spending
a few hours in the system.

But those few hours of precious data
have allowed us to begin

to understand the mysteries that lie
there on the outer edges

of the solar system.

We have ignition
and we have liftoff.

Reports coming back indicate
those twin, large, solid motors

are functioning perfectly.

Our first mission into the dark
was only possible thanks to some

help from the planets themselves.

Each planet orbits
at a different speed.

And, for most of the time,
they're scattered around the Sun.

But once every 175 years...

.. something rather wonderful
happens.

The outer planets align...

.. creating a path...

.. that leads from Earth past Jupiter
and Saturn...

.. all the way to Uranus and Neptune.

Voyager 2 had been launched
at precisely the right moment

to take advantage of this rare
planetary alignment.

It arrived at Jupiter
in a little under two years.

Two and a half times more massive

than all the other planets
combined...

.. Jupiter gave Voyager
a gravitational kick.

Another two years on,
and Voyager witnessed

one of the most beautiful sights
in the solar system.

The icy rings of
another gas giant...

.. Saturn.

And, then, on and on...

.. further into the darkness.

Almost nine years after leaving
Earth, Voyager approached

an entirely new class of planet.

Just like Jupiter and Saturn,
the planet's upper atmosphere

is composed mostly of swirling
hydrogen and helium gas.

And hidden beneath lies
an exotic, icy mix of methane,

ammonia and water.

But, unlike the other gas giants,
Uranus is almost featureless.

For all the time that Voyager
stared at the planet,

it saw just ten cloud formations.

And we soon discovered why.

Uranus, at -224 degrees Celsius,

is the coldest planet
in the solar system.

The first of the ice giants,

in a permanent state of deep freeze.

Voyager spent just
six hours with Uranus...

.. and, as its gaze widened,
it took in the entire system.

Just like Saturn...

.. Uranus has rings.

The rings are so dark, so faint,
that they're very difficult

to see from Earth.

In fact, we didn't
discover them until 1977.

They must be made of some kind of
material that doesn't reflect

a lot of sunlight back,
and we don't know what that is.

It can't be water ice
like the rings of Saturn,

because they would be much brighter.

They're also extremely
delicate and thin,

and, again, that's a great mystery,
cos you'd expect the particles

to collide and the rings
to spread out over time.

That doesn't happen.

So, something must be holding
those rings in place,

and the answer can be seen
in this photograph

taken by Voyager 2.

You can see a bright, thin ring,
which is known as the epsilon ring,

and, above it and below it,
you can see two moons.

The inner one is called Cordelia,
and the outer one is called Ophelia.

Now, the thing you need to know
about orbits, a moon or a ring

particle that's orbiting closer
to a planet is moving more slowly

than a moon or ring particle
that's orbiting further away.

And, so, if there's a particle
in the epsilon ring that maybe

has a collision, loses a bit
of energy and starts to drop down,

then Cordelia tends to accelerate
that particle back up into the ring

again to speed it up
by its gravitational influence.

Whereas, if it's a particle on the
outer edge of the epsilon ring,

it gets a bit more energy
and raises up in altitude,

then Ophelia tends to slow it down,

and, so, it falls back
into the ring again.

And, by that mechanism,
these moons keep the ring nice

and neat around Uranus,
and, for that reason,

these moons have become
known as shepherd moons.

Now, we haven't discovered
any more shepherd moons.

Voyager was only in
the system for a few hours.

But we assume that they're there,
orbiting around Uranus,

keeping the other rings
precisely in place.

The missing moons are not the only
mysterious thing about the planet.

Uranus orbits the Sun
in a unique way,

a consequence of events
that happened long ago.

Uranus was born from the vast cloud
of material

that surrounded the young Sun.

Over time, this material grew
together

under the influence of gravity...

.. forming each of the planets...

.. all orbiting in the same
anticlockwise direction

as the primordial cloud
of gas and dust.

This rotation remains to this day.

Almost all the planets spin
on their axis in the same

anticlockwise direction.

But, for reasons we don't
yet fully understand,

Venus and Uranus spin on their axes
in the opposite direction.

And Uranus is even more curious.

The entire planet is on its side.

It's not known for certain why
Uranus spins in this way.

But it seems likely
that, at some point,

probably in the distant past,
it was hit by another planet,

possibly the size of
Earth or even larger,

which knocked the planet over,
and, indeed, modern computer

simulations suggest
that when you do that to a planet,

all the moons follow,
and you end up with the planet

and its system of moons,
and now its rings,

sort of corkscrewing
around the Sun on its side.

Voyager had taken us to the most
distant planets we had ever visited.

And, now, an almost unthinkable
distance of 1.6 billion kilometres

of icy cold space
lay between the spacecraft

and its next encounter.

The size and scale of the solar
system is impossible to imagine.

But, when we scale everything down,
then it becomes easier to visualise.

The Sun is 1.4 million
kilometres in diameter.

But scale that down by a factor
of around 600 million,

and it becomes the size
of that light.

And the Earth, Mars and the rocky
planets

are about that big on this scale.

And the distances are,
I think, even more remarkable.

So, Mercury, the closest
planet to the Sun,

would be all the way
over there by the sea.

So, this would be where
Mercury orbits.

And Mercury is 96 metres away.

It's about just under
a centimetre in diameter.

And the next planet, Venus,
is somewhere over there.

We'd fly past Venus about
180 metres away from the Sun.

And then head onwards
and outwards towards the Earth.

Here, at the Earth's orbit,
the true scale of our fragility

and isolation becomes apparent.

The Sun there, 250 metres away
with the twinkling stars

of the galaxy beyond,
and here's the Earth.

Two centimetres across.

On that pebble, all of
human history play out.

At around 380 metres,
we pass by Mars,

the last of the rocky planets,
and then onwards into the darkness.

Crossing the harbour
takes us to the fringes

of the outer solar system.

And, now, the distances
and the sizes

are beginning to get really big.

Jupiter, the largest planet
in the solar system,

is over ten times the diameter
of our Earth, and there's the Sun,

1.3km across the harbour.

At almost twice the distance
from the Sun to Jupiter,

we reach Saturn.

2.4km on our scale.

And we really are now entering
the twilight of the solar system.

Light levels are like early morning
or late evening here on Earth,

but we can still see Saturn
in the night sky,

because the planet and its rings
are highly reflective.

As far again as all the rocky
planets, and Jupiter and Saturn,

we reach Uranus.

3 billion kilometres from the Sun,
and we begin to fully appreciate

the distances Voyager
had travelled...

.. on its journey to the farthest
reaches of the solar system.

Journey's end, Neptune,
a ball of gas about

7.5 centimetres across.

7.4km away
from the Sun on this scale.

And we are still down there
on the island in Reykjavik harbour.

Little, microscopic ants crawling
around on the surface of a pebble.

It had taken 12 years,
but Voyager had finally crossed

the great expanse of space
between Earth and the most

distant planetary system.

Neptune.

17 times the mass of the Earth,
and even more massive than Uranus.

But, in contrast to its sister
planet, Neptune's atmosphere

is bursting with activity.

What we found was a planet
of extreme weather,

where high altitude winds whip white
methane clouds around at speeds

of over 2,000km/h.

Those are the highest wind speeds
anywhere in the solar system.

And Voyager saw a great, dark spot,

not unlike Jupiter's
great, red spot.

A storm system the size of Earth,
that, when we looked only four

or five years later,
we found had vanished.

But, over the years,
we've observed more storms raging

over the planet.

It's one of the great mysteries
of planetary exploration,

why a planet so far from the Sun,
with so little energy falling

into its atmosphere from sunlight,
can have the most extreme winds

in the solar system.

And Voyager made yet
another puzzling discovery.

Although further from the Sun,
the planet is warmer than Uranus.

The source of this extra
heat remains a mystery.

But, for some reason, Neptune emits
over two and a half times

the heat it receives from the Sun.

And this internal heat helps
to explain the ferocious storms.

As the heat makes its way
from the core of the planet

and out into space...

.. it churns up the entire
atmosphere...

.. creating winds unlike anything
seen elsewhere in the solar system.

The probable reason that wind
speeds can be so high

is because there's no solid surface
on a planet like Neptune.

It's exotic liquids further down,
and gases further up.

But there's no rocky surface.

There are no mountains
and continents to break up the flow

of atmospheric gases,
and, so, the winds can just whip

around the planet
at supersonic speeds.

Voyager 2 had almost completed its
grand tour of the solar system.

But before it began
its lonely journey

out into interstellar space...

.. it had one, last world to visit.

Triton, a vast moon covered
in a sheen of frozen nitrogen.

We expected it to be a silent,
still world,

but Voyager was in
for one, last surprise.

But, when Voyager arrived,

we saw geological activity
on that frozen world.

We saw geysers erupting up
into space 8km high,

and then carrying dark material
100km downwind.

Now, the key to understanding
what process caused those eruptions

was that we noticed that the geysers
tended to erupt on a place

on Triton's surface
below the faint Sun,

even though it's so many
billions of miles away.

What's happening is the sunlight
is falling on the thin layer

of nitrogen ice, going through,
and heating up a layer of darker

particles a metre below the surface.

A difference of just
four degrees Celsius

is enough for the heat
to radiate up

and vaporise the frozen nitrogen,
creating gas.

That gas is under pressure,
and, ultimately,

it punches through the nitrogen ice

carrying those dark particles up and
making the geysers of Triton.

In the furthest reaches
of the solar system,

the faint light of the Sun
is still enough to power the most

distant of geological wonders.

But this can't explain the rest
of Triton's mysterious features.

Terrain on this scale could only
be created

by a much more powerful force.

And a clue seems to be lurking
in Triton's odd orbit.

Unlike every other large moon
in the solar system,

Triton orbits in the opposite
direction to the spin of the planet.

And that means it's highly unlikely

that Triton and Neptune formed
at the same time.

If you formed a planet and a moon
out of the same collapsing cloud

of gas and dust, then they tend
to spin and orbit in the same

direction as the spin
of the initial dust cloud,

so, the most likely explanation
for that is that Triton is a visitor

to the Neptunian system, that,
unlike Voyager, never left.

One theory is that, billions of
years ago,

Triton was not a moon at all.

It grew up in a region
of space way beyond Neptune...

.. the Kuiper belt.

Here, trillions of frozen lumps
of water, ammonia

and methane circle the Sun.

The frozen leftovers from
the formation of the solar system.

Perhaps Triton ended up on the inner
edge of this region...

.. and ventured close enough

to Neptune to be drawn
in by its gravity...

.. until, eventually, it was plucked
from the Kuiper belt,

and forever trapped in orbit
around the distant, blue planet.

Today, Triton orbits in a nice,
regular circle around Neptune,

but it wouldn't have
started out that way.

When it was first captured,
it would have glanced

past the planet and ended
up in a wide, elliptical orbit,

sometimes being far away from
the planet, and sometimes close.

That means that, as Triton
orbited around Neptune,

the gravitational pull
was constantly changing,

and that stretches and squashes
the moon, and heats up

the interior by friction.

It's a process called tidal heating.

The molten interior would
have exploded up through cracks

and faults in Triton's crust...

.. creating the ragged surface
we see today.

And what you're left
with is a cooling moon,

orbiting in a circle the wrong
way around the planet,

the dramas of its past
hidden from view.

All that remains are the geysers,
powered by the faint light

of the Sun, painting dark streaks
across the now-quiescent surface.

Voyager had fulfilled its mission,

and now headed onwards,
out of the solar system...

.. where it would never
encounter another planet.

But, elsewhere, another world
awaited our arrival...

.. far out in the darkness.

Mission... And lift off of Nasa's
New Horizons spacecraft

on a decade-long voyage to visit
the planet Pluto and then beyond.

The New Horizons spacecraft,

on its way to the very edge
of our solar system.

Our ambition to learn
more about a small,

intriguing world depended
on the success of New Horizons.

As the tiny probe,
less than three metres in length,

was sent out into the abyss...

.. most of its electronics
and all nonessential systems

were shut down to save power.

It would hibernate for almost its
entire 4.8 billion kilometre trek.

Whilst New Horizons continued
its voyage to Pluto,

a controversy arose here on Earth
about the definition

of Pluto itself.

See, we always knew
that Pluto is small,

smaller than Neptune's moon Triton,
but it was discoveries made

in the vicinity of Pluto
by the Hubble Space Telescope

that really challenged
its status as a planet.

From Earth, Pluto, far away on
the inner edge of the Kuiper belt,

is so distant that, even with
Hubble, it appears

as nothing more than a fuzzy image.

But Pluto was not alone.

We began to detect
other distant objects.

Then, Hubble discovered
that another world

is almost the same
size as Pluto.

The discovery of these new worlds
has forced us to reconsider

what we mean by the word "planet".

The International Astronomical Union
has a three-part definition

of a planet. Firstly, it has to
orbit the Sun,

and Pluto certainly does that.

It goes round once every 248 years.

Secondly, it has to be massive
enough for gravity to sculpt it

almost into a sphere, and Pluto
indeed is almost spherical.

But, thirdly, it has to clear
its own orbit around the Sun,

and that's essentially
a mass constraint as well.

It has to have enough
gravity to throw things out

from its orbital path.

And that's where Pluto falls down.

Now, why that extra idea?

Well, ultimately,
it's because if Pluto is big enough

to be a planet, then Eris is big
enough to be a planet,

and Makemake is big
enough to be a planet,

and tens or even potentially
hundreds of objects out there

are big enough to be planets.

Now, does it really matter?

Well, I don't think so.

Ultimately, Pluto is a world, and,
therefore, we should explore it.

And, in July 2015,
that's exactly what we did.

After nine years of flight,
New Horizons awoke.

Copy that.

We are locked by telemetry
with the phase path.

We had our first glimpse of the most
distant world ever visited.

Pluto is beautiful.

And, far from being a frigid,
featureless world,

it turns out that it
has all the characteristics

of a dynamic, living planet.

And, yes, it is cold on the surface,
about -230 degrees Celsius.

I'd be walking
over solid nitrogen.

But the blotches
that could be made out

in the grainy Hubble Space Telescope
images turned out to be

a tremendous variety
of geological form and structure.

But by far the most recognisable
feature is the region known

as Tombaugh Regio,

or to give it its more popular
name, Pluto's Heart.

The western lobe of the heart
is called Sputnik Planitia.

A giant plain of frozen nitrogen,
methane and carbon monoxide...

.. that stretches for a million
square kilometres.

And, at its edge, lies a range
of mountains made of pure,

frozen water ice...

.. that rise up to 6km
above the plain.

But there's something
very strange about the region,

something that sets it apart
from the rest of this dwarf planet.

The surface of Pluto is covered
in craters, the scars

of impacts that have taken place
over many billions of years,

just like the surface of our moon.

Except, if you look on Sputnik
Planitia, it is absolutely smooth.

There are no craters there at all,
not a single one.

From space, the lack
of craters is striking.

And, the closer you look,
the stranger the mystery gets.

Detailed imagery beamed back
by New Horizons revealed a network

of hexagon and pentagon shapes...

.. that crisscross the frozen
nitrogen surface.

A clue to what might be happening
can be seen in these images.

Those kind of patterns are found
elsewhere in nature.

Here, for example,
on the surface of the Sun.

Or here, in a liquid that's
been heated from below.

These patterns are
characteristic of convection.

There's a heat source which is
causing material to rise up,

and then it cools and falls back
again, and in those circulating

convection currents, you tend to get
patterns like this.

So, what could be happening below
the surface of Sputnik Planitia

is that there is a heat source deep
down which is melting the nitrogen

ice and causing it to rise,
cool and then fall again,

and those convection currents
are constantly resurfacing the area.

That a small world like Pluto is
still active was a huge surprise.

Our best theory of how this could be

is that somewhere deep
in its interior...

.. there are radioactive elements
that generate heat as they decay.

This heat warms a sunless,
half-frozen ocean of water

that has existed for billions of
years beneath the surface of Pluto.

Now, the evidence from Sputnik
Planitia that there is still

an internal heat source, and studies
of other geological features

on Pluto, have led some scientists
to suggest that

that ocean of water
might still be there.

And the relatively warm ocean

could explain the lack of craters
on Sputnik Planitia.

The entire area is constantly
being repaved as the nitrogen

surface is slowly turned over.

But why all this activity
here and nowhere else?

Perhaps Pluto's heart is the site
of a huge impact...

.. that, long ago, punched a large
hole in the surface,

almost down
to the vast ocean beneath.

A hole that's slowly filled
with soft nitrogen ice...

.. that now gently churns
just above a warmer ocean.

Imagine that.

An ocean billions of
miles away from the Sun,

on the frozen frontier
of the solar system.

On this most surprising of worlds,

there may still be
an ocean of water.

New Horizons' closest encounter
with Pluto lasted just hours...

.. during which only one
hemisphere was visible...

.. leaving the other side a mystery.

Pluto held a final surprise.

As New Horizons turned its camera
back for one, last look...

.. it captured an image
of Pluto's atmosphere,

glowing in the dark.

A thin, blue sky over
a hidden ocean of water...

.. 4.8 billion kilometres from Earth.

We've come a long way.

After just half a century
of space exploration...

.. we've reached every one of
the planets in the solar system...

.. and begun to tell their story.

What's emerging is a tale
of never-ending change...

.. of dramatic origins...

.. moments of hope...

.. and loss.

Where just eight motes of rock,
ice and gas...

.. set against the dark backdrop
of space...

.. have conspired together to
produce life on at least one world.

"Why do we explore?"
some people ask.

Shouldn't we deal with the problems
here on Earth before committing time

and energy and resources
to exploring the stars?

Well, I think focusing
entirely on our mote of dust

would be a profound mistake.

It would mean that we've taken
the decision to sit huddled

in a tiny corner
of the solar system,

wondering what we're doing here.

It would mean that we've taken
the decision to fight amongst

ourselves for ever-more-precious
resources,

confined below a thin shell
of air on a small rock,

rather than following the
three-dimensional path marked out

by the lights in the night.

We live in
a solar system of wonders,

of planets of storms and moons
of ice and landscapes and vistas

that stir the imagination
and enrich the soul,

a system of limitless resources,
limitless beauty

and limitless potential.

A system that we've only
just begun to explore...

.. in a journey that has already
rewarded us with so much.

Pluto was the most distant world
that Nasa had ever planned to visit.

Ten, nine, eight, seven...

The launch of New Horizons
was incredibly dramatic.

Mission... And liftoff.

And it just shoots up so fast.

It's the fastest launch
ever from Earth.

About ten miles per second.

In order to ensure that it was going
to last for the decade across space

and actually work there,
they built in a lot of redundancy.

There were sort of two of
everything, computers

and the guidance systems,

and they figured out
how to hibernate.

The first spacecraft
that hibernated.

To put the spacecraft largely
to sleep let the spacecraft

just silently coast,

then we could extend the life
of the electronics.

No spacecraft mission had ever
really used hibernation

as a day-to-day way
to cross the solar system.

As the spacecraft slept,

it sped away from Earth
towards its destination...

.. to a world that had lain hidden

amongst the stars for almost
all of human history.

The discovery of Pluto was in 1930,
when Clyde Tombaugh,

a Kansas farm boy,

was using the telescope at
the Lowell Observatory in Arizona.

Every clear night,

Clyde would go out and take images

of the sky on photographic plates,
and when it was cloudy

or during the day,
he would compare those plates.

And Clyde Tombaugh noticed
that there was this little speck

moving across the stars.

And, then, in 1930,
Clyde Tombaugh discovered Pluto.

But Tombaugh would never
see Pluto up close.

He died nine years before
the launch of New Horizons,

at the age of 90.

He had asked if a mission ever
did get launched some of his ashes

could be sent on the journey.

And to think that Clyde's ashes,
you know, flew by Pluto,

and, you know, he was finally
visiting the place

that he discovered,
that was pretty cool.

Wow!

Now, I mean, Pluto was a shock
and a revelation in so many ways.

Our jaws just dropped to the floor.

This amazing world that we would
never have known about,

unless we sent
a spacecraft out there.

But New Horizons didn't stop
after its flyby of Pluto.

It continued on
for three more years,

ever deeper into the Kuiper belt.

The temperatures out there
are almost absolute zero,

so everything is so well preserved.

The Kuiper belt completely
changed everything.

It's an archaeological dig,
if you will, into the early history

of our solar system,
so it's a scientific wonderland.

On New Year's Day, 2019,

New Horizons had reached
the centre of the region...

.. and was about to encounter the
most distant object ever visited.

This object may be the most
primitive object

ever encountered by a spacecraft.

By looking at Ultima Thule today,
we think we're looking back in time

to the origin of the solar system.

Ultima Thule's formation was perhaps
just one of the countless collisions

that formed the planets themselves.

But, somehow, it escaped
and was flung far out

into the Kuiper belt.

The worlds out there,
which are billions of miles

from the Sun, there are so many
mysteries kept in these objects,

and we can really go out
and explore them, and start

to understand these mysteries.

Like every mission before it,
New Horizons has helped write

the story of the planets.

A story that we're only just
beginning to tell.

The planets of our solar system,
they're in our back yard.

We have the technology
to be able to go

and stand on these surfaces.

To be able to go out there
and be able to continue doing

something that is very natural
and intrinsic to all of us,

and that is answering the big
questions and exploring the unknown.

I feel as though we're absolutely
just at the beginning

of planetary exploration.

There's a lot of exploration
left for us.

Journey through our solar system
with this free poster

produced by the Open University,

and discover more about its
planets and moons.

Order your free copy by calling...

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