Original subtitles

MAN: T minus 40 seconds. Everything looks good for launch.

NARRATOR: Want to get away from it all?

Pluto is one of the loneliest places you'll find.

Smaller than our own moon,

Pluto has never had a single visitor from Earth...

until now.

WOMAN: We keep saying, "This is it, this is humanity's trip to Pluto."

NARRATOR: For the first time in human history,

Pluto is about to be revealed.

MAN: We can expect surprises. It's almost guaranteed.

NARRATOR: Until then, we can only guess what Pluto looks like.

And recently, astronomers decided not to call it a planet anymore.

MAN: | will probably go down in history as the guy who killed Pluto.

NARRATOR: Once thought to be an isolated oddity,

Pluto now marks the beginning of a whole new frontier.

WOMAN: It's as if the explorers had climbed up to the top of the Rockies

and were looking over

and could see the lands of California on the other side.

NARRATOR: And now, as we begin to gaze beyond our own neighborhood,

the hunt is on for Earthlike worlds around other stars.

MAN: | have no doubt that within a few light years of our Earth

there are other life forms.

NARRATOR: There has never been a better time to venture

where no human has gone before...

to follow in the footsteps of our robot pioneers

and explore the planets of our solar system.

MAN: T minus two minutes and counting.

MAN: Go at throttle up.

NARRATOR: Ever wanted to be an astronaut?

Imagine it was you who was heading to the edge of our solar system.

Where would you go?

What would you see?

And how would you survive?

Recent discoveries have revealed much more about these frozen depths.

Armed with this new knowledge,

think of this as your personal travel guide to Pluto and beyond.

A fan of winter sports?

Like the feeling of isolation?

Don't mind the cold?

Welcome to ice land!

Out of sight-- and until now, out of reach--

Pluto is the solar system's most puzzling world.

Why is it pink?

Where did it come from?

And how are we connected to it?

JOHN SPENCER: Pluto is such a mysterious place,

we really have no idea what it's going to look like when we get there.

And that really adds to the wonder of it.

NARRATOR: Pluto is uncharted territory.

We know so little about this place, but it's not a complete mystery...

FRAN BAGENAL: Well, we know that it's an object that is roughly round,

it has ice and a lot of dirt on the surface.

HAL WEAVER: Pluto is a very cold world for one thing, you know,

roughly minus 390 degrees Fahrenheit on the surface.

KEVIN SCHINDLER: If you could somehow have your hand exposed,

if you hit it, your hand would shatter like an ice cube, it's so cold out there.

NARRATOR: To get a sense of Pluto's scale,

imagine a world that's half the width of North America,

sprinkle it with ice and toss it 3.7 billion miles into space.

Unlike the mostly circular orbits of its big brothers,

Pluto's journey around the sun is extremely elliptical and lengthy.

BAGENAL: Pluto's year is 248 Earth years.

So it's a very, very long time for Pluto to orbit the sun.

HAL LEVISON: And we really didn't know, understand, how small Pluto really was

until 1977, when its satellite was discovered.

NARRATOR: Charon, the largest of Pluto's three moons,

isn't huge-- just 737 miles in diameter--

but compared to Pluto, it's a whopper.

WEAVER: Charon is unusual

because even though we think of it as a moon of Pluto,

it's really more of a companion object that's almost the same size--

about half the size of Pluto.

NARRATOR: Named after the god of the underworld,

for more than 70 years school kids are taught

that Pluto is the ninth planet.

And it's a class favorite.

MIKE BROWN: Pluto has this place in people's heart

that it's hard to understand,

except that even | sort of feel it's the distant one,

it's the runty underdog,

it's the one that has a cartoon character named after it.

Everybody loved Pluto.

WOMAN: Pluto is a planet!

NARRATOR: But recently this small-sized planet

has been the subject of a neighborhood dispute

of cosmic proportions.

WOMAN: Pluto is a planet! Size doesn't matter!

GEOFF MARCY: It's a highly controversial topic, even today.

Poor little Pluto, of course, being demoted.

And frankly, rightfully so.

NARRATOR: What on Earth caused Pluto to be kicked out of the planet club?

Just what's going on at the end of the solar system?

One mission is about to find out.

But to get there means setting out on a three-billion-mile journey

to the edge of our neighborhood.

NARRATOR: Aiming for Pluto?

You'd better shoot straight.

Miss your target, and you could end up a very long way off course.

It's something that Hal Weaver thinks about often.

He's the Project Scientist on NASA's current mission to Pluto,

New Horizons.

WEAVER: Oh, yes!

When you're shooting a basketball,

you have to get the angle and the speed just right

in order to get the ball through the hoop.

And that's what we had to do for the New Horizons spacecraft.

MAN: Two, one, zero, ignition, liftoff.

NARRATOR: Launched in 2006, New Horizons is set to become

the first visitor ever to Pluto, due to arrive in 2015.

WEAVER: We had the most powerful rocket available on the Earth,

the Atlas V 551.

It was the fastest spacecraft ever to leave the Earth,

36,000 miles per hour.

It was screaming out.

WOMAN: We keep saying, "This is it, this is humanity's trip to Pluto."

NARRATOR: Miss it, and we'll miss the only opportunity

to explore Pluto in our lifetime.

WEAVER: Because it's heading farther and farther away from the sun.

And if we didn't do this now,

we would have to wait probably another 250 years.

We get one shot, | have to get it right.

This is our one chance to go to Pluto in my lifetime.

Whoa! There it goes.

And we made it!

WOMAN: Mon, this is Pluto Ace, go ahead.

Station 43, Pluto Ace.

NARRATOR: Keeping vigil over New Horizons' long, lonely journey

is the job of these flight controllers at the Applied Physics Lab

at Johns Hopkins University in Maryland.

WOMAN: Could you please tell us

what our current downlink configuration is?

ALICE BOWMAN: Mission duration for this mission is very long.

And so one of the things we do to save costs

is to put the spacecraft into hibernation a good portion of the year.

NARRATOR: Beyond the orbit of Saturn,

New Horizons awakes from its slumber for its regular check-up.

WOMAN: That's affirmative, we are go for command.

NARRATOR: Ever have problems with your computer?

Try loading software to a hard drive that's 1.4 billion miles away.

BAGENAL: The difficult thing about Pluto is the fact that it's so far from the sun,

we're so far away from home.

We have to design and plan a sequence of observations,

and they've got to work.

We don't get a second chance.

NARRATOR: Today's upload goes without a hitch.

But as the spacecraft travels farther away,

the communication times get longer.

When New Horizons phones home from Pluto in 2015,

its signal will take more than four hours to reach Earth.

WEAVER: Encounter day is going to be nerve-wracking and exhilarating.

You know, it's Bastille Day in 2015.

It will be roughly a day after we fly by Pluto

that we should have our first glimpse at what Pluto really looks like.

NARRATOR: And that glimpse will be brief.

After ten years getting there,

New Horizons has only a few months to explore Pluto in detail.

WEAVER: This is the model of the New Horizons spacecraft,

one eighth of the actual size, and it's about the size of a grand piano.

We have the long-range reconnaissance imager,

our "eagle eyes" on the New Horizons spacecraft.

This is the highest resolution telescope that we have.

NARRATOR: Other instruments will peer at Pluto in ultraviolet light,

collect dust in its upper atmosphere

and measure charged particles from the solar wind

to determine if Pluto has a magnetosphere.

WEAVER: The New Horizons mission

is something that future generations are going to look back on

and Say that this is one of the marks that we made in civilization,

the initial exploration, the initial reconnaissance

of this whole new region of the solar system

that had never been explored before.

BAGENAL: | have a bumper sticker on my car

that says, "My other vehicle is on its way to Pluto."

WEAVER: | love this thing!

NARRATOR: July 14, 2015, is a date to remember--

when a tiny world at the end of the solar system

finally gives up its secrets.

In 1930, a young American researcher, Clyde Tombaugh,

scanned the skies for Planet X...

a world that was thought to exist beyond Neptune.

CLYDE TOMBAUGH: It was a very exciting time,

and | remember it like it happened yesterday.

There was never another moment in my life like that.

DAVID LEVY: | knew him very well.

He was a wonderful man with a sublime sense of humor

and a lovely sense of the ridiculous.

We had a lot of fun together.

SCHINDLER: Discovering Pluto was very tedious,

| think that's the best word to describe it.

During the nighttime he would take pictures of the sky

on these photographic glass plates.

And then during the daytime

he would mount the plates onto this machine,

look through this eyepiece.

And then, when you turn this on, there's a mirror that flips back and forth.

And what you're doing is comparing one picture to the other

to look for any changes.

NARRATOR: The machine allowed Tombaugh

to flip between his time-lapsed photographs of the night sky.

What he was searching for was a single tiny pinprick of movement

against a sea of countless stars.

SCHINDLER: He would just plug away at this, hours a day,

stare through this microscope eyepiece looking at dots.

Hundreds of thousands of dots he looked at.

LEVY: And on the 18th of February, 1930, about four in the afternoon,

he saw the two images of the planet appear and disappear alternately.

NARRATOR: These are the actual images that show the discovery of Pluto:

First in one part of the sky...

And then another.

It's proof of a planet on the move.

TOMBAUGH: And | recognized it immediately.

| knew instantly it was beyond the orbit of Neptune

because of the small shift.

It made my day.

Ha!

NARRATOR: For the next 76 years, Pluto is counted as the ninth planet.

But what does this tiny dot in the sky really look like?

WILL GRUNDY: We do know a lot about Pluto now,

a whole lot more than we knew, say, 20 years ago.

And in large part that's driven by the advance in technology

of telescope detectors and telescopes.

NARRATOR: Peeling back the layers of Pluto

is a little like solving a cosmic whodunit...

and it's a mystery that stretches

back to the beginning of the solar system.

GRUNDY: Kind of like the crime scene photographer,

you see a few blood spatters on the wall,

you've got to try to work out what happened,

based on that little evidence that's still left over.

NARRATOR: One clue to unveiling Pluto

comes from NASA's orbital telescope, Hubble.

These are some of the best images we have of Pluto.

What the blotchy pixels show

is a world that's stamped with light and shade.

What causes such dramatic contrasts?

It could be that Pluto has patches of ice and rock,

much like Colorado's Rocky Mountains.

JOHN SPENCER: We do have some idea what it looks like.

We know that it's a very contrasty place.

There's black or very dark brown areas.

And then there's some extremely bright areas,

kind of like the snow on the mountains here.

NARRATOR: As a member of New Horizons' imaging team,

it's part of John Spencer's job to guess what the surface of Pluto is like.

One piece of advice-- pack your snow boots.

SPENCER: Oh, dear. Ha ha!

So the snow on Pluto,

if you were to sit down next to it like this,

would kind of look like this, maybe, we don't really know.

It would be made of nitrogen, not water.

But you might expect big banks of snow

left over from the previous season.

NARRATOR: Spectroscopy, or the analysis of an object in different wavelengths,

has revealed that Pluto is made of three types of ice...

mostly nitrogen, and a little carbon monoxide and methane.

But less is known about Pluto's large expanses of dark material.

SPENCER: The dark stuff on Pluto is more mysterious.

We think it's probably some sort of hydrocarbon gunk

from the methane that's in the atmosphere

being processed by ultraviolet light.

NARRATOR: Pluto's not alone in having a mysterious dark coating.

When the Cassini spacecraft flew past Saturn in 2004,

we saw something similar on moon Phoebe.

Here, scientists believe, is a thin carpet of carbon-bearing compounds...

the precursor for the evolution of life.

BAGENAL: If you take methane,

which is organic because it's got carbon and hydrogen,

if you expose it to sunlight, you expose it to energetic particles,

it goes a sort of pinkish color.

You have chemical reactions,

so it becomes a slightly more complicated hydrocarbon.

NARRATOR: These slowly simmering hydrocarbons

could explain Pluto's other feature that spectroscopy has revealed--

its color.

BAGENAL: Well, it has a bit of a pink tinge to it.

| don't think it would be a flaming pink, but a slight pink tinge.

SPENCER: It might even have this kind of brownish color

that we have here on the snow due to dirt mixed into it.

We do think the snow on Pluto is kind of a creamy, browny color.

BAGENAL: Well, | wonder what it would be like

to walk on the surface.

The ice--would it be crunchy, or would it be soft?

Are there big cliffs?

Is the sort of snow just sort of beautifully sitting on the surface

like a Christmas morning?

LEVY: Just an absolute cold, sublime, terrible beauty out there

that | can't wait to see.

But | think it's going to be one of the most spectacular things

humanity has ever gazed upon.

NARRATOR: And there is one tourist sight you'll see nowhere else in the solar system--

Pluto's largest moon, Charon, in a fixed position against the heavens.

BAGENAL: So we have Pluto and Charon,

each orbiting around the center of mass between the two systems,

so this is the balance point right here between the two objects,

and they orbit around that point.

So both Pluto revolves around Charon, and Charon revolves around Pluto.

NARRATOR: The cause for this orbital ballet is Charon's comparatively large size.

WEAVER: They're almost like a double planet.

The only example of that kind of an object that we know in the solar system.

SPENCER: On Pluto, there's one side of Pluto

where you would never see Charon

and one side where you would always see it.

And they're rigidly locked.

You could build a bridge between the two of them if you were so inclined.

So that's really a remarkable and unique thing.

NARRATOR: So does the double planet with three types of ice have four seasons?

We'll see, but it's best to be prepared.

What we do know is that Pluto has surprisingly complex weather.

Barely contained by its low gravity,

Pluto's whisper-thin atmosphere

is the first thing a visitor will encounter here.

BAGENAL: Well, we think the atmosphere is probably a bit like our atmosphere,

made out of nitrogen,

with some methane, and maybe some carbon dioxide.

SPENCER: And then as you come down towards the surface,

you'd probably encounter some layers of haze.

NARRATOR: But watch where you land.

On summer days, the Plutonian ice below will literally vaporize into thin air.

BAGENAL: You can think of the atmosphere being a bit like a frosty morning,

with evaporating stuff coming off an icy surface.

NARRATOR: Come winter, gentle Plutonian breezes

carry the atmosphere to the dark side,

where it freezes and falls to the ground again.

Camp out here, and you'll soon be covered in a dusting of nitrogen frost.

SPENCER: The temperature there in the icy regions,

which will be the coldest regions,

is probably only about 40 degrees above absolute zero.

NARRATOR: Absolute zero, or minus 460 degrees Fahrenheit,

is as cold as it can theoretically get.

Summer days aren't much warmer.

But it's during this time when Pluto's fragile atmosphere comes to life

and is eroded away by the solar wind.

WOMAN: It probably has lost maybe up to half a mile of ice

over the age of the solar system,

which is kind of interesting.

It means that when New Horizons flies by Pluto and takes close-up pictures,

we might see what we call “sublimation scarps,"

so huge cliffs left from where the ice has evaporated away.

NARRATOR: It could be that Pluto has spectacular cliffs,

like those seen on Uranus' moon Miranda...

carved by millions of years of seasonal erosion.

WOMAN: We can actually with a straight face

talk about trying to understand weather on Pluto.

And that's one of the most fantastic things for me in the last few years.

NARRATOR: Planetary scientist David Jewitt couldn't understand

why the outer solar system seemed so empty,

why Pluto seemed alone.

DAVID JEWITT: So the inner parts of the solar system in the 1980s

were known to be full of comets and asteroids

and all the planets and so on.

But the outer parts basically had nothing.

And beyond Neptune there was just Pluto.

And that seemed very strange.

NARRATOR: Then, in 1992, he makes an extraordinary discovery:

the first of a vast army of frozen bodies beyond the orbit of Neptune,

now known as the Kuiper belt.

JEWITT: The Kuiper belt is basically a warehouse of icy bodies,

things that have been preserved at temperatures

maybe 200 degrees colder than the room I'm standing in right now.

NARRATOR: Out here keeping Pluto company

is an immense ring of ice and rock

that redefines the backyard boundary of our solar system.

BAGENAL: It's as if the explorers had climbed up to the top of the Rockies

and were looking over

and could see the lands of California on the other side.

NARRATOR: We now know these Kuiper belt objects

number in the millions.

Many are the size of mountains...

some as large as Cities...

and some much larger, like Pluto.

JEWITT: We know there are about 50,000 or more

larger than 60 miles in diameter.

And there's probably 100 million objects bigger than a mile or so in this Kuiper belt,

and maybe even ten times that many.

So there's a huge number of bodies

that previously were just completely unknown

in the outer parts of the solar system.

NARRATOR: But what exactly are these objects?

And where did they come from?

JEWITT: We think they formed in the very early stages of the solar system.

NARRATOR: Dating back to the time when the planets formed,

the objects of the Kuiper Belt are what's left over,

planetary spare parts that were hurled to the edge of space

to form an immense rubble pit.

JEWITT: And out in the Kuiper belt it's really, really cold.

And so everything that was trapped in the objects in the beginning

is still there.

It's all frozen in place, in solid ice.

NARRATOR: For years, it seemed that Pluto was the biggest object out here.

But some believed there could be others...

even larger.

BROWN: | had actually been wondering

about whether there was another object out there

as big as Pluto, or bigger than Pluto, or even much bigger than Pluto.

And for a long time nobody had the technology

to really do the job right to look for these.

And finally, when we got it,

we ran to the telescope as fast as we could, basically,

and started scanning the skies.

NARRATOR: In the same way that Clyde Tombaugh

searched the skies for Pluto more than 70 years earlier,

Mike Brown hunted the heavens for an even bigger prize.

BROWN: Our job is much easier than Clyde Tombaugh's.

He did everything himself.

He did the telescope all by himself, he developed the plates by himself.

| did almost none of it.

The telescope is run by a robot and a computer.

And | only would come in at the very end

and just double check what the computer had done

and see what was really there and what was really real.

NARRATOR: In 2003, this image shows what could be a new planet.

But the movement is so tiny, the computer misses it.

BROWN: So this is the actual discovery image.

There it is.

To your eye it's really obvious, and you can see it moving quite clearly.

But that was a little too slow for the computer to get the first time.

That's the only little thing in there that's moving across the sky.

NARRATOR: Mike Brown and his team name the object "Eris."

1,500 miles across, it's nearly a third more massive than Pluto...

and three times the distance from the sun.

BROWN: First thing | did is picked up my phone

and called my wife and said, "| just found a planet!"

NARRATOR: No longer alone-- and now overshadowed by Eris--

Pluto's tenuous status as a planet began to slip even more.

BROWN: My first thought was, "| will probably go down history

as the guy who killed Pluto."

NARRATOR: In 2006, the International Astronomical Union

asked an important question:

Just what is a planet?

For the first time ever, the rules for being a planet are laid down.

BAGENAL: A planet has to be big enough to be round.

Pull itself into a sphere.

It has to be in its own orbit around the sun.

So we can't count moons that orbit planets.

NARRATOR: Planets should also have sufficient gravity

to spit out or suck up all the debris around them.

LEVISON: Their definition is if you put down a bunch of small objects

in an orbit similar to the orbit of the planet,

if the planet either throws them out of the solar system

or eats them in the age of the solar system,

four billion years, then it's a planet.

And if it doesn't, it's not a planet.

NARRATOR: Surrounded by the icy debris of the Kuiper belt,

Pluto is too small to clear its orbital path.

MAN: Meeting in Prague Thursday,

the International Astronomical Union voted it out.

NARRATOR: 76 years after its discovery, the solar system loses a planet.

MAN: Pluto not a planet? What on Earth is going on here?

BOY: Everyone grew up thinking Pluto was a planet.

Because everyone was told that.

But now it's the other way around.

It will confuse people.

BAGENAL: | was pretty astounded how the public responded.

Not only were all the young kids upset that their favorite planet was demoted,

but there was a lot of debate and discussion.

SPENCER: People learned at school there were nine planets,

Pluto was the furthest from the sun, and people are very comfortable with that.

And it's like someone rearranges the furniture in your solar system

without your permission,

and that got people quite upset.

NARRATOR: So if Pluto is no longer a planet, just what is it?

WOMAN: Members of the International Astronomical Union

redefined the body as a "dwarf planet."

NARRATOR: Including Pluto and Eris, there are now five official dwarf planets...

but perhaps there are hundreds more waiting to be discovered in the ice.

BAGENAL: | don't think it really matters whether we call it a planet

or whether we call it a dwarf object or a Kuiper belt object,

in the same way it doesn't matter

whether we call a tomato a fruit or a vegetable.

It could probably be one or the other.

NARRATOR: Whatever we call Pluto and its ring of icy neighbors,

one thing is becoming clear:

we have more in common with these distant objects

than ever imagined.

LEVY: Welcome to my house of magic.

These are the telescopes that | enjoy using to search for comets.

NARRATOR: Comet hunter David Levy devotes his life to watching the skies.

LEVY: Comets are basically beautiful things that are not dangerous at all.

But once in a long, long time,

a comet will threaten the Earth or one of the other planets.

NARRATOR: But where do these intruders come from?

Many comets originate in the Kuiper belt,

where every so often they're nudged out of orbit.

SPENCER: We have direct connections to the Kuiper belt

because a lot of the comets that come into our neck of the woods

here in the inner solar system

are originally derived from the Kuiper belt.

NARRATOR: Even Pluto,

with its highly elliptical orbit and streaming atmosphere

behaves more like a comet than a planet.

BAGENAL: So we think of Pluto as a colossal comet

because its atmosphere is escaping.

But it's much, much, much bigger, it's huge by comparison.

NARRATOR: Most of these visitors from the Kuiper belt

harmlessly pass us by.

But others make a big impact.

It's believed that a large comet

ended the reign of the dinosaurs 65 million years ago.

LEVY: This is maybe a once in 100,000-year event,

or even a million-year event, that a comet is likely to hit the Earth.

But it has happened before.

And believe me, it will again at some point.

NARRATOR: In recent years we've taken a good hard look at these tourists

as they come in from the cold,

hoping to find out what they're made of.

NARRATOR: In 1986, the European Space Agency's Giotto

made a brave rendezvous with one of the most famous comets of all--Halley.

DAVID SOUTHWOOD: We did the most challenging thing,

which was to fly very close to the cometary core

to see the nucleus, an environment that was really unknown.

NARRATOR: In 2005, NASA made an even more daring mission

with Deep Impact--

deliberately smashing into a speeding comet.

MAN: Oh, yeah!

Oh, my God, look at that!

[applause]

NARRATOR: And in 2014, Europe's Rosetta mission

will be the first to make a long-term study of a passing visitor

by landing on its surface and then surfing it around the sun.

SOUTHWOOD: At the same time

we will actually land something down on the surface

to really, truly scratch and sniff what the comet is made of.

NARRATOR: So what secrets have these comets been hiding?

It now seems that the outer solar system is flooded with carbon-bearing molecules,

which are also found in the DNA of every living creature on Earth.

WEAVER: It turns out that the outer solar system

is one of the most prodigious sources of organic material in the solar system,

as well as plain old water ice.

SOUTHWOOD: Once you start discovering organic material

sitting on something like a comet,

you've got to really ask, well,

to what degree did the Earth just pick up life?

NARRATOR: What does all this mean?

The makings for life on Earth, and even its water,

may have originated in the Kuiper belt.

WEAVER: And fortunately nature has provided a way

to transport that rich, organic material from the outer solar system

into the inner solar system through the comets.

And we think that that may have been a significant source

of the seeds for life that we have here on the Earth.

NARRATOR: And if water and pre-organic molecules

came to Earth this way,

there's every chance they were delivered elsewhere in the solar system, too.

MARCY: There's no doubt that there are some lovely environments suitable for life,

right in our own backyard, the solar system.

The aquifers under the surface of Mars, the ocean under the ice crust of Europa,

the reservoirs of water on the moon around Saturn called Enceladus,

not to mention Titan with its liquid methane.

Some exquisite destinations

where we humans will be searching for life on them

| suspect for the upcoming hundred years and beyond.

NARRATOR: It could be that our own backyard is abundant with life.

We just need to go find it.

CHRIS McKAY: Finding evidence of a second type of life

on a nearby planet

would have broad implications, first just philosophically.

We would now know that life is common in the universe.

If it started right here in our little solar system twice,

then certainly the universe is full of life.

NARRATOR: And if we ever do find life in our solar system,

we can probably thank the vast army of ancient bodies in the Kuiper belt.

But where exactly does our neighborhood end?

And what lies beyond it?

NARRATOR: One mission that began more than 30 years ago is about to find out.

Launched in 1977, the twin Voyager spacecrafts

were the first to visit the gas giants of the outer solar system.

And now, after powering down their cameras, they're still traveling.

EDWARD STONE: Well, Voyager 1 is about 110 times

as far from the sun as the Earth.

And Voyager 2 is about 90 times as far from the sun as the Earth.

Neptune's at 30.

So you can see, we are many times as far away as Neptune is,

the outermost planet.

NARRATOR: Although flying blind, both spacecrafts can still hear.

And on December 15, 2004, Voyager 1 recorded something never heard before.

[chirping and hissing]

NARRATOR: This is the sound of the solar wind

as it abruptly slows down from the pressure of interstellar gases--

8.7 billion miles from the sun.

STONE: The sun has a solar wind

blowing radially outward from it in all directions

at about 1.5 million kilometers per hour,

and that wind creates a bubble around the sun.

NARRATOR: This bubble, called the heliosphere,

marks the end of our sun's reign.

NARRATOR: Both Voyager spacecrafts have now crossed into this invisible region.

STONE: We are now on the final outer layer of that bubble

where the supersonic wind from the sun has finally slowed down

as it turns around and meets the interstellar wind.

NARRATOR: So what's there to see beyond the heliosphere?

Out here there is believed to be another field of ice.

Not a disk, but a huge sphere known as the Oort cloud.

LEVISON: Probably one with 12 zeros is the number of objects in the Oort cloud.

And it's a circular structure that extends 10% of the way to the nearest star.

It's huge.

NARRATOR: If a traveler makes it this far out,

they will be teetering on the brink of interstellar space.

Because the outer edge of the Oort cloud

marks the city limits for our tiny patch of the universe.

From here, you can look back and see what makes for a good home...

A star that's not too big and not too hot.

You'll want a rocky planet with liquid water and protected by a magnetic field.

And a good home should have at least one giant gas neighbor,

whose immense gravity can vacuum up rogue comets.

You wouldn't want to live anywhere else, or would you?

Because it now seems that beyond our solar system

is a whole universe of possibilities.

MARCY: There's a wild international competition,

a race, if you will, to find the first planets around other stars.

By far the most successful technique

is something called the wobble technique.

It's very simple.

We can't see the planet going around the star,

so instead we watch the star itself.

And a star, of course, will wobble, move in space,

because it's yanked on gravitationally by the unseen planet.

NARRATOR: Using this method, we've found more than 400 exoplanets--

alien worlds around other stars.

But can these worlds support life?

McKAY: If we find such a planet, we'll want to zoom in

and look to see if we can detect key molecules in the atmosphere,

like oxygen or ozone.

Almost certainly that is evidence of life on that world.

MARCY: | have no doubt that within a few light years of our Earth

there are other life forms.

The much more difficult and daunting question

is whether or not intelligent life is common in our galaxy and in our universe.

NARRATOR: So how will we know if intelligent life exists

on other Earthlike worlds?

What will be the telltale signs?

MARCY: You'll point your radio telescope at that Earth,

to try to pick up the radio waves, the television transmissions,

any other transmissions that intelligent species might be broadcasting,

perhaps serendipitously.

NARRATOR: Trying to tune into ET's TV may sound far-fetched...

but who knows, maybe they've been watching ours all along.

ANNOUNCER: You are there!

NARRATOR: Whether a human traveler

will ever make it to the edge of the solar system

remains to be seen.

But one man has begun the journey.

On board New Horizons is a small urn

holding the cremated remains of Clyde Tombaugh,

the man who discovered Pluto.

SPENCER: He died in the late '90s,

and it's very cool that some of his ashes

are now onboard that spacecraft on their way out to Pluto.

And who could have predicted when he discovered Pluto in 1930

that part of him could have actually gone there in person?

And | think that's a really wonderful tribute

to the person who made this whole thing possible.

NARRATOR: But Clyde Tombaugh will not rest here.

After flying past Pluto in 2015, New Horizons will keep going.

WEAVER: The spacecraft will continue moving.

Newton's laws tell us that, you know,

the spacecraft is just going to keep heading out.

It won't be operating, we won't be communicating with it.

So it will just be another piece of space junk.

We'll just, we'll say goodbye.

NARRATOR: Clyde Tombaugh may be disappointed

to learn that Pluto is no longer regarded as a planet...

but to become humanity's longest ever space traveler

would surely make up for it.

And with an estimated 1,000 billion billion stars in the universe,

where he travels to we can only imagine.