Original subtitles

There's something out there in the darkness.

A world with no name.

Its surface is ancient and probably pink.

We don't know where it came from or how long it's been there.

But we do know it's not alone.

It's a dwarf planet, only discovered in 2018.

Now, you might reasonably ask,

how could thousands of astronomers, both amateur and professional,

miss a world that is pink and 400km across?

Well, the answer might lie in its nickname,

which is "FarFarOut", because it really is far, far out.

You might have thought of the solar system as the sun

and then all the planets lined up, all the way out to Pluto,

but we now know that that's just the tip of the iceberg.

This is a journey to the least explored regions

of our solar system...

..as we probe the mysteries of the asteroid belt...

..visit frozen worlds that we're discovering beyond Pluto...

..and reveal a vast hidden kingdom

that even our most powerful telescopes can't see...

..lost...

..in the dark.

It was the sound that most people noticed.

DEEP BOOM

A sonic boom in the night.

Travelling at almost 50,000km per hour,

a rock tore through the atmosphere...

..broke apart...

..and rained down across Winchcombe.

A visitor from a distant realm.

On the 28th of February 2021, a rock from space landed there,

on this driveway,

and it shattered into hundreds of pieces, most of it went onto

the lawn, bits of it went onto the neighbour's driveway,

and the indentation in the driveway that was here is

now in the Natural History Museum.

And here is a piece of that rock.

And you see that it's very dark, almost black,

very different to the rocks you find naturally around here.

So the question is, other than the rather nonspecific

"it came from space",

what exactly is this and where exactly did it come from?

The cows weren't the only ones watching that night.

Doorbell and CCTV cameras never sleep.

A network of specialist meteor cameras also captured it.

And that meant it was possible to calculate its trajectory...

..back over the skies of England...

..out of Earth's atmosphere...

..past Mars...

..and into the darkness.

In between Mars and Jupiter lies a realm of rocky worlds.

This is where that rock came from...

..the asteroid belt.

Asteroids are rubble left over from the formation

of the solar system...

..the remnants of planets that never were.

It's thought there are at least a million out here...

..so dark they're incredibly difficult to see.

But asteroids don't always stay in the asteroid belt.

EXPLOSION

Every now and then, two asteroids collide...

..causing fragments, big and small, to be lost from the belt.

This can create havoc for neighbouring planets...

..including...

..our own.

We've flown several spacecraft straight

through the asteroid belt...

..but only one has stayed to explore the region itself.

Most asteroids are like misshapen boulders...

..but one stands out from the rest...

..much bigger than the others and almost perfectly spherical.

Dawn's target world.

At first, Ceres appears dark

and heavily cratered, like its fellow asteroids.

But Dawn has discovered that it's different.

Its surface is peppered with bright white crystals.

So what are they?

Now, there is another world in the solar system where white

crystals form on the surface - it's here, it's Earth.

This is a mountain composed almost entirely of salt.

It's mainly sodium chloride, actually, table salt,

there's a bit of magnesium and potassium in there,

which gives it those pinky colours.

This is enormous.

It extends hundreds of metres down into the ground.

Now, it formed because, around 40 million years ago or so,

this was the Atlantic Ocean.

And then, as the climate changed, the oceans receded,

evaporated away, and left the salt behind.

And then, plate tectonics raised the ground up to form salt mountains.

Now, salt crystals like this only form

in the presence of liquid water.

In this case, it was the Atlantic Ocean.

That raises an intriguing question.

Are those crystal deposits on the surface of Ceres salt?

And, if so, does that imply there was once an ocean on Ceres?

Dawn entered into a close orbit.

And by measuring sunlight reflected off the crystal deposits,

it could determine what they're made of...

..sodium carbonate, a common type of salt...

..a tantalising sign that Ceres had an ocean in its past.

Orbiting just 35km from the surface, Dawn found another

clue that suggests an ocean may still be there today.

It detected a different type of salt crystal that has only been

found in one other place in the solar system.

When saltwater evaporates, then it leaves these things behind,

salt crystals, essentially sodium

and chlorine ions bonded together into a crystal lattice.

But under the right conditions, when it's very cold,

then a different sort of crystal can form when sea water freezes.

It's called a hydrohalite, essentially water

molecules are incorporated into the crystal lattice.

Now, we see hydrohalites here on Earth.

But the fact that we also see them on Ceres presents a mystery

because hydrohalites are unstable in a vacuum.

It's been calculated they should only last around 100 years

on the surface of Ceres before the water escapes out into space.

So the fact that we find them

implies that they must be constantly forming.

And that, in turn, must imply that, below the surface of Ceres,

there is a reservoir of saltwater today.

Ceres is a world of water.

It's thought salty, subterranean lakes 40km deep

exist beneath the surface today.

But how did the salt end up on the surface?

The answer lies in Ceres's location.

Incoming asteroids can punch straight through to the lakes below.

Exposed to the vacuum of space, the water vaporises instantly...

..leaving its cargo of salt behind

and peppering Ceres with bright white spots.

The discovery of water beneath the surface means this asteroid

is now a candidate in the search for life.

The more we've explored the asteroid belt,

the more we've come to realise that it's not just a band of rubble.

The dark worlds hiding here are worthy of exploration

in their own right.

And the asteroids that leave the belt can transform

the planets nearby.

Mars orbits closer to the asteroid belt than any other planet.

And around 100 fresh impact sites appear on Mars every year.

Nasa's rovers have found some spectacular meteorites...

..scattered across its surface.

And one even spotted what looks like an asteroid in the sky...

..floating in front of the sun.

But it's actually one of Mars's two moons.

Phobos isn't spherical, like our moon,

it's kind of a strange irregular shape.

So, there's a theory, simple theory,

maybe Phobos is an asteroid that came in from the asteroid belt

and was captured by Mars's gravity.

Well, actually, wonderfully, turns out things aren't that simple.

The European Space Agency has a probe in orbit,

busily surveying the Red Planet.

Mars Express orbits Mars three times every day,

regularly passing beneath Phobos.

Its high-resolution cameras captured the most detailed

images of the moon ever seen...

..showing a surface covered in strange grooves.

Phobos looks like someone's ridden a bike all over it.

But perhaps more intriguing

were the measurements Mars Express made of the tiny moon's mass.

It's pretty hard, actually, to measure the mass of a small

moon, but the way it was done was to measure how the path of the Mars

Express spacecraft was deflected by the weak pull of Phobos's gravity.

Now, once you have the mass,

and we have a reasonable idea of the volume, we calculate the density.

And that came as a surprise because it turned out that Phobos

is more like a kind of a gravel pile weakly held together.

It's not very dense at all.

And that does support the theory that maybe Phobos was

formed from the debris of some kind of collision.

So it's possible that Phobos is not a captured asteroid after all...

..but was in fact formed after a massive asteroid hit Mars.

But there's still something about Phobos

that neither theory can explain...

..its strange grooves.

This landscape was sculpted by the elements, by the wind and the rain,

but on Phobos there is no wind and rain,

and so the landscape is sculpted by something else.

It's actually sculpted by gravity.

When you've got a planet and a moon orbiting around each other

and spinning on their axes,

very complex gravitational forces come into play -

they're called tidal forces -

and they act to deform the moon and also actually

slightly deform the planet, and they also act to change the orbits.

But in the case of Mars

and Phobos, what's changing is that Phobos is descending towards Mars in

its orbit, so it's only about 6,000km above the surface now.

And, ultimately, over time, Phobos gets closer to Mars

and that means the tidal forces get stronger and stronger.

The grooves on the surface are stretch marks and fractures.

Mars's gravity is tearing Phobos apart.

Eventually, Phobos will drift

so close that Mars's gravity will destroy it.

But from this destruction, something beautiful will emerge.

As Phobos begins to break apart...

..most of the debris will fall to the surface of Mars.

But the rest will remain in close orbit,

spreading out to encircle the entire planet...

..leaving Mars with a spectacular set of rings.

So what is Phobos?

The truth is, and I think this is wonderful,

we still don't know the full story of the origin of Phobos.

But what we can say is that this little moon is a wonderful place...

..because in the future it's going to turn the Red Planet

into a ringed world.

Leaving the asteroid belt behind, we travel out,

further into the darkness.

Beyond Jupiter, the gaps between the planets get ever wider...

..and the temperature plummets.

We push on, through millions of kilometres of empty space...

..until we reach the furthest planet from the sun.

Neptune is wrapped in a dense blue blanket...

..clouds of methane and ammonia thousands of kilometres thick

beneath which there is no detectable surface.

But there are worlds out here that you could stand on.

Neptune has at least 16 moons.

And one of them is very unusual.

Triton's surface is coated with pale nitrogen ice.

But unlike the other moons which are frozen to the core,

Triton is an active world.

Geyser-like plumes of gas and dust 8km high

stretch into Triton's upper atmosphere,

which flattens them abruptly at 90 degrees...

..creating a vista so strange...

..it's hard to believe it's real.

How did a moon, 4.5 billion km from the sun...

..become so active?

In the 10th century, monks built a chapel on this hillside.

It's an astonishing achievement, they carried all the stones

up by hand, and when they'd finished, a solitary monk...

..a hermit, lived here,

contemplating the great mysteries of existence.

And every morning, he would have seen the sun rise in the east.

Now, we know the reason for that.

It's because the Earth is spinning on its axis

and, in fact, everything spins in the solar system,

and almost everything spins in the same direction.

Now, it's a fundamental property of nature that,

once things are spinning, they continue to spin,

and continue to spin in the same direction, unless something happens.

Triton doesn't spin in the same direction as almost everything else.

It goes in the opposite direction in its orbit around Neptune,

so there must be a reason for that.

Triton's unusual orbit,

which takes it around Neptune in the opposite direction to the

inner moons, suggests that Triton didn't form alongside Neptune.

It came from somewhere else.

Sometimes in science, things are quite simple, there are sort

of rules of thumb that apply and are useful and one of them in the solar

system is that planets and moons that form in the same region around

the sun, the same distance from the star, are made of the same stuff.

So, for example, here's Neptune, which is

mainly made of hydrogen, helium and a bit of methane.

And there is Uranus, which is made of hydrogen, helium and a bit

of methane, cos they formed in the same icy region far from the sun.

Here's Mars, formed much closer in, and it's silicates

and iron oxides, aluminium, magnesium. Things like that.

And there's the Earth, roughly the same place, same stuff again.

Here is Pluto, made of water ice,

nitrogen ices, methane ices, carbon monoxide.

And here is Triton...

..same.

So, we have a hypothesis, which is a guess I suppose that we can test.

The hypothesis is that maybe Triton and Pluto

formed in the same place.

To understand Triton, we have to look deeper into the darkness.

Neptune might be the furthest planet from the sun,

but it's not the edge of the solar system.

Not even close.

Over a billion kilometres further out lies Pluto.

And it's not alone.

There are hundreds of thousands of other worlds out here.

This is the Kuiper belt...

..a vast, frozen realm billions of kilometres wide.

It's home to an exotic collection of icy worlds...

..including some 200 dwarf planets.

Some have rings and moons.

Others are bizarre and misshapen.

But they all share one thing in common.

They're made of similar material to Triton.

It seems, then, that the Kuiper belt is where Triton belongs.

So how did it end up in orbit around Neptune?

The answer might lie in the strange geography of the Kuiper belt itself.

In one region, everything orbits the sun in pretty much circular

orbits, like almost everything else in the solar system, but

there's another region where things orbit in ellipses. It's almost

as if something ploughed through the Kuiper belt and disturbed it.

There is one prime suspect.

Neptune.

It's thought Neptune formed much closer to the sun...

..then slowly drifted out.

Its gravity disrupted the Kuiper belt and scattered the icy worlds...

..but Triton was unable to escape.

To this day, Triton is trapped in a backwards orbit around Neptune.

And being this close to a giant has consequences.

Just as our moon raises tides on the Earth,

Neptune raises tides on Triton...

..stretching and squashing it...

..heating up its rock and ice.

It's thought that this is what melts its frozen interior...

..and powers the spectacular plumes.

So here is the wonderful hypothesis of what might have happened

to explain what we see on Triton.

So, as Neptune ploughed through the Kuiper belt, in the ensuing

gravitational chaos, Triton got captured into its reverse orbit.

And even to this day,

that orbit is ejecting energy into the moon by the tides raised

upon it by Neptune's gravity, and that's driving the geology.

And isn't that a wonderful example of how you build a hypothesis

in science? You put together the clues. So you have the composition

of Triton being the same as Pluto, we have the strange geography

of the Kuiper belt, and we know that gas giants can change their orbits.

Put those together and we have a hypothesis of why Triton is

the strange magical twilight world that we see today.

Leaving Triton behind, we travel into the Kuiper belt.

We know very little about the worlds out here.

They're so far away, we can't make them out in much detail.

But we have sent one spacecraft to explore this region.

It took over nine years for New Horizons to get here

and hone in on its primary target.

The spacecraft captured the first close-up

images of a Kuiper belt world ever seen...

..revealing mountains on Pluto as high as the Alps...

..made from water frozen hard as granite.

But there's something else here.

Another world...

..hanging uncomfortably close in the sky.

Charon doesn't rise or set, and it's only visible from one side of Pluto.

Understanding why these two are so close together...

..reveals something unique about this dark and distant realm.

When Charon was first discovered in 1978,

we just assumed that it was a moon, Pluto's moon.

But the more we've learned about Charon

and, crucially, the more we've learned about the Pluto-Charon

system, the more we've come to realise that there's more to it.

All large moons in the solar system, including our own,

have something in common.

They only show one face to their parent planet...

..and the fact that all moons do this is not a coincidence.

It's to do with the complexity of orbits.

So let's say this is the Earth and I'm the moon,

and we tend to think of the moon just orbiting around the Earth

and the Earth sitting still - that's not what happens.

Actually, both moon and Earth orbit around what's called the

common centre of mass of the Earth-Moon system.

It's just that, because the Earth is way more massive than the moon,

the centre of mass is actually inside the Earth,

so the Earth is kind of doing that.

Let me show you what I mean.

- Bueno.

- So here's the Earth, spinning on its axis once every 24 hours,

that's one day.

And there is the moon orbiting around the Earth,

once every 27 and a bit days.

The gravitational interactions between Earth

and moon cause their orbits to synchronise.

The moon rotates on its axis exactly once for each lap

it makes around the Earth...

..with the result that the same face of the moon always

points to the Earth.

But in the case of Pluto and Charon,

the centre of mass of the system is outside of both bodies,

and so they both orbit around the centre of mass,

which is somewhere in space. It's where I'm standing.

Now, Pluto orbits once every 6.5 Earth days or so

and spins on its axis once every 6.5 days or so.

And Charon also orbits once every 6.5 days or so

and spins on its axis once every 6.5 days or so.

And the result of all that is that Charon always presents the

same face to Pluto...

..and Pluto always presents the same face to Charon.

Both worlds face each other in a synchronous dance.

Now, this might seem really strange and unusual - it isn't.

It's a consequence of all those forces acting.

This is what would happen to our Earth and moon, given enough time,

but enough time would be many times the current age of the universe.

The difference here is that this is not a moon

orbiting around a planet, it really should be thought of as two

objects of similar mass orbiting around each other.

Thank you.

Pluto and Charon are a double planetary system...

..also known as a binary pair.

Just 20,000km apart,

they gaze at each other without ever turning away.

This is the first double planetary system we've observed,

but we think there could be many more.

New Horizons is still out there today,

searching for more Kuiper belt worlds.

So far, it's encountered just one.

Arrokoth is formed from another pair, now so close they're touching.

So why do the worlds out here in the Kuiper belt

form these rare partnerships?

The further from the sun planets are, the slower they move.

And way out here, they move very slowly indeed.

But, occasionally, collisions do occur.

It's thought that in the past Pluto and Charon crossed paths...

..but their glacial movement meant that this was less of an impact

and more of an embrace.

Instead of destruction, the pair became partners for life.

Now, we've only actually visited three

worlds in the Kuiper belt - Pluto, Charon and Arrokoth -

but we have seen more using the Hubble Space Telescope.

And what's interesting is that, when we zoom in on those

what look like single pixels of light, many of them

aren't single pixels, they're binaries just like Pluto and Charon.

So this is a pair of worlds called Typhon and Echidna,

and it's a binary.

And this is another pair of worlds, Vanth and Orcus.

And, again, you see them orbiting around each other.

Orbiting around their common centre of mass.

And this complex gravitational dance, far from the sun,

means that the Kuiper belt has a very complex structure

we don't really understand.

So the Kuiper belt does, in a very real sense, represent

the frontier of our knowledge about the solar system - almost.

Past the Kuiper belt, we enter a realm of true darkness.

All we see are the bright stars of the Milky Way.

Even our most powerful telescopes struggle to see

anything in the solar system out here.

But every now and then,

things do show up in the darkness.

In 2018, a faint point of light was detected...

..10 billion km beyond the Kuiper belt.

It's a dwarf planet...

..nicknamed FarFarOut.

But we think there's more out there.

A lot more.

There is another realm of our solar system, far beyond the reaches

of any spacecraft or telescope we've ever built.

So vast, so strange

and so deep into the darkness...

..it's almost impossible to imagine.

Imagine that pinnacle is the sun,

and I'm standing somewhere around the orbit of Neptune,

and let's head out to the Kuiper belt.

Now, Neptune is about 30 metres away from the sun

so, on our scale, that means that the Earth would be one metre away.

One metre, that's called one astronomical unit,

the distance of the Earth from the sun.

It's about 150 million km.

Well, now I'm inside the Kuiper belt,

traversing through that realm of icy worlds,

and I reach Pluto, the furthest extent of its orbit at the edge

of the Kuiper belt, 50 astronomical units, so 50 metres away.

And then to the most distant object we can see.

Well, we have to get a move on because it's a long way away.

It is, of course, FarFarOut.

That is the most distant object we have ever seen.

And it is, well, 130 astronomical units away,

130 metres.

But, remarkably, we strongly believe that there's another region,

another realm to the solar system.

But to go to the edge of that, we have to go far, far, far away.

Here we are, 2km away from the sun.

That's 2,000 astronomical units.

And this is where we think another realm begins.

And, astonishingly, to me,

we think it extends 100km further.

100,000 astronomical units. That's over a light year.

Just imagine how many strange worlds that contains.

This distant realm is known as the Oort cloud,

a mighty kingdom at least a trillion strong...

..hidden in the dark.

Most are thought to be icy objects the size of mountains.

But it's likely some are made of rock.

And if we were able to go there and visit them,

we'd discover that they are pristine.

Barely changed since the dawn of the solar system.

Stretching out into interstellar space...

..and held in place by the sun's gravity...

..the Oort cloud is our solar system's largest

and most mysterious realm.

Well, here's the thing, no telescope has ever seen

anything in the Oort cloud, and the most distant

spacecraft from Earth, Voyager 1, is all the way back there.

It's just beyond FarFarOut.

It is travelling at around a million miles a day and it will take

300 years to get here and 30,000 years

to cross the Oort cloud.

But science doesn't deal in fairy stories.

It's not as if somebody just said,

"Well, that'd be a good idea, let's invent it."

So why are we so sure that it's there?

For as long as we've been looking into the night sky,

there have been signs that the Oort cloud is real.

And this is one of them.

It's existed for billions of years.

And for most of that time, it's been locked in a deep, frozen slumber.

But now it stirs.

As it nears the sun, warm rays bathe its surface...

..and it begins to thaw.

So much material is torn from the surface.

It stretches out to form a tail...

..20 million km long.

It's a comet.

Comet Nishimura was visible from Earth for just a few weeks.

And it's by no means the first of its kind.

Hale-Bopp...

..and NEOWISE lit up our skies for weeks on end.

And when we followed their trajectories back,

we discovered that they all came from the same place...

..300 billion km from the sun.

Countless comets throughout recent history have orbits that

started in the Oort cloud.

And that's doubly surprising because comets have a very short lifetime.

Many of them only last a few orbits around the sun.

So that implies that there must be a vast reservoir of comets out here.

And that is a central piece of evidence for the existence

of the Oort cloud.

But what does the Oort cloud look like?

Well, here's a fascinating thing,

the comets all come from different directions.

From over there and over there and down and across,

from everywhere.

Now, if the Oort cloud was flat, like the Kuiper belt,

you'd expect them to come from similar directions...

..which implies that the Oort cloud is not a flat disc...

..but some kind of giant sphere surrounding the sun.

Now, just imagine the scale of the Oort cloud,

a giant sphere enveloping the entire solar system,

stretching more than a light year, every direction.

We tend to think of the solar system as the sun

and all the planets and moons...

..but, as technology's improved, we've discovered more...

..and more.

Our exploration of the solar system has only just begun.

We know that we've only explored the tip of the iceberg,

a fraction of what's out there.

Most of the solar system lies, at least for now, way beyond our reach.

But by just making some observations of a few comets and asteroids

and even captured moons that've ventured

inwards from the outer solar system, and, by doing science,

we've been able to build a picture

and tell a story of our solar system.

And we've discovered that it isn't just a few planets orbiting

close to a lonely star.

It is a vast structure that stretches outwards,

maybe halfway to the nearest star.

- There's a real mystery surrounding Phobos,

which is we don't really know what it is.

It looks exactly like an asteroid, but it orbits Mars in a very

circular orbit around its equator, which is not what you would expect.

- Phobos looks like an asteroid, but it behaves like a moon.

So which one is it?

- We may soon have an answer.

The Japanese space agency is planning an audacious mission

to Phobos in 2026.

- The MMX mission, the Martian Moons eXploration mission,

aims at going to Mars's moon and bringing some samples

of Phobos back to the Earth.

- I'm really confident that, as soon as we have a sample of it on Earth,

we'll know almost straight away what Phobos is.

- But landing on Mars's tiny moon, with its low gravity,

drilling into its surface and getting the samples all

the way back to Earth in a single round trip...

..is no mean feat.

- People have wanted to explore the Martian moons for a long time,

have tried to send missions there,

but they've failed for a number of reasons.

- MAN TALKS IN RUSSIAN

- Unfortunately,

Phobos 1 didn't work on its way to the Mars system.

And then Phobos 2 got there and sent back a little bit of data

and then was lost contact with.

- In 2011, the Russian space agency, Roscosmos, made a third attempt.

REPORT: - The Phobos-Grunt probe blasted off

from Baikonur cosmodrome.

But the Russian spacecraft failed to fire its own engines to set

it on its path to Mars.

- When a mission fails it's devastating, it's disappointing.

But we've got to learn from mistakes.

We need those failures to help us develop our instruments,

to develop our mission in order to make it a success.

- If all goes to plan, the Japanese mission will

transform our understanding of what Phobos is,

giving new insight into how the moons and planets

of our solar system formed.

- The MMX spacecraft is amazingly capable.

It's got cameras and spectrometers and a rover

and two sampling mechanisms.

But it's also doing something that's never been done before,

which makes it hard.

And so, to be ready for that,

you have to put things through a whole array of tests.

They'll vibrate everything the way it will when it launches.

They'll put it through hot and cold temperatures like you'll

experience from direct sun or from being in the cold of space.

- If the mission succeeds,

the return spacecraft is due home with its samples in 2031.

- When the sample gets back to Earth, we'll analyse it in our laboratories

to find out what it's made of and therefore what Phobos is.

- Understanding the origin of Mars's moon Phobos will help us

to take a step forward to understand the evolution

and the formation of our solar system.

- Next time, our solar system's most violent worlds...

..where we encounter 20km-tall whirlwinds,

lightning ten times more powerful than that on our planet,

and slow-motion rainstorms on a distant moon.

These are the Storm Worlds.