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NARRATOR: Our solar system is filled with mysterious worlds...

...wandering between and beyond the planets.

[collision pounds]

MALENA RICE: The objects that lie between the planets

are actually the key to understanding

how our solar system formed.

NARRATOR: Imposters, and oddballs,

rewriting our understanding of our place in space.

JESSICA SUNSHINE: It is two pieces that are stuck together.

It was really quite remarkable.

[explodes]

QUEENIE HOI SHAN CHAN: It is possible that after 30 million years,

Mars might have a ring of its own.

♪

NARRATOR: Studying these wandering worlds

allows scientists to explore regions of the solar system

we have no chance of visiting.

WANDA DIÍAZ MERCED: I cannot go to the asteroid belt,

so when a meteorite lands on Earth,

we will have a chunk of history right in our hands.

ASHLEY KING: The Winchcombe meteorite

was like a little treasure box for planetary scientists.

[geyser bursts]

NARRATOR: And the misfits of our solar system

are full of surprises.

VERONICA BRAY DURFEY: There are many, many worlds out there

that we are yet to discover

and that we haven't even imagined yet.

SCOTT SHEPPARD: It's like a box of chocolates.

You never know what you're gonna get in our images.

This model of the solar system,

it's missing all of the most interesting bits.

NARRATOR: What secrets do these wandering worlds

reveal about our solar system?

And what else is lurking out there

in the dark?

[bursting]

"Solar System: Wandering Worlds,"

right now on "NOVA."

♪

♪

NARRATOR: There's something out there in the darkness.

We don't know for sure where it came from

or how long it's been there.

But we know it's not alone.

It was discovered in 2018.

A world with no official name.

So distant, it has been called simply

FarFarOut.

SHEPPARD: FarFarOut,

as its name implies, is very far away.

So, it's just a, a very faint point of light

that, uh, we discovered

with one of the largest telescopes in the world.

JANE LUU: Its notoriety comes from the fact

that it's the most distant object

that we have found in the solar system.

SHEPPARD: And the big question is, what do we call the next one?

Do we just add another "far" or not?

LUU: "ExtremelyFarOut," and then "StupendouslyFarOut,"

and it just keeps going. [laughs]

♪

NARRATOR: 12 billion miles from the sun,

FarFarOut is not alone.

Out here, there could be countless other worlds

yet to be discovered,

wandering in the dark.

Even closer to home, we are only just beginning

to explore the vast spaces between the planets.

Home to a myriad of worlds,

many too small or too dark to see.

NANCY CHABOT: When we think about our solar system,

we think about the sun, it's in the center,

everything else is kind of going around it,

and we've got the big planets, Jupiter and Saturn,

and the most important planet, Earth.

But there are so many more secrets and mysteries

than just these planets that are shown in the model.

So, to me, the most exciting thing

about studying the solar system is not the planets themselves,

but it's all the bits in between the planets.

NARRATOR: These worlds could provide clues to our own planet's story.

RICE: In a lot of ways, studying the solar system

is a way of trying to understand ourselves.

How did we come to be in the first place,

and what does it mean that we're on this rock

that is sailing through space?

NARRATOR: With distances so vast, there's still much to discover.

KING: There's so much stuff

that we just don't know about our solar system,

and what we do know is, it is constantly surprising us.

So we need to go out there, look at these wandering worlds.

NARRATOR: And many questions remain unanswered.

LYNNAE QUICK: Are these lost and wandering worlds

worthy of exploration in their own right?

CHABOT: How far do these dark regions extend

beyond, to the edge of our solar system?

Are there more planets in the dark region

waiting to be discovered?

♪

[dog barking in distance]

NARRATOR: It was the sound that most people noticed.

[explosion echoing in distance]

A sonic boom in the night.

But this was no fighter jet.

Traveling at around 30,000 miles per hour,

it tore through the atmosphere.

♪

Broke up.

And rained down across the fields.

A visitor from a distant realm.

One of the biggest chunks

landed on a driveway in front of a house.

KING: When I first saw it, it was kind of a splat.

I mean, it's so soft, it kind of just made a powder

and it threw fragments all over the driveway.

SARA RUSSELL: The Winchcombe meteorite fall

was the first U.K. meteorite fall recovered for 30 years,

so we've been waiting such a long time.

KING: I couldn't believe that we had a new meteorite in the U.K.

Yeah, it was incredibly exciting.

And all meteorites are scientifically priceless.

RUSSELL: I feel amazed and privileged

to be able to hold something

that may tell us about thesecrets

of the origins of our solarsystem,

and how we got to be here.

NARRATOR: It's estimated as many as 50 meteorites hit Earth every day.

Many burn up in our atmosphere,

creating shooting stars and meteor showers.

But some of the largest ones do make it to Earth.

In 2018, the Hamburg meteor fireball

streaked across the American Midwest sky.

WOMAN: What was that?

Did you see a light flash out there?

It's particularly pristine, because it landed

on a frozen lake, so that keeps it in cold storage.

It was like the meteor selected where to land,

because it wanted to be studied.

NARRATOR: A few years later,

a meteorite the weight of a grand piano

exploded in the skies near McAllen, Texas.

[explosion pounds in distance]

MERCED: Ah!

It's like a, like an explosion of dynamite.

[explosion echoes]

That big explosion, that was a sonic boom.

♪

NARRATOR: The cows weren't the only witnesses

that night in Winchcombe.

Doorbell and CCTV cameras never sleep.

And networks of dedicated meteor cameras

captured it from different angles.

That meant it was possible to calculate its trajectory.

[cows mooing]

Back over the skies of England,

out of Earth's atmosphere,

past the orbit of Mars,

into the dark.

This is where the meteorite's journey began,

a mysterious realm of countless rocky worlds,

rubble left over from the formation of the solar system:

asteroids.

There could be at least a million out here.

But they're so dark, reflecting little light,

they're incredibly difficult to detect.

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

[collision pounds]

When two collide,

they blast fragments in all directions,

sending some out of the asteroid belt forever.

Sometimes wreaking havoc on planets they encounter.

♪

Including our own.

And asteroids that leave the belt

can transform planets nearby.

♪

A planet covered in scars from the asteroid belt is Mars.

Around 80 fresh impact sites are found on Mars every year.

♪

And some amazing meteorites

have even been snapped by NASA's rovers.

But there's another dark rock here,

imaged by the Perseverance rover,

not on Mars's surface, but orbiting overhead...

♪

...captured during an eclipse,

as it passed in front of the sun.

This is Mars's moon Phobos.

Today, the orbiting probe Mars Express

regularly flies between Mars and Phobos.

♪

Its high-resolution camera

captured incredibly detailed images of Phobos,

showing a surface covered in strange grooves.

Phobos looks like someone has ridden a giant bike all over it.

Astrophysicist Sean Raymond is investigating

how objects like Phobos could have formed.

Phobos is a strange-lookingmoon.

Like this beach, it's covered ingrooves.

You might say it's

the grooviest moon in the solar system.

NARRATOR: The magnificent grooves here in Spain

are formed by geological processes

and erosion from relentless, crashing waves.

RAYMOND: It's so cool-- the grooves are amazing here.

NARRATOR: But Phobos isn't anything like Earth,

so what's going on?

RAYMOND: Phobos certainly looks like an asteroid.

It's covered in craters, and it's dark in color.

So you might think it's just an asteroid that got

too close to Mars and then was captured in orbit.

NARRATOR: It turns out it's not that simple.

Phobos orbits Mars in an almost perfect circle,

right along Mars's equator,

and this makes us question whether Phobos

is really the asteroid it appears to be,

because captured asteroids usually orbit planets

on very elliptical orbits

that sometimes take them very far from their planet

and sometimes being much closer.

NARRATOR: So, if it's not an asteroid,

where did this funny little moon come from?

CHABOT: So, in planetary science,

the joke is that you just add an impact event,

and an impact event can explain everything.

That might actually be true in this case, though.

It's this one-off event, and it might have been

that an object hit Mars,

and Phobos is the result of that impact collision

between those objects, and that's what we're seeing.

♪

NARRATOR: Phobos may not be a captured asteroid at all,

but formed from debris thrown into orbit

after a massive asteroid impact.

However, this still doesn't explain

why it's covered in grooves.

Phobos is about 3,700 miles from Mars's surface,

and every hundred years, moves closer by about six feet.

And as it does, the tidal forces caused by Mars's gravity

become stronger.

RAYMOND: So, just like the moon pulls on the Earth,

dragging the oceans and causing the tide,

Mars pulls on Phobos, causing tides, as well.

So, as you can see behind me right now,

the tide is coming in as the moon is pulling on

the water on the Earth.

The same process happens on Phobos, due to Mars,

except on Phobos, it's sand and rock that's moving,

instead of water.

NARRATOR: Those tidal forces are pulling Phobos apart.

And with every orbit, the grooves widen and deepen.

Eventually, Phobos will drift so close,

Mars's gravity will destroy it.

As Phobos breaks 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,

giving Mars a wispy ring.

CHAN: It would be quite impressive.

I love ring planets, I love Saturn,

and it's not bad to have another ring planet in our solar system.

NARRATOR: It seems Mars will one day be transformed

by the asteroid belt.

But the asteroid belt itself is still an enigma.

We've flown several spacecraft

through this mysterious region,

but only one has orbited the rocks

in the asteroid belt itself:

NASA's Dawn probe.

Most of these ancient remnants are misshapen boulders.

But one stands out.

Ceres is much bigger than the others

and almost perfectly spherical.

QUICK: I joined the Dawn mission

right after it reached Ceres,

and it was so exciting.

Dawn collected thousands of images of Ceres.

They showed us areas of the surface

that we'd never seen before.

We thought that it's kind of

a dead planetary body.

Looking at those images,

we could not, basically, believe our eyes,

and it was, like, "Uh, what's going on over there?"

♪

NARRATOR: At first, it appears dark and heavily cratered,

like its fellow asteroids.

But Ceres is different.

Its surface is peppered with white crystals.

But what are they?

Clues come from another planet in our solar system,

where white crystals are also found on the surface.

Earth.

HAKEEM OLUSEYI: This mountain is made up almost entirely of sea salt,

the exact same stuff that you put on your food

at the dinner table.

NARRATOR: 40 million years ago,

this whole area was under an ancient sea.

Over time, it dried out,

leaving behind a layer of salt a mile thick.

Holy moly!

Whoa!

♪

It's definitely salty.

And the thing about salt deposits like this

is that they're only formed in the presence of water.

So, that raises an intriguing possibility.

Could the white spots on Ceres also be salts

that were deposited by water?

NARRATOR: Finding water on Ceres today

would be an astonishing discovery.

♪

Dawn made tight orbits of Ceres.

And by bouncing light off the crystal deposits,

it determined what they were made of:

sodium carbonate, a common type of salt.

A tantalizing sign that Ceres,

which is 13 times smaller than Earth,

had an ocean in the past.

Flying just over 20 miles from the surface,

Dawn finds something else in the white spots:

hydrohalite crystals,

another salt found on Earth in the presence of water.

Could Ceres still be a water world today?

So, "hydro" is water and "halite" is salt--

table salt.

So, if you imagine a table salt

that's got water molecules in the structure,

that's what hydrohalite is.

It was very surprising that there were hydrohalites

on the surface of Ceres.

CHAN: Hydrohalite would be very unstable

at the surface of Ceres.

At that pressure,

the water molecule would have been vaporized.

That implies that water was recently emplaced on Ceres,

bound to salt crystals within the past few hundred years.

This replenishment of hydrohalite

at Ceres' surface leads us to wonder

if Ceres might be an ocean world

that's masquerading as an asteroid.

♪

NARRATOR: Scientists suspect that pools of salty water exist

about 25 miles underground,

raising another question:

how did these salts end up on the surface?

A clue lies in Ceres' location,

right in the heart of the asteroid belt,

where collisions are common...

[collision pounds]

...and incoming asteroids can form cracks in the surface

reaching the pools below.

Exposed to the vacuum of space,

the water vaporizes,

leaving its cargo of salt behind

and peppering Ceres with white spots.

QUICK: There's still one big twist

to the story of Ceres' past.

The Dawn spacecraft found ammonia salts on the surface.

NARRATOR: Ammonia salts could be traces of ammonia ices

that once covered Ceres' surface.

Frozen ammonia is common in colder parts

of the solar system, but not here.

CHAN: Ammonia doesn't really exist as ice at where Ceres is now.

It has to be formed at a distance

further away from the sun,

where the distance is cold enough

for ammonia to exist as ice.

This suggests that Ceres didn't form

where it stands currently in our solar system.

NARRATOR: Instead, scientists think it formed much farther out,

and as it wandered towards the sun,

the ammonia ices melted,

leaving behind the ammonia salts we see today.

An imposter lurking in the asteroid belt,

exposed by its salty secrets.

Ceres has a journey of relocation

just written all across the surface.

NARRATOR: But just how did Ceres end up where it is today?

OLUSEYI: The best explanation is that it wandered.

Well, actually, it was pulled.

So let's say this is Ceres.

Okay?

We think that Ceres formed beyond

what is known as the ice line

for ammonium ices.

NARRATOR: Beyond this ice line,

ammonia freezes,

and this is where Ceres likely started forming,

alongside Neptune and Uranus.

OLUSEYI: And here we have Saturn and Jupiter.

Now, normally, these planets

aren't lined up like this.

They're moving around the solar system.

And what happens is, as Ceres

orbited the solar system,

Jupiter tugged on it with its gravity,

and Ceres ended up right there,

in the asteroid belt, where we find it today.

NARRATOR: In the heart of the asteroid belt,

Ceres, once a much icier world,

has migrated far from where it formed,

evidence that our solar system is ever changing.

♪

Ceres isn't the only displaced world.

Farther from the sun, there is another icy world

that doesn't seem to belong where it's found today.

Beyond Jupiter,

the space between the planets gets wider

and wider.

And temperatures plummet.

♪

And a billion miles past Uranus,

we finally reach the farthest planet from the sun.

Neptune.

♪

Wrapped in a dense blue blanket,

Neptune has no detectable surface,

with clouds of methane and ammonia.

But there are rocky worlds nearby.

Neptune has at least 16 moons.

And one is very unusual.

Triton is the largest of Neptune's moons.

Its surface is coated with ice.

And, unlike the other moons,

Triton is an active world.

[geyser bursts]

Geyser-like plumes of gas and dust

stretch five miles high into Triton's atmosphere...

...which flattens them abruptly

by 90 degrees,

creating a vista so strange,

it's hard to believe it's real.

How did a moon 2.8 billion miles from the sun

become so active?

A clue lies with how the planets and moons

move around each other in the solar system.

CARLY HOWETT: Every day, the sun rises in the east

and sets in the west.

And the moon follows the same course.

And there's a reason for that.

NARRATOR: To understand why,

we need to go all the way back.

To the birth of the solar system.

DURFEY: In the beginning of our solar system,

the sun was surrounded

by a disc of dust and gas.

And it was within this spinning disc

that the planets formed.

PAREKH: So that's the reason

the planets and the moon continued

orbiting also in the same direction

around the sun.

NARRATOR: Moons that form around their planets

tend to follow this pattern,

orbiting and spinning in the direction

of the planet's rotation.

But while the inner moons orbit Neptune in the same direction,

Triton goes the other way,

suggesting it didn't form alongside Neptune,

but came from elsewhere.

HOWETT: What a planet's made from

can tell you where it formed in our solar system.

And that's because, in our early solar system,

the ingredients that a planet could be built from

varied as the distance from the sun.

So if we take Earth, for example,

it's got a lot of rocks and metals, too:

aluminium and iron.

And Mars is prettysimilar.

Iron, metals, and other rocks.

So these can be paired together.

If we do the same thing

for the ice giants, Neptune and Uranus,

with big atmospheres made of hydrogen,

helium, and a bit of methane,

they're huge, too.

So we can pair those together.

So how does Triton fit in?

Well, its surface is mainly nitrogen ice,

and it's much smaller.

It doesn't fit with either of these two groupings.

To understand where it could fit,

we have to look at Pluto.

Pluto has a, a nitrogen ice surface with methane,

carbon monoxide, and water ice.

And it's about the same size as Triton, too.

These are a good pairing.

NARRATOR: With such similar ingredients,

Triton and Pluto could've formed in the same place,

but they aren't anywhere near each other today.

HOWETT: Triton's located one billion miles from Pluto.

So how did that happen?

NARRATOR: Neptune might be the farthest planet from the sun,

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

Not even close.

Almost a billion miles farther 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, doughnut-shaped ring of icy bodies

billions of miles wide.

So dark and distant,

it remained undiscovered until 1992.

LUU: We had been searching for, uh, the Kuiper Belt

for, uh, for five years before we finally found it.

So when we spotted the first Kuiper Belt object,

1992 QB1,

there was jumping up and down, and there was...

I think we gave each other a high five. [laughs]

The discovery of the Kuiper Belt

helped us to better understand our solar system

as it is now, and also its history.

LUU: The Kuiper Belt is really the frontier

if you want to understand the solar system.

But it is very difficult to study it,

because it is so far away.

NARRATOR: Despite the challenges,

more than 4,000 Kuiper Belt objects

have been discovered to date.

And scientists estimate

there may be 200 dwarf planets.

Some have rings and moons.

Some are bizarre and misshapen.

But they all share one thing in common.

They're made of icy materials, similar to Triton.

This is where Triton belongs.

So how did it end up around a billion miles away,

in orbit around Neptune?

♪

HOWETT: So, to understand Triton,

you have to understand the Kuiper Belt.

If this is our sun,

the one population of Kuiper Belt objects

orbit it in a nice circle.

Another population of Kuiper Belt objects

have a highly elliptical orbit.

So the question is, why are these so different?

What caused them to be in this weird orbit?

Well, the answer is, some of these objects

are in resonance with another planet, Neptune.

Resonance is when two objects have paths

that meet up occasionally.

And we see this throughout the solar system.

If you look, for example, at Pluto,

it'll go around

on this elliptical orbittwice

in the same time Neptune has gone around three times.

And this resonance is an important clue

in understanding how disruptive

Neptune is to Kuiper Belt objects.

♪

NARRATOR: Scientists think Neptune formed

much closer to the sun,

then slowly drifted out.

Its huge gravity disrupted the orbits

of the Kuiper Belt worlds it encountered,

kicking them into the elliptical orbits

we see today.

DURFEY: The early solar system was complete chaos,

filled with small bodies growing larger

and smashing into each other,

sometimes destroying.

Complete chaos.

NARRATOR: During all the commotion,

Triton became trapped by Neptune's gravity,

slipping into its backward orbit.

And being this close to a giant has consequences.

Just as our moon raises tides on Earth,

Neptune raises tides on Triton,

stretching and squashing it like a stress ball,

heating it up.

HOWETT: Triton being captured by Neptune

fundamentally changed how it works.

On Earth, we experience energy every day.

You might be able to hear it in the roar

that comes from the tide coming in underneath us.

[waves pounding]

Tidal energy can force water up through cracks

in the Earth, like the one in front of us,

creating plumes erupting up to the sky.

NARRATOR: On Triton, the plumes are even more spectacular.

HOWETT: The plumes on Triton would be magnificent to behold.

Erupting from the surface, cracks like this,

but going five miles into the sky.

I mean, it'd just be absolutely phenomenal.

[geyser bursts]

NARRATOR: What creates Triton's plumes is a mystery.

But one theory lies in its unusual orbit.

HOWETT: It's highly tilted, and Triton orbits backwards.

Meaning as it goes around Neptune,

its pull and push that it receives

from Neptune and its moons changes.

This creates a kind of friction,

similar to the one that you get

when you rub your hands together on a cold day.

Your hands warm up, and so might Triton.

We call that kind of energy tidal heating.

NARRATOR: As Neptune's gravity stretches and squashes

its giant moon...

[plume roaring]

...the tidal heating produced

melts Triton's frozen interior...

♪

...powering its spectacular plumes.

DURFEY: Triton is a great example

of how interconnected our solar system is.

And it also serves as a goodexample

of how a large, giant planet

straying into a belt of objects

can really mess up your system.

NARRATOR: Triton was plucked from the Kuiper Belt,

a distant realm filled with mysterious worlds

so far away

we can't make them out in much detail.

But we have sent one spacecraft.

It took over nine years for New Horizons to get here

and home in on its primary target.

♪

The tiny spacecraft captured the first close-up images

of this Kuiper Belt world,

revealing unexpectedly complex

and dynamic surface features:

mountains as high as the Alps, made from water,

frozen as hard as granite.

But there's something else odd out here.

Another world close by.

How did Charon and Pluto end up almost touching?

Charon orbits about 12,000 miles

from the surface of Pluto,

which sounds like a really largenumber,

but is actually really close

for the scale of the Kuiper Belt.

It's very common in our solar system

for moons that are close enough to their parent body

to become tidally locked.

And that is where one orbit

will also be one spin,

so that the same face of that moon

is presented to the planet at all times.

NARRATOR: All large moons in the solar system,

including our own, are tidally locked,

only showing one face to their parent planet.

But in the case of Pluto and Charon,

it isn't just Charon that is tidally locked to Pluto.

Pluto is also tidally locked to Charon.

Both worlds constantly face each other at all times.

RICE: And what that means is, if you're on the side of Pluto

where you're able to see Charon,

then it'll look like it's just hanging there all the time.

It's not going to rise, it's not going to set.

It's just going to constantly be in your sky.

♪

NARRATOR: This isn't just a dwarf planet and its moon,

but a binary pair.

♪

New Horizons is still studying Kuiper Belt worlds today.

During the most distant fly-by in history,

it encountered Arrokoth,

a bizarre snowman-shaped object.

SUNSHINE: It is two pieces that are stuck together,

and those two pieces must have come together very gently.

To actually see it was really quite remarkable.

Any time you see a new world, it's remarkable.

NARRATOR: Why do the worlds out here in the Kuiper Belt

form these partnerships?

LUU: Things become slower and slower

as you go further away from the sun.

In the Kuiper Belt, things might collide

at the speed of a few hundred feet per second.

They occur at such low-impact velocity

that a lot of the collisions are constructive

rather than destructive.

NARRATOR: At some point in their history, Pluto and Charon collided.

But their slow movements

meant this was less of an impact

and more of an embrace.

The Kuiper Belt is so vast and far from the sun

that many mysteries still remain.

I am often pretty amazed that we're actually able

to study things like the Big Bang

and the early universe and distant galaxies,

and yet we still actually don't know everything

that there is to know about our own solar system

and our own backyard.

I'm very confident that there are many, many worlds

out there that we are yet to discover,

and that we haven't even imagined yet.

♪

NARRATOR: We can see distant stars in the night sky

because they burn so bright.

Telescopes can detect planets

as they pass in front of their parent stars.

Seeing worlds in our own solar system

is much harder,

but not impossible.

♪

In 2018,

a faint point of light was detected

about 12 billion miles from the sun.

It's thought to be a dwarf planet

nicknamed FarFarOut.

This is the most distant object yet seen in our solar system

by some of our most powerful telescopes.

But it's not the limit of what could be out there.

♪

OLUSEYI: How far do you expect the solar system to extend?

You may imagine that it ends

at the last major planet, Neptune.

However, that's not exactly the case,

and let me show you why.

Here I'm gonna make a model of the solar system.

This rock is my sun,

and I'm gonna place it right there,

and an inch away, I'm gonna place the Earth.

And so this will be my scale, one inch equals

the average distance between the Earth and the sun.

So now, if I go a second inch,

I arrive at the asteroid belt.

And to get to the most distant major planet, Neptune,

that's gonna be about 30 inches.

Now, we have to keep going and going,

because out here is

the most distantly observed object ever

in the history of our solar system,

11 feet away from the sun,

132 inches,

12 billion miles,

is FarFarOut.

NARRATOR: But the solar system doesn't end here.

OLUSEYI: I have to walk another 50 paces...

...another 186 billion miles,

and now we've reached our destination,

a vast region known as the Oort Cloud.

♪

NARRATOR: Clinging on at the very edge

of the sun's gravitational influence,

the Oort Cloud is our solar system's largest

and most mysterious realm.

It's so deep into the darkness,

it's almost impossible to imagine,

let alone see,

filled with billions upon billions

of strange worlds.

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, we'd discover they are pristine.

♪

Barely changed since the dawn of the solar system.

It's thought some could be older than the sun.

The Oort Cloud's existence is hypothetical,

but science doesn't deal in fairy tales,

so how do we know it's actually there if we've never seen it?

♪

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

we've seen signs that the Oort Cloud is real.

And this is one of them.

For hundreds of years,

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.

[hissing]

♪

So much debris is torn from the surface,

it stretches out, forming a tail...

...around 14 million miles long.

This is the comet Nishimura.

It's visible from Earth for just a few weeks,

before eventually heading back into the darkness,

just the latest in a long line of icy visitors.

Hale-Bopp.

Neowise.

They lit up our skies for weeks, or even months, on end.

Humans have been captivated by comets

because, frankly, they're spectacular.

CHABOT: And the fact that they do wander

around the solar system from time to time

really is a unique scientific opportunity.

NARRATOR: When scientists followed the trajectories

of these comets back,

they discovered they came from the same region,

more than 200 billion miles from the sun.

SUNSHINE: If you calculate

where the orbit took you back to,

it was way out,

unbelievably far out, what we now call the Oort Cloud.

NARRATOR: It's thought the gravity of other objects in the galaxy

can occasionally knock these icy objects inwards.

SUNSHINE: But the really interesting thing

was that they were coming from all directions in space,

and there had to be a cloud.

♪

NARRATOR: Although we can't see this far out,

comets with orbits starting in the Oort Cloud

streak across our skies.

RICE: The Oort Cloud is kind of incredible.

No one's actually been there,

no one's actually observed anything within the Oort Cloud.

They've only seen the wanderers that have escaped

from the Oort Cloud for a brief moment,

which I think is what makes it

so spectacular and so exciting to me.

CHABOT: If I could visit one of those dark regions,

I would have to choose the Oort Cloud.

We don't know that region at all,

we haven't ever explored it, so for me,

that would be the most exciting place to go.

♪

NARRATOR: We think of the solar system

as the sun and all the planets.

But that's just what's close to Earth.

As wandering worlds give us rare glimpses

into what is beyond

and technology improves,

we're discovering more and more.

RUSSELL: We're really only scratching the surface,

so there are millions of these tiny bodies out there,

and we still have so many secrets to unlock from them.

MERCED: There is so much information in darkness.

If we pay more attention

and we trust what we perceive

with all our senses,

we will gain much more information

from those dark regions of not only our solar system,

but of the universe.

This model of the solar system,

it's missing all of the most interesting bits.

♪