NOVA

From the comfort of our home world,

bathed in the warm light of our life-giving star...

The 8-4-4 century interface...

we've set out to explore every one of the planets.

We've pushed further into the dark of space.

But after decades of exploration,

just two probes have explored the most inhospitable

and distant reaches of the solar system's strangest realm.

This is a region of the solar system

that we really had very little information about.

Their instruments waking up to find astonishing vistas...

unlike anything encountered before.

It's at that moment you realize

you're looking at something that's very different.

Places where all the rules are broken.

This was a really strange system.

So why is Uranus so tipped on its side?

Where everything we thought we knew is being rewritten.

Geysers like this one

even exist on Triton,

where it's 30 times farther from the sun.

Incredibly, we have pushed the frontier

all the way to the solar system's most distant outpost...

Pluto.

A small planet should be simpler,

and they should run out of energy in their internal engines

and stop evolving,

like the moon did, billions of years ago.

Pluto didn't read any of those textbooks.

"The Planets: Ice Worlds"...

Right now, on "NOVA."

Major funding for "NOVA" is provided by the following:

Far, far away...

beyond Mars...

Past the storms of Jupiter...

and the rings of Saturn...

Where the distance from the sun is measured not in millions

but billions of miles...

Lies a region unlike anywhere else in the solar system.

Out here, the planets are utterly bizarre...

even unnerving.

Uranus is an oddity even in a solar system of oddities.

Temperatures approach absolute zero,

at which everything grinds to a halt.

Yet, somehow, the worlds here are still teeming with activity.

Neptune was a great excitement,

all sorts of strange things going on.

A place where cold doesn't always mean frozen.

It's at that moment you realize

you're looking at something that's very different.

And even deeper into the darkness,

past where the planets end...

a vast expanse scattered with mysterious bodies awaits.

Our jaws just dropped to the floor.

If ever there were an object that turned out to be

far more surprising than we ever would have imagined,

that's Pluto.

There is no part of the solar system more puzzling.

This is the realm of the ice worlds.

60 seconds, all continuing to go well.

Travelling to the most distant planets,

Uranus and Neptune, is so hard,

we've only managed it once.

We have ignition,

and we have liftoff.

This first mission

to the farthest part of the solar system

is only possible thanks to some help

from the worlds closer to the sun.

25 seconds into the launch,

all continuing to go well.

Making the most of a rare planetary alignment.

People realized that there was an opportunity coming up

in the 1970s

that only comes up every couple of hundred years,

where the planets in their orbits kind of line up

in just the right way

so that you can slingshot from one to the next.

The guidance data coming back shows

that we're right on the money.

The key idea here is that you could use the gravity

of each planet to accelerate the spacecraft,

speed it up, and shorten the trip time.

If it works, this series of planetary boosts

will help Voyager cross nearly 900 million additional miles

of space

before reaching the first of the ice worlds.

This is a region of the solar system

that we really had very little information about.

We had excellent cameras and sensors and so on.

And we used that to go out into the solar system

and see things for the first time

we had never seen before.

Almost nine years into its mission,

Voyager begins its approach to an entirely unexplored planet

and reveals a world weirder than we had ever imagined...

A vast ball of gas,

14 times the mass of Earth and four times its diameter.

Sensors detect an atmosphere of 83% hydrogen and 15% helium.

But it's the 2.3% methane,

that by absorbing red light and reflecting blue,

lends Uranus its color.

Just this giant,

pretty much featureless, blue-green ball

that seemed unreal.

They could see none of the activity

that you're familiar with on Jupiter.

And it's at that moment you realize

you're looking at something that's very different.

Poor Uranus, it's got rather boring clouds, truth be told.

So there wasn't a lot to see with the cameras.

Uranus spins on its axis once every 17 hours,

which should generate dramatic storms.

So why is it so featureless?

Voyager detects a clue.

Temperatures here are the coldest

of any planet in the solar system.

There's simply not enough heat

to drive the storms seen on Jupiter and Saturn.

Uranus is an entirely new class of planet:

an ice giant.

And more surprises are to come.

Uranus has rings.

We knew long ago from telescopic observations

the rings were there at Uranus,

we didn't know their configuration

or their shape.

Voyager begins to explore the ring system

and spots something curious in the shadows...

Two moons...

Cordelia on the inner edge of the brightest ring

and Ophelia on the outer edge, help to organize the system.

Particles within the rings can be moved inwards or outwards

as these tiny objects called shepherd moons,

whose gravitational force

can shape and sculpt the rings themselves

But it's the orientation of the orbits of these moons

and the rings themselves that singles out Uranus

as the strangest of worlds.

Uranus looks bizarre.

The rings, rather than looking like that

when you look through a telescope,

they're actually orbiting up and over the top of the planet.

The whole thing is tipped, like a spinning top.

A bizarre orientation that hints at an unusual past.

Since the beginning,

everything in the solar system has been circling

in the same direction.

It begins with the vast cloud of material

that orbits the young sun.

Over time the material draws together...

forming each of the planets.

Today they all orbit the sun

and spin on their axes

in the same counterclockwise direction

as that primordial cloud...

Except for Venus and Uranus,

which spin in the opposite direction.

But Uranus is even stranger,

because the entire planet is on its side.

This was a really strange system.

So why is Uranus so tipped on its side?

If a roughly Earth-sized object smashed into Uranus

late in its formation history,

then that would've had the proper amount of momentum

to basically knock it on its side.

Not only did the collision tip the planet over,

but it may also help to explain another of Uranus's mysteries:

why the planet is so cold.

That impact presumably relinquished

all of that internal energy that the planet had

when it first formed

and has left it as a relatively dead and sluggish world.

We learn so much during this brief encounter.

But Uranus still holds many mysteries.

And Voyager's strangest discoveries are yet to come.

After Uranus,

it must cover another billion miles of empty space

before it will see another world.

We can say the numbers,

but can we really wrap our heads around something that large?

In order to help ourselves do this,

we can make scale models.

At the center of the solar system is our sun,

a ball of fire 850,000 miles in diameter.

But if we scale the whole system down

by a factor of 600 million,

the sun becomes the size of this light.

And walking out, you pass all the rocky inner words

within the first 500 years.

But to get to the outer solar system

is far harder.

On our scale, we have to cross the harbor

to reach the next planet, nearly a mile away.

And we've reached the orbit of Jupiter,

the largest of the planets and the first of the gas giants.

Even farther,

at almost twice the distance from the sun as Jupiter,

we reach Saturn.

Now as we approach Saturn,