Original subtitles

COMMS: Lift-off of Messenger on NASA's mission to Mercury.

BRIAN COX: Far, far away...

...beyond Mars...

...past the storms ofJupiter 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 everfind 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 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

evervisited.

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.

COMMS: We have ignition and we have lift-off.

Reports coming back indicate those twin large solid motors

are functioning perfectly.

BRIAN COX: 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.

...something rather wonderful happens.

The outer planets align...

...creating a path...

...that leads from Earth, pastJupiter and Saturn...

...all the way to Uranus and Neptune.

VoyagerZ had been launched at precisely the right moment

to take advantage of this rare planetary alignment.

It arrived atJupiter in a little undertwo years.

Two and a half times more massive than all the other planets combined...

...Jupiter gave Voyager a gravitational kick.

Anothertwo 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 likeJupiter 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 sawjust ten cloud formations.

And we soon discovered why.

Uranus, at minus 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 spentjust 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 discoverthem 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.

(WIND HOWLS)

50 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 ora 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 there's a particle on the outer edge of the Epsilon Ring

and 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 forthat reason,

these moons have become known as shepherd moons.

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

I mean, 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 drew together

underthe 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 axes

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 a 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 planet 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 if we scale even/thing down, then it becomes easierto 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 aboutjust under a centimetre in diameter.

And the next planet, Venus, is somewhere overthere.

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,

And here's the Earth - two centimetres across.

On that pebble, all of human history played out.

At around 380 metres, we pass by Mars,

the last of the rocky planets,

and then onwards, into the darkness.

(BOAT CHUGS)

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 overten times the diameter of our Earth.

And, I mean, there‘s the Sun, 1.3 kilometres across the harbour.

At almost twice the distance from the Sun toJupiter,

we reach Saturn...

...2.lr kilometres 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 nightsky

because the planet, and its rings, are highly reflective.

As far again as all the rocky planets,

and Jupiter and Saturn,

we reach Uranus.

Three billion kilometres from the Sun,

and we begin to fully appreciate the distances Voyager had travelled...

...on itsjourney to the farthest reaches of the solar system.

Journey's end - Neptune,

a ball of gas about seven and a half centimetres across,

7.4 kilometres 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.

(EXCITED CHATTER)

(EXCLAMATIONS AND APPLAUSE)

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.

(THUNDER CRASHES)

(WIND HOWLS)

What we found was a planet of extreme weather,

where high-altitude winds whip white methane clouds around

at speeds of over 2,000 kilometres per hour.

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

And Voyager saw a great dark spot, not unlikeJupiter‘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 overthe 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 furtherfrom the Sun, the planet is warmerthan Uranus.

The source of this extra heat remains a mystery.

But for some reason,

Neptune emits overtwo 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 lonelyjourney 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,

eight kilometres high,

and then carrying dark material a hundred kilometres 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,

then heating up a layer of darker particles a metre below the surface.

A difference ofjust four degrees Celsius

is enough forthe heat to radiate up...

...and vaporise the frozen nitrogen, creating gas.

That gas is under pressure,

and ultimately it punched through the nitrogen ice,

carnring 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.

That means that it‘s highly unlikely

that Triton and Neptune formed at the same time.

If you formed a planetand 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 to 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.

COMMS: We have ignition 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.

(CHEERING)

BRIAN COX: 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 non-essential 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 even! 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 is 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.

(FAINT COMMUNICATIONS CHATTER)

And inJuly 2015, that's exactly what we did.

(FAINT COMMUNICATIONS CHATTER)

After nine years of flight, New Horizons awoke.

COMMS: Copy that. We are in lock with telemetry with the spacecraft.

(CHEERING AND APPLAUSE)

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

Pluto is beautiful.

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 minus 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 six kilometres 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 strangerthe 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.

And 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‘s 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 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 mystew.

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.

Afterjust 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...

...wherejust 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'd 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'd 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 onlyjust begun to explore...

...in ajourney that has already rewarded us with so much.

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

COMMS: Ten, nine, eight, seven....

DR DAVID GRINSPOON: The launch of New Horizons was incredibly dramatic.

COMMS: Ignition and lift-off of NASA's...

DAVID: And itjust shoots up so fast.

It's the fastest launch everfrom Earth.

About ten miles per second.

In orderto 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.

ALAN STERN: To put the spacecraft largely asleep,

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.

BRIAN COX: 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.

Clyde Tombaugh,

a Kansas farm boy who was using the telescope

at the Lowell Observatory in Arizona.

Even/ 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.

HAL WEAVER: And Clyde Tombaugh noticed that there was this little speck

moving across the stars.

And then, in 1930, Clyde Tombaugh discovered Pluto.

BRIAN COX: But Tombaugh would never see Pluto up close.

He died nine years before the launch of New Horizons,

at the age of 90.

ALAN: He had asked ifa mission everdid 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.

(EXCITED MURMURING)

(CHEERING AND APPLAUSE)

No, I mean, Pluto was a shock and a revelation in so many ways.

Ourjaws just dropped to the floor.

This amazing world

that we would never have known about

unless we sent a spacecraft out there.

(CHEERING)

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

It continued on for three more years,

ever deeper into the Kuiper Belt.

ALAN: 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.

BRIAN COX: 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.

HAL: 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.

BRIAN COX: Ultima Thule's formation

was perhapsjust one of the countless collisions

that formed the planets themselves.

But somehow it escaped

and was flung far out into the Kuiper Belt.

DR NOAH HAMMOND: 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.

BRIAN COX: Like every mission before it,

New Horizons has helped write the story of the planets.

A story that we‘re onlyjust beginning to tell.

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.

Tobe able to go out there and be able to continue doing something

that's 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 absolutelyjust at the beginning

of planetan' exploration.

There's a lot of exploration left for us.