All language subtitles for Solar.System.S01E02.Dark.Worlds.1080P.WEB-DL.x264-skorpion_track3_[eng]

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.