Solar System (2024) s01e02 Episode Script
Dark Worlds
1
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.