All language subtitles for How The Universe Works S04E02 Edge Of The Solar System_ENG_Subtitles01

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NARRATOR: This is space,

but not as you know it.

Trillions of icy rocks,

a dark, mystical star,

giant magnetic bubbles,

each one more than
100 million kilometres across...

..and a cosmic war zone where
fields of deadly radiation collide.

This isn't
a distant part of the galaxy.

This is the edge of
our own solar system...

..and what's going on here

determines whether we live or die.

Over the last 30 years,

our understanding of the universe
has exploded.

We've discovered billions of stars
in our galaxy

and billions of new galaxies
across the universe.

We've seen black holes
shooting out cosmic jets...

..exploding stars...

..and new planets
in distant solar systems.

But when it comes to
our own cosmic postcode,

we know remarkably little.

When we study space,
the closest thing to us

is our solar system, so that's what
we should understand the best.

But it turns out
we're just getting started.

We don't understand
everything in our solar system.

The solar system is our home,
our local neighbourhood,

but it may surprise you to know
that we don't really understand

how far it extends.

PHIL PLAIT: When I was a kid,
I was taught that

where the planets were
was the solar system,

so that the farthest planet -
at the time, Pluto -

was where the solar system stopped.

And it turns out
that's completely wrong.

If you think that
the solar system stops

where the planets stop,
where Neptune and Pluto are,

you're missing
the big picture, literally.

90% of our solar system
lies beyond the planets.

Out here, the sun could be mistaken
for just another star.

In this dark region
of the solar system, giants lurk,

their very existence only confirmed
by sightings of a few of them

that strayed into
the inner solar system.

They are the comets.

It was proposed that comets came
from a mysterious disc

out beyond the orbit of Neptune,
but no-one could prove it.

The Kuiper Belt was theoretical.
It was thought to exist,

but it was also thought that
it would be too hard to observe.

You're talking about objects
which may be 100 miles across,

but they're billions of miles away.

Trying to find something
that's much, much smaller

would be incredibly difficult,
and no-one thought you could do it.

In 1986, a team of astronomers

based at Hawaii's
Mauna Kea observatory

decided to prove them all wrong.

I'm a bit of a hoarder.

These are the notes that we took
while we were observing.

I used to write lots of things
in a notebook to try to keep track

of everything that was happening
on the mountaintop.

Here's the name of the telescope -
the 88-inch telescope on Mauna Kea,

also known as the Rust Bucket.

Visitors to that telescope
always think it's the ugliest thing
they've ever seen.

It's painted brown.
It looks like it's totally rusty.

Parts of it are rusty. So it's
a horrible-looking telescope,

but we used it for all of our work.

Jewitt's goal was to locate
an object beyond the planets,

an object that inhabited
the ghostly Kuiper Belt.

It was like searching for
a rusty penny

dropped somewhere
in the Pacific Ocean.

For six years they found nothing.

We spent many years looking for
objects in the outer solar system.

Lots of pain, frankly.
Lots of disappointment.

MICHELLE: You have to really have
some respect

for the scale of objects.
Yes, we can see stars

that are hundreds or thousands
of light-years away,

but they're these giant
glowing objects.

How do you see something
that's only 100 miles across?

And it's dark. It's not giving off
any visible light of its own.

That's why finding these things
has been difficult.

Kuiper Belt objects
are made mostly of ice...

..but they don't sparkle
like icebergs.

We're in a volcanic country,

so some of these icebergs
are actually dark, covered in ash.

An ash-covered iceberg
makes a good analogy

for a Kuiper Belt object.

I really wanted to get close to
this one,

but my guide's just told me
it's too big.

We can't get close.
It might flip over.

All right.
We've found a really cool one.

It's really dark.
Let's get over here.

So, you see how black
this thing is on the outside?

Well, comets are also black,
but actually,

they're even blacker than this.

We think that comets, typically,

are blacker than asphalt,
blacker than tar.

The ash that covers these icebergs
comes from a volcano.

But the ink-black skin
that surrounds comets

seems to originate from
its surface being cooked by the sun.

So where does this black come from,
if it's not ash?

Well, we now know that comets have
a lot of organic molecules -

that means molecules with carbon.
And when these things get blasted

by ultra-violet radiation
and cosmic rays,

they kinda get charred
and turn black.

So it's a lot like
you had a piece of meat

and you leave it on the barbecue
too long and it gets charred black.

It's the same thing
going on with comets.

With their jet-black coating,
comets and other Kuiper Belt objects

blend into the dark background
of space.

HAKEEM: The problem with studying
these objects

is that many of them are
very, very dark in colour,

they're very, very small,
and they don't emit their own light,

so they're just
very difficult to see.

It requires a very large telescope
to do so.

After a six-year search,

Jewitt and his team
spotted something,

and the solar system
would never be the same.

So, here's the log sheet, the notes
that we keep while we're observing.

29th August, 1992.

We started as normal,
looking at the images

as they came out of the telescope,
and found this first object.

It was a single tiny dot,
moving very slowly across the sky.

Its path showed
it was orbiting the sun

from much further out
than any of the planets -

an orbit that placed it
in the mysterious Kuiper Belt.

It was moving incredibly slowly
across the sky,

and therefore was incredibly
far away, and we knew -

just from the slow speed,
we knew immediately -

it was the most distant object
ever seen

in the solar system
up to that point.

Here's this note - "SMO?"
Slow-Moving Object, question mark.

So that's the discovery
of the Kuiper Belt, right there,

with those three
tiny little letters.

In an instant, the Kuiper Belt
was no longer just theory -

it was real.

It was ground-breaking,
because now we have this...

this whole part of the solar system
that we had predicted,
but had no evidence for.

Boom! Evidence.

The discovery of
the first Kuiper Belt object
was a clear indication

that there is this new population
of objects out there.

Basically, overnight we doubled
the diameter of the solar system.

The team later identified
several other Kuiper Belt objects.

They calculated
there must be billions

orbiting the sun in a vast disc.

The discovery of
the first Kuiper Belt object

was a fantastic achievement.
It was observational proof

that there was an entire population
of objects out in the solar system

that we thought might exist,
and then found out truly did.

It would be as big as discovering
another full-fledged planet.

The Kuiper Belt extends

50 times farther away from the sun
than the orbit of Earth...

..a vast ring

populated with
mountain-sized lumps of black ice.

The material in the Kuiper Belt

really is leftover building blocks
of the planets.

This stuff should have
formed into larger planets,
like the Earth or Jupiter,

but some of it
actually got thrown out

and it stayed that way,
small, little chunks of stuff,

remnants from the formation
of the planets.

The discovery of the Kuiper Belt

instantly inflated
the size of our solar system,

but deflated the status
of one member.

Because some of
the objects in the Belt

were bigger than our smallest planet,

Pluto was demoted to a dwarf planet.

I have really enjoyed
the controversy about,

"What do you call Pluto?
A planet? A dwarf planet?
Is it something in between?"

It shows people still care.
Pluto is still fascinating,

is still an important part
of our solar system.

But now we see it in the context
of its nearest neighbours.

Let's not get angry that Pluto's
no longer labelled a planet.

Let's get interested in the fact
that Pluto is a dwarf planet

and can teach us lots of things
about all of its new friends

we're discovering
year after year after year.

Now Pluto has
over 1,400 brothers and sisters,

called plutinos.

If Pluto is a planet,
all the plutinos are too, so...

I have a daughter.
It would be very painful for her
to try to remember all those names.

The discovery of the Kuiper Belt
opened up the solar system,

but it was only the start.

What has been discovered farther out
is even more remarkable.

PHIL: The Kuiper Belt extends about
50 times farther out from the sun

than the Earth's distance
from the sun. It's huge.

But if you think
the edge of the Kuiper Belt

is the edge of the solar system,
you're thinking way too small.

Far beyond the Kuiper Belt,

the sun has deployed
a deflector shield

around the solar system.

Our star is waging a cosmic war

against the biggest guns
in the galaxy,

and our lives depend on it winning.

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For a long time, we assumed

the solar system ended
where the planets ended.

But the discovery of the Kuiper Belt
in the '90s

doubled the reach
of the sun's influence,

and led scientists to ask -

could it reach further?

We now know that it does,
much further,

and we wouldn't be here if it didn't.

If you put a Geiger counter
in this room, it would click.

If you put a Geiger counter
in space, it would click a lot more.

Geiger counters detect a deadly form
of radiation called cosmic rays.

Cosmic rays are essentially
sub-atomic particles,

like the nuclei of helium atoms
and protons and electrons,

that are zipping through space
at almost the speed of light.

And these can travel across
great distances -

hundreds, thousands of light-years -
and it turns out

they hit us here on Earth.

Cosmic rays are blasted out of
exploding stars deep in the galaxy.

And they're incompatible with life.

They can rip out electrons.
They can, in fact,

change the chemical configuration
of cells, causing mutations.

They can, ultimately, kill you.

If the Earth were continually
blasted by cosmic rays,

without any protection,
it's not clear

that advanced forms of life
could survive.

Very few of the cosmic rays
aimed at us actually hit us.

14 million kilometres away,
a deflector shield is protecting us

from the full onslaught
of this deadly radiation.

The shield is powered by the sun...

..which fights the cosmic rays
by sending out

its own stream
of super-fast particles,

known as the solar wind.

When you think of wind, you think of
the air of the Earth

moving through your hair.
The solar wind is nothing like that.

It's actually a stream
of high-energy particles

coming off the sun -
things like hydrogen.

And it's substantial.
It's billions of tons every second.

Some parts of the solar wind
are going as fast as

about two million miles an hour.
It can pack quite a wallop.

The wallop blows by us here on Earth

because our planet's magnetic field
deflects it.

The solar wind doesn't just stop
at the Earth.

It's out in space. It keeps going.
It blows past the inner planets.

It blows past the outer planets -
Jupiter, Saturn, Uranus, Neptune -
and keeps going.

The solar wind rushes 5.5 billion
kilometres past the Kuiper Belt,

carrying the sun's magnetic field
with it,

creating a huge defensive shield
around the solar system,

called the heliosphere.

The solar wind is blowing a bubble
around our solar system.

The sun is like a big mother
that literally creates

a bubble of charged particles
and associated magnetic fields

that protect us from cosmic rays
coming from outside
the solar system.

The incoming cosmic rays
hit the outgoing solar wind,

and most of the harmful rays
are deflected into space.

Without this shield, complex life

might not have had the chance
to evolve on Earth.

Like everything else in nature,
the solar wind is a two-edged sword.

It can harm us here on Earth,
but it also protects us

from this galactic material.

There are so many instances
in the universe

where something you'd think would be
very dangerous and destructive

can even be protective, and
the solar wind is one such thing.

Yes, there are these high-energy
particles coming from the sun,

but the effect of the solar wind is
to protect our entire solar system

from even more dangerous
kinds of particles.

The protective bubble battles its way
through the dust and gas

as the solar system slowly orbits
the centre of the galaxy.

It's ploughing through it
like a boat moving through water,

and there's a wave where
the solar wind has slowed to a stop

as it's slamming into the material
outside of the solar system,

into interstellar space.

And it was here that NASA's
Interstellar Boundary Explorer,

or IBEX, found a completely strange
and unexpected phenomenon.

The IBEX mission actually found
a giant streamer of hydrogen gas,

the largest thing we've ever seen
in our solar system,

and this may be
due to the interaction
of our solar system's magnetic field

with the larger magnetic field
of the galaxy.

Scientists think that the interaction

between the sun's solar wind
racing out

and the interstellar medium
of the galaxy pushing in

traps the hydrogen particles
into a giant ribbon.

The ribbon is
a million kilometres wide

and three billion kilometres long...

..making it the biggest object
in our solar system.

It's fundamentally so exciting
to learn about

these extended structures
at the edge of the solar system

because no-one expected this.
No-one ordered this.

And so, by seeing things
that we weren't expecting before,

that's how we really learn
and how science progresses.

25 years ago,
we thought that Pluto was the edge

of, at least, the planets
in our solar system.

But now we know that there are many
more objects like Pluto out there,

so not only has the edge
of the solar system moved out,

but we've become much more aware of
how much more we have to learn.

The latest eyes in the sky

peer into the most distant regions
of our solar system.

We are just learning

about the vast array of objects in
the outer part of our solar system.

As our technology has increased,
we keep discovering

more and more exotic objects,

and our picture of
what the solar system really is

has changed as a result of
our ability to improve our eyes.

But it's old technology that's
actually taking us to the edge.

When NASA launched Voyager 1
in 1977,

they thought it might work
for five years.

37 years later,
the little space probe that could

is still sending us
mind-boggling information.

In August 2012,
18 billion kilometres from Earth,

it left the heliosphere and entered
the utterly mysterious heliopause,

the junction between
the furthest reach of the solar wind

and interstellar space.

JAMES: That heliopause is, sort of,
where those two boundaries meet.

It's where our neighbourhood,
kind of, stops

and the general
neighbourhood begins.

PHIL: I was excited
when the news came back

that Voyager was in the heliopause.

This is an entire region
of the solar system
about which we know almost nothing,

and for the first time
in human history

we had a robot probe
sitting in the middle of it,

measuring what was going on
around it.

It was ground-breaking.

For the first time, we can say,
even to a limited extent,

that humanity is
an interstellar species.

We are now out between the stars.

When this veteran probe took a look
back at the solar system,

it had quite a surprise for us.

Astronomers had thought
the heliosphere was shaped
a bit like a comet,

with a giant tail extending back
hundreds of billions of kilometres.

But Voyager 1 found something new
and extraordinary...

..not a comet-shaped heliosphere
with a giant tail,

but a crescent-moon shape
with two short tails.

It's a really important observation
because it teaches us something new.

It teaches us what the fundamental
shape of the heliosphere is.

A long time ago
we just had the simplest picture,

but now we have some detail,
we know it's got structure,

and we can start with that
and push forward

and begin a deep understanding
of the region around the sun.

The "tails" are actually jets
of energised solar wind.

The cause of these jets
is fundamentally the tension,

or the interaction,
between the background
magnetic field of the galaxy

and those streaming, fast,
charged particles

that are coming from the sun.

Voyager also sent back shocking news

about the very edge
of the sun's influence.

The heliopause
may not be a clear barrier.

It's not just a hard edge. It's not
just something really sharp.

It's actually
a fuzzy extended region.

It's hundreds of millions
of miles wide.

And right in the middle,

Voyager detected
something truly bizarre:

a sphere of giant magnetic bubbles.

We expected that
the solar magnetic field

would just, sort of, disperse
at that point,

but that's not really what happened.
What they found is that

the magnetic field lines
from the sun have tangled up,

creating a frothy, foamy
magnetic structure with bubbles.

That was quite a surprise,
and a very exciting one.

As the sun's magnetic field
ripples out into deep space,

it twists and tangles.

The lines criss-cross and reconnect,
forming giant magnetic bubbles.

Each foamy sphere/bubble thing
is about 100 million miles across,

which is tremendous.
That's the distance
from the Earth to the sun.

And there are thousands of these
surrounding the solar system.

No-one is sure
what these magnetic bubbles do,

but it's possible
that this cosmic bubble-wrap

is the first line of defence
against galactic radiation,

helping protect the solar system
from the cosmic onslaught.

JAMES: Think about how much
technology has changed

since Voyager was launched,
and today.

No-one had phones in their pockets.

I mean, no-one could even make
long-distance phone calls

without it being an ordeal.
But yet, with this old technology

we're now learning about
the very edge of our solar system.

Just imagine what we're going to
learn over the next 50 years.

Voyager is now beyond the heliopause,

but has it really left
the solar system?

Not really. There is still
plenty of solar system out there

for Voyager to pass through.

And it will still be going
for many centuries, many millennia,

before it leaves
the solar system proper.

Voyager may be outside
the range of the solar wind,

but it's not beyond
the sun's gravity.

If we go out,
we run into the Kuiper Belt

beyond Pluto's orbit, right?
This is very, very far away.

And then, even beyond that,
we have the heliopause,

the area of the sun's
magnetic field.

So you might think you're at the end
of the solar system. But guess what?

There's still stuff out there

that's gravitationally bound
to our solar system.

Every once in a while,

one of these objects falls
towards the sun -

and sometimes
Earth gets in the way.

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Deep in the outer reaches
of the solar system,

far beyond the reach
of the solar wind,

we are blind.

Even the most powerful telescopes
are unable to discern

what lurks in the darkness.

But sometimes, mountain-sized rocks
appear from beyond.

These potentially
life-threatening wanderers

are the long-period comets.

There's basically
two kinds of comets.

There's comets that are called
short-period,

and the other kind,
called long-period.

Short-period comets
come from the Kuiper Belt,

the disc of space junk
just beyond the planets.

They travel around the solar system
in relatively small circles,

returning at regular intervals
of 200 years or less.

Long-period comets
are more mysterious.

The long-period comets
can come back around

something like every thousand years
or even million years,

and unlike the short-period comets
that come in on sort of circles,

the long-period ones come in
on really plunging orbits,

and come in from all over the place
in all different directions.

These comets are on oblique,
elliptical orbits.

Starting at the edge
of the solar system,

it can take thousands of years
for them to reach us.

By the time they do,
they've picked up a lot of speed.

These things can move as fast as
100 miles a second

when they get close to the sun,
so just imagine that.

100 miles in a second.

Comets travel fast
because they're falling,

dragged in by
the sun's immense gravity.

And the longer and farther
something falls,

the faster it goes.

Physicists like to talk about
gravity wells,

and the sun is at the bottom
of a deep gravity well.

A skater at the top of the ramp,
before he falls down into the pool,

is going slow, but by the time
he gets to the bottom

he's going as fast as he's gonna go.
It's the same way with the sun.

The comet is plunging
into the very depths

of the sun's potential well.

When it gets very close,
it's going extremely fast,

and as it comes back out,
it's going slow,

just like a skater
at the top of this pool.

Their orbits are a clue
to the comets' mysterious origin.

The long-period comets, we think,

are coming from someplace
much, much farther away,

and it's more like a spherical cloud
of very distant things

that are now plunging down
into the sun every now and again.

When a comet comes in,
the sun heats it up.

The ice turns into a gas,
and the comet loses mass.

Over time,
these comets will disappear -
and yet they keep coming.

That means that there must be
a reservoir of them out there.

The question is

where are they coming from?

One of the first explanations
for these strange comets is,

"Maybe they're not
from our solar system.

Maybe the galaxy is full of
these icy chunks

that occasionally rain down on us."
But then we thought,

"Maybe they come from
another part of the solar system."

Perhaps, at the edge
of the solar system,

there's debris left over from
the very first days of its creation -

a hypothetical dumping ground
known as the Oort Cloud.

PHIL: It was theorised that
there must be a reservoir of them,

a giant cloud surrounding
the solar system

with millions, billions, maybe even
trillions of these icy bodies.

But we've never seen it.

So this Oort Cloud,
this theorised population of comets,

is that - it's theoretical.
It's never been directly observed.

The objects in the Oort Cloud are
so far away and so small and dark

that we've never observed something
actually out in the Oort Cloud.

Something has to fall in.

So you see things falling in
from all of these different angles.

From there, you can deduce
what must be out there.

It turns out that deducing
what the Oort Cloud might look like

is a staggeringly complicated task,

requiring one of the world's
most powerful number-crunchers.

Luckily, Kevin Walsh has
just the machine for the job.

Here we are in the guts
of a modern supercomputer.

On these racks are thousands of
processors running day and night,

analysing computer models
to understand

the evolution of our solar system
and what's out there at the edge.

Meet the Janus Supercomputer
in Boulder, Colorado.

In supercomputers like these,
there are thousands of processors,

tens of thousands
of computing cores.

You can do over 100 trillion
calculations per second

in a machine like this.
And we need this,

cos we're trying to understand
the evolution of

hundreds of millions of comets
over billions of years.

Compared to what I can do with
a pen and paper, it's no contest.

Using the supercomputer,
astronomers can begin

to piece together a picture
of the giant, invisible Oort Cloud.

And they start with
what we can see -

the comets that race in
toward the sun.

We never see the Oort Cloud
until a comet comes by,

so we need to estimate
how frequently

one of them is going to be perturbed
in just the right way

that'll make this close passage by
the sun so that we can observe it.

We can then work backwards and say,
"Well, we see about one

every ten years
of about this size,"

and then say,
"Therefore, there must be

billions and billions of these
making up that Oort Cloud."

Calculations indicate the Oort Cloud
is home to two trillion icy objects

that extend out
far beyond the heliopause.

PHIL: It's hard to grasp just
how far out the Oort Cloud extends,

but the edge of it probably is

10,000 times farther away
from the sun than the Earth is.

A trillion miles -
a 1 followed by twelve zeroes.

It's so big, it is so vast,

so much larger
than the solar system of planets,

that if you removed
all the planets from the sun,

the Oort Cloud wouldn't even notice.

So how exactly did the cloud's
icy bodies get out here?

A fun way to think of the Oort Cloud
is that these are pieces of planets

that never got used.
They really are leftovers.

CAREY: The Oort Cloud
actually didn't come with
the original package, if you will.

The Oort Cloud formed when
they tried, but miserably failed,

to join the party
and become those giant planets,

and instead got thrown
in all different directions,

and they now exist
halfway to the next star,
a big, giant spherical distribution.

The sun burst into life
4.6 billion years ago.

A cloud of dust and gas
circled the infant star.

Then gravity started to suck in
gas, dust, ice and rocks

to form the infant planets.

The planets formed
out of building blocks,
smaller chunks of material.

Rock, ice...

..stuff that came together
over time.

The frenzy of swirling,
colliding matter

left behind debris
as rocky rubble in the asteroid belt

and ice in the Kuiper Belt.

Then Neptune and Uranus
scattered the debris out further.

The thing about what happens when
you have bodies falling together,

all kinds of small rocks
under the influence of gravity,

some of that will get slingshot
around the planet

and thrown out into space.

Most of them got thrown
out of the solar system for ever,

but the ones that didn't
go on this big, long,
million-year looping orbit,

and that's the Oort Cloud.

Trillions of icy rocks were ejected.

Some fell into the sun,
some flew into outer space,

and some formed the Oort Cloud.

Oort Cloud comets are dinosaur bones
of solar-system formation.

Contained in them
are the ingredients
that went up to make our planets.

They are a time capsule
of what the solar system was like

4.5 billion years ago.

Most of the Oort Cloud objects
will stay in this icy cloud for ever,

orbiting the distant sun.

But a few get nudged loose,

and the sun's gravity
pulls them inwards

like moths to a flame.

If such a comet
could sneak up on you,
you won't even know it's there.

These comets can be
many miles across,

and they're moving
extremely rapidly,

sometimes as much as
100 miles per second.

This makes them pretty dangerous.
How dangerous? Ask a dinosaur.

Some people think that the massive
impact that killed off the dinosaurs

may have been caused by
a huge Oort Cloud comet.

These comets are potentially
much more dangerous

than the asteroids usually blamed
for extinctions on Earth.

Comets are falling
from so far away

that in general, they're moving
faster than asteroids,

and their impact energies
are that much larger.

Even something as small
as a mile or two across

could cause a lot of hurt,

but a lot of these Oort Cloud comets
that come into

the inner solar system
are very large, and you can imagine,

if one of these things is
30 or 50 miles across,

this would be a global event.

Understanding the Oort Cloud better
could help us predict

if one of these comets is on the way.

Some have suggested
that the Oort Cloud hides

the solar system's
best-kept secret,

something that's responsible
for hurling these high-speed comets
at us -

a sinister twin to our own sun,

a second star called

Nemesis.

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Our theoretical knowledge
of the solar system

extends 14 trillion kilometres
past the planets,

but at this distance
we're working in the dark.

Carl Sagan said that
we're in the age of Magellan,

we're not in the age of Columbus.
What does that mean?

That means that we know the basic
outlines of our solar system,

we know the planets, we know there's
a Kuiper Belt and an Oort Cloud,

but we don't know the details.
We haven't actually visited
these places

and we don't know the geography.

So we're really filling in the map
right now.

We've worked out that there are
around two trillion objects
in the Oort Cloud,

a sphere of junk left over from
the formation of the solar system,

and that every now and then

one of those objects
comes hurtling in towards us.

A big question is why would one
suddenly decide to come in?

What is perturbing 'em?
What's poking 'em

so that they fall in
and approach the inner solar system?

Well, there are a lot of ideas of
what it could be.

There may be clues
in the series of catastrophes

that nearly wiped out life on Earth.

There appeared to be,
in the fossil record,

a periodic series
of mass extinctions

roughly every 26 million years.

It seemed that something was jostling
the Oort Cloud periodically,

spraying comets towards us
every 26 million years -

but what could it be?

MICHELLE: When we look out at stars,

we notice that most of them
occur in binary pairs.

In some cases,
they can be very far away.

They can take millions of years
to orbit each other.

So the question seemed
sort of natural -

does the sun have a companion
that we haven't found yet
because it's so far away?

CAREY: It's fascinating
to think about

whether our sun may have
a brother or sister currently,

cos we know that more than
half the stars in our galaxy,

100 billion strong, are in binaries
or trinaries or quaternary systems.

So, actually, a lone star
is a little exceptional.

If the sun does have a twin,
it could be responsible

for disturbing the Oort Cloud.

What would happen is that
every 26 million years,

this binary star partner
would pass through the Oort Cloud,

upset orbits and send comets
streaming in all directions,

and some of them would come
into the inner solar system,

strike the Earth,
cause a mass extinction event.

And the name of
this really dangerous binary star,

partner of the sun,
was Nemesis. Ooh!

The Nemesis theory has its problems.

Certainly, the gravity
of a mystery star

passing through the Oort Cloud

could periodically send comets
raining down

towards the sun and Earth.

But if our sun has a companion star,

it should be visible.

So, you'd think, of course,
this Nemesis star should be
very easy to see,

because, after all,
stars are bright.

But the idea here is
maybe Nemesis is a dwarf star.

Maybe it's a brown dwarf.
So it'd be really small,

and therefore it would not be
very bright.

A brown dwarf isn't a star
or a planet.

It's something in between -
a failed star that never got started.

Stars ignite when gravity crushes
vast clouds of hydrogen gas together,

generating huge pressures
and temperatures.

If there's not quite enough gas,

then there's not quite enough gravity
to light a nuclear fire.

LAWRENCE: So it doesn't
become a star.

On the other hand,
it's much bigger than Jupiter,

which is only 1/1000
the mass of our sun.

These objects heat up, but never

go boom like stars do.

They're difficult to see
because they're very dim,

but they could contain
a significant amount of mass.

A brown dwarf that periodically
skirted the edge of the solar system

and nudged comets towards us
would be too dim to see...

..at least, not without night vision.

With infra-red telescopes, you can
pick up the heat of something

even if it's not giving off
any light.

So something like a brown dwarf,

even though it's not glowing
in visible light,

should be giving off a lot of heat.

PHIL: There have been missions
which have surveyed the whole sky,

and if there had been
a four-billion-year-old brown dwarf

anywhere near the sun,
it would have been spotted.

So far, no Nemesis has been found.

So if there's no Nemesis star
out there,

is something else
messing with the Oort Cloud?

Another idea is there could be
a relatively massive planet -

Earth-sized, for example -
orbiting way out past Neptune.

Detecting a planet that far away

is much more difficult
than detecting a dwarf star.

The farther a planet is
from the sun,

the dimmer it is
and the harder it is to see.

So you could have a planet
the mass of the Earth,

several billion miles out
from the sun,

and it would be basically invisible
to our telescopes now.

There is still a chance
that something the size of the Earth

might be out there, far away in
the Oort Cloud, disrupting things.

That's the last gasp
of the Nemesis theory.

We may never know what's nudging
Oort Cloud objects out of orbit,

and it's not the only mystery
out there.

We still don't know
where the solar system ends.

Finding the edge of the solar system
helps us with the bigger problem

of understanding the solar system
as a whole.

How do you know you have the whole
system until you've found the edge?

But perhaps
the biggest mystery of all

is that our solar system,
with a medium-sized star
at its centre,

seems to contain far too many objects
in its outer reaches,

which raises the prospect
that many of these icy lumps

originated from elsewhere
in the galaxy.

Theoretically, we can calculate
how many comets should have formed

when the solar system itself formed,

then look at the number of comets
coming in from the Oort Cloud

and try to figure out
how many are actually out there.

It turns out
the two numbers are way off.

The number that formed with the sun
is much, much smaller

than the number of comets
we infer are out there.

There's simply too much material
in the Oort Cloud

to belong to one average-sized star.

The only way we can balance
these two numbers

is if the sun, when it was young,
was a thief.

Five billion years ago,
a supernova exploded,

sending a shockwave through
a vast cloud of dust and gas.

It compressed the cloud,
creating regions of dense gas...

..a star nursery.

Hundreds of pockets of gas
collapsed under their own gravity,

giving birth to
hundreds of sibling stars.

We think our sun formed with
brothers and sisters in clusters,

and the sun, we think, formed in
a cluster with 400-500 other stars.

If you can picture
these stars forming,

they're very small compared to
the distance between them,

but their own cloud of comets
is very large.

So it's possible
to steal comets from each other.

Well, the sun was smart,
and walked away when it was ahead.

It got a bunch of comets
from other stars

and then went off on its own way.

MICHELLE: Some of the stuff
orbiting us right now, far away,

could have come from
those other young stars.

This cosmic sibling rivalry
may still be happening.

Our sun could be stealing objects
from nearby stars

while they are stealing
from our Oort Cloud.

The nearest star that our Oort Cloud

butts right up against
the solar system of
is Alpha Centauri.

There's probably another Oort Cloud
around Alpha Centauri.

Maybe there are objects that even
switch between the two stars.

So halfway in between
these two stars

is where we find
the true edge of our solar system.

What happens at the edge
could spell disaster.

Alpha Centauri could have
a real effect on life here on Earth.

It could be that that star

and the objects around it
in its Oort Cloud

have some gravitational effect
on our Oort Cloud.

When something falls in,
that may have been due to
the influence of our nearest star.

So it's hard to know
where our solar system ends

and a new one begins.

We humans love hard edges.
We like boundaries.

But the solar system...
it's not like that.

It just fades away. It blends into
the galaxy at large.

The sun does not live alone,
and nor does our Oort Cloud.

We're interacting continuously,
and in that way we're connected

to the much larger environment
of the Milky Way.

This no man's land
could help us travel to the stars...

..because where our solar system ends
we might find stepping stones

that could be the launch pad
for a new interstellar journey.

One interesting thing
about knowing, now,

the structure of our solar system

is that there are many more objects
that we can use for resources

as we venture out
into interstellar space.

There's rock out there. There's ice.

We might be able to replenish
our spacecraft as we go past.

Maybe something as distant
and tenuous as the Oort Cloud

will be our next jumping-off place
to truly explore the galaxy.

We have much to learn
about the solar system,

with many new surprises to discover.

LAWRENCE: Every time we open
a new window on the universe

we're surprised,
and that's what excites me.

Every morning,
I'm surprised if I'm not surprised.

And as new technologies open up,

so too will our understanding
of what's out there.

We're building better telescopes.
We're sending probes out into space.

We have so much more to learn,
and that's the real exciting part.

Our new eyes to the universe

are showing our solar system
in a whole new light.

Slowly, we're piecing together
the complex connections

that bind it to our lives
and to the universe beyond.

Every new discovery we make
about the universe has told us

that no solar system is an island,

that material comes from outside
and comes inside

and from inside goes to the outside,

and so as we probe
the outer edges of our solar system

we're changing what we mean
by "solar system".

We are taking the first baby steps
off the planet

and starting to look
around our environment,
and that's pretty neat.

PHIL: The sun is like
one voice in a vast chorus

of hundreds of billions
of other stars

and their particular solar systems.

And I find it very poetic
that, in fact,

we are part of a much larger
population out there.

To think that we've found
everything in our solar system,

to assume we know everything,
is not the way science works.

Science works
by pushing the frontiers,

and we continue to do so,

and in ten years,
if we do this programme again,

I wouldn't be surprised
if we talk about new things.