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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.