Space's Deepest Secrets

Uranus and Neptune.

Two of the largest,

farthest

and strangest planets in our solar system.

Of all the big planets,

Uranus and Neptune remain the biggest enigmas.

Today, astronomers are finally unlocking

why these supersized snowballs are so weird.

In Uranus and Neptune, we have two planets

that just don't like playing by the rules.

Can an ancient mega-collision

reveal what knocked Uranus onto its side?

Can a missing twin world

unlock why one of Neptune's moons orbits the wrong way?

This is a large moon

but yet it's orbiting the planet backwards.

That's a mystery that needs explaining.

And what is the energy that drives the fastest winds

in the solar system?

It's a rare planet that has winds faster than the speed of sound.

To find out, we venture to these frozen planets

explore their alien underworlds

to reveal what makes these ice giants

the weirdest worlds in the solar system.

At the limits of our solar system lie two huge planets.

Uranus

and Neptune.

Both are over 14 times the mass of Earth.

They orbit more than 2.8 billion miles away from the Sun.

This makes them the farthest out, confirmed planets in the solar system.

And the most mysterious.

We have sent orbiters

to every major planet in the solar system

except Uranus and Neptune.

Uranus and Neptune are really far away

and sending a space probe out there is pretty hard.

Only one NASA mission

has ventured out for a closer look.

Voyager 2 flies past Uranus in 1986

and Neptune in 1989.

The spacecraft reveals that these planets,

sculpted from frozen water, methane and ammonium,

are far from dead.

The thing that's so poignant about the ice giant worlds,

is that we've only visited them

for a couple of days.

The Voyager spacecraft just flew on by.

And yet, in that brief encounter,

we saw things we never expected.

Instead of cold, boring worlds

like we maybe thought they would be,

they were exciting. They had weather.

They turned out to be much more dynamic than we ever imagined.

As Voyager 2 flies by,

it captures detailed photographs,

of the strange moons that orbit the ice giants.

Uranus has 27 moons.

Neptune has 14.

The ice giants have

some of the most fascinating moons in the solar system.

You have Naiad and Thalassa,

two moons of Neptune that are locked

in this gravitational dance with one another.

And they zig-zag up and down

unlike any other moons in the solar system.

Uranus has Miranda,

this moon that seems to have been cracked apart and reassembled,

with vast craters and giant cliffs and fields of ice.

The weirdest moon of all is Triton.

Triton is around half the size of Earth's moon.

But it is 400 times more massive than Neptune's next largest moon, Proteus.

Two-thirds of Triton is rock and metal.

The rest is a crust of water and nitrogen ice,

frozen to a temperature of minus 390 degrees Fahrenheit.

This ice world is unlike any other large moon in the solar system.

Triton is really unique

in that it's the only large moon in the solar system

with a retrograde orbit.

That's an orbit that is opposite to the direction of the planet's spin.

Scientists think that most planets and their moons

form from the same spinning cloud of gas and dust.

The center of the spinning cloud

collapses to form a rotating planet

while the outer edge condenses into the planet's moons.

Everything turns in the same direction

as the original cloud.

So, how does Triton end up orbiting Neptune in reverse?

Today, new technologies and discoveries

make it possible to finally unlock

the mysteries of the ice giants.

A clue about Triton's strange orbit

lies in the data collected by one of NASA's most ambitious missions

to the outer solar system.

In 2015,

NASA's New Horizons probe flies past Pluto.

Pluto is a dwarf planet.

It is the largest object in the Kuiper belt,

a massive ring of billions of space rocks

that lies beyond the orbit of Neptune,

on the fringes of the solar system.

New Horizons beams back to Earth

breathtaking images of this frozen world.

Pluto just blew me away.

We saw glaciers made of nitrogen ice.

And one of my favorite images from the past decade

is sunlight glinting off of mountains of pure water ice.

The scientific instruments on board the probe

analyzed Pluto's vital statistics

to unlock what the dwarf planet is made from.

The instruments revealed

that Pluto and Triton are almost exactly the same.

The bodies are roughly the same size.

Both Pluto and Triton have cores made from rock and metal.

And their crusts consist of layers of frozen nitrogen, water and methane.

This evidence points to an incredible possibility.

One that could solve the mystery

of why Triton orbits Neptune backwards.

Triton and Pluto

are in many ways geological twins.

One of the simplest explanations that could account for that

is that they originated from the same place.

That place is the Kuiper Belt.

Why did Triton break out

from this ring of rocks?

Astronomers think that Neptune's size

could reveal the answer.

Neptune is 17 times more massive than Earth.

This mass gives the planet

a super-strong gravitational influence.

It is possible that over time

Neptune's huge gravity

gradually pulls Triton away from this ring of rocks

to eventually capture it as its moon.

But there is one problem with this theory:

the speed of the objects inside the Kuiper Belt.

If you do the calculations what you will find is,

by the time a solitary Triton passes by Neptune,

it's moving too fast to be captured.

Neptune's gravity would have deflected it,

but it would have just kept on going.

In order for Triton to end up in orbit around Neptune,

something would have had to hit the brakes and slowed it down.

What slowed down Triton enough

for Neptune to capture it?

And how did this dwarf planet

end up orbiting Neptune backwards?

The giant ice planet, Neptune,

has a weird moon.

Triton orbits in the opposite direction

to Neptune's rotation.

One explanation is that

the planet pulled Triton from the Kuiper Belt.

But what slowed down Triton enough

to allow Neptune to capture it

travelling the wrong way?

Astronomers believe that one of Pluto's moons

offers a clue.

Charon is half the size of Pluto.

This closeness in size

makes the pair dance around each other

as they orbit the sun.

Pluto is not much bigger

like all the other planets and moons.

So instead of being like, a single large body

with a smaller one orbiting it,

it's more like a dumbbell,

and this dumbbell orbits as one thing.

Astronomers call two orbiting bodies of a similar size

a binary system.

The strange orbit of binaries

could unlock how Triton slows down enough

for Neptune to capture it travelling the wrong way.

When objects in the Kuiper Belt pick up a partner,

they completely change the way they move

through the cosmos.

Once they've caught each other,

they start revolving around their common center of gravity.

This disrupts their trajectory,

as on every turn,

one partner must move backwards

against their orbit.

Once every pirouette,

they can slow down so much

that they become vulnerable to capture.

Some astronomers believe that

Triton once had its own binary partner,

just like Pluto has Charon today.

They think that this missing binary

is the key that unlocks how Neptune captures Triton.

The discovery that

ten to fifteen percent of Kuiper Belt objects are binary

may solve the Triton problem.

If Triton itself was a binary,

it had a large companion

when it passed by Neptune.

That could have put the brakes on it

and allowed it to orbit Neptune.

Triton and its companion

orbit each other inside the Kuiper Belt.

Over time,