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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 knocks 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 3 billion kilometers 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 ammonia, 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 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 had ever imagined.

As Voyager 2 flies by, it captures detailed photographs of the strange

moons that orbit these 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 Nyad and Thalassa, two moons of Neptune that are locked in this

gravitational dance with one another and they zigzag 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's 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 235 degrees Celsius.

And 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 analyze Pluto's vital statistics

to unlock what the dwarf planet is made from.

The instruments reveal 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.

Their crust consists 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.

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'll 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 slows Triton enough for Neptune to capture it?

Astronomers believe that Pluto's moon might solve the mystery.

Charon is an icy world of frozen methane and water.

The moon 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 traveling 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 10 to 15% 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, Neptune's gravity gradually pulls the pair away from the belt.

They fall towards the giant planet and slingshot around it.

Thanks to their binary dance, there's a point where Triton moves slowly enough

for Neptune to capture it.

This breaks the gravitational bond between Triton and its partner.

and hurls away its companion across the solar system.

Triton is left alone, orbiting Neptune the wrong way.

This companion was kind of a sacrificial lamb that got kicked out into the depths

of space in order for Triton itself to be captured into Neptune's orbit.

Two frozen and mysterious worlds lie at the farthest reaches of our solar

system.

Uranus and Neptune.

Voyager 2 is the only probe to visit these ice giants.

The spacecraft sees Neptune's moon Triton up close in 1989.

The camera on board Voyager 2 captures something weird on the surface of this

strange world.

Hidden in this grainy photograph is a mysterious plume that rises from the

surface of Triton.

The plume is a geyser of frozen nitrogen eight kilometers high.

Winds in the moon's thin upper atmosphere blow the vapor into trails

that are hundreds of kilometers long.

When Voyager flew past Triton, it saw the unexpected.

There were jets of gas shooting out from the moon's surface.

No one expected that. It's out of the far reaches of the solar system. We

expected to find a cold, dead moon.

What the heck is going on?

Why do geysers blast from the surface of Triton?

And what is the power source that drives them?

Astronomers think that a big clue lies at the center of our solar system.

The Sun.

Neptune and Triton are between 3 and 5 billion kilometers from the Sun.

They get around just 0.001% of the amount of sunlight that Earth receives.

Today's astronomers think that a quirk of Triton's chemistry unlocks how this

weak solar energy

powers the geyser that Voyager photographs in 1989.

Out in deep space, on the moons of these planets, ice behaves in ways you don't

expect, and we have to understand these to understand what's going on out there.

In 2019, investigators used the Gemini South telescope in Chile to study

Triton's icy crust.

This telescope captures the sunlight that reflects off the surface of Triton.

But certain chemicals in the moon's crust absorb specific wavelengths of

this sunlight.

This creates gaps in the signature of the light that Triton reflects.

Astronomers analyze these gaps to unlock the moon's chemistry.

Researchers discover that the ice on the surface of Triton is a special cocktail

of carbon monoxide and frozen nitrogen.

In some ways you can think of Triton as having a frozen atmosphere.

Our atmosphere on Earth is mostly nitrogen, and that's true on Triton too,

but the nitrogen is frozen into a layer of ice.

That ice is actually transparent to light, so sunlight can get through that

transparent ice and heat up little pockets in it.

Some types of ice transform directly from a solid into a gas when they heat

up inside an atmosphere that is super thin.

Astronomers believe that this volatility of Triton's ice unlocks the mystery of

its mega geysers.

At a point in Neptune's year, Triton's North Pole leans towards the Sun.

This gives one hemisphere of the moon a summer that lasts 41 Earth years.

The rays from the sun are weak, but they can pass through Triton's nitrogen ice.

Here they hit a dark layer of dust, which heats up by just a few degrees,

enough to turn the ice above into gas.

The pressure rises as the gas builds up.

until it erupts as powerful geysers of nitrogen.

Voyager 2 reveals that ice giants contain many secrets.

The data it beams back to Earth reveals another mystery for astronomers to

solve.

The magnetometer on board the probe detects that the magnetic field of

Uranus is super strange.

The field lines continually twist and shift.

The magnetic poles are offset from the planet's axis of rotation by a massive

60 degrees.

Voyager 2 discovers later that the magnetic field of Neptune is also

irregular and misaligned.

When you think about the magnetic field of a planet, sometimes you get something

very simple, like the Earth.

It's almost as if there's a giant bar magnet going right through the core of

our planet with a very clear north pole and south pole.

With the ice giant, something altogether different is going on.

The magnetic fields are chaotic.

The magnetic field lines twist and turn like a bag of writhing snakes.

And the south pole of the planet is actually along the equator.

Why are the magnetic fields of Uranus and Neptune so twisted?

Scientists believe that the answer lies in discovering what inside the ice

giants generates their magnetic fields.

A mass of molten iron and nickel sits at the center of a rocky planet like Earth.

This super dense liquid metal forms a thick and compact shell wrapped around

the planet's core.

Heat drives powerful currents inside this concentration of molten material.

These deep currents generate a magnetic field.

The rotation of the Earth twists the currents into orderly streams that move

in the same direction as the Earth turns.

The result is that the axis of the Earth's magnetic field almost matches

the axis of the planet's rotation.

Well, the magnetic field is generated by rotating liquids inside the planet, and

those liquids are going to rotate along with the planet itself.

So it makes sense that the planet's geographic north and south pole align

with the magnetic north and south pole.

However, with Uranus, we don't see that.

An ice giant contains no moving metal at its core.

Scientists believe the cores of these planets are made from a mixture of rock,

water, methane and ammonia.

So what happens at the center of ice giants?

How do these planets create their chaotic magnetic fields?

Scientists at the University of Rochester in New York believe that the

huge pressure at the core of ice giants helps to unlock the mystery.

In 2018, they test how this pressure transforms water into weird states of

matter with strange properties.

They use an anvil made of diamonds to compress a sample of water to a pressure

similar to that at the core of an ice giant.

The crushed water transforms at the molecular level.

and turns into a super hot jet black material called super ionic ice.

Its oxygen atoms form a rigid lattice that hydrogen atoms can flow through,

meaning that it can become electrically charged.

But this weird ice is rigid.

So if it is at the core of the ice giants, it creates a fundamental

problem.

Only material flowing inside a planet can act like a dynamo and generate a

magnetic field.

The Rochester experiment shows that an ice giant's core could be static.

This means that the planet's magnetic field can't come from there.

Water ice, under the right conditions, very compressed to the core of a planet,

is actually stopping the magnetic field going through the very center of the

planet.

So there's going to be no magnetic field based in the core of Uranus.

If the super pressurized ice core of an ice giant doesn't generate the planet's

magnetic field, then what does?

One theory is that a liquid ocean of compressed water exists in a thin shell

around the solid core of an ice giant.

Farther up from the core, there's a sort of a Goldilocks zone. Not too

compressed, the water can flow around yet still be electrically charged, and

there's your magnetic field.

How this layer of pressurized liquid moves around the solid, super-ionic ice

core could unlock why the magnetic fields of ice giants are chaotic.

Deep inside Uranus,

lies a compressed superionic ice core that's a scorching 5,000 degrees

Celsius.

Somewhere in between this rigid ball and the surface of the planet is a shell of

highly pressurized water that can flow.

But the disconnected currents inside this layer do not generate a magnetic

field that matches the direction of the planet's rotation.

So that's how the magnetic fields of the ice giants can slip and become

misaligned

Uranus

and Neptune lie at the farthest reaches of our solar system

Voyager 2 remains the only mission to visit these ice giant planets

The spacecraft discovers that they have strange moons and even stranger magnetic

fields Voyager 2 sees up close arguably the single weirdest

characteristic of either of these distant planets There's a lot of weird

stuff about Uranus, but maybe the weirdest is that it orbits the Sun on

its side

Uranus leans over 98 degrees from the vertical.

It is the only planet in the solar system that is orientated this way.

Why does this ice giant roll around the Sun like a giant beach ball?

Astronomers think that a planetary system starts life as a disk of gas and

dust orbiting a young star.

This dust and gas combine to form protoplanets.

These young worlds inherit both their direction and axes of spin from the

angle of the disk.

This means that all of the planets should be born with a spin axis that is

roughly 90 degrees to the disk.

This model predicts that Uranus starts life upright, just like all the other

planets in the solar system.

What tips Uranus onto its side after it forms?

Scientists believe that a clue lies in our solar system's violent past.

The early days of the solar system were a chaotic maelstrom of smash and grabs.

We see evidence of this, of course, in the form of craters on many bodies in

the solar system.

Impact craters form after collisions with asteroids.

But evidence exists that planets also collide in the early solar system.

Samples of moon rock reveal that Earth and a protoplanet called Theia smashed

together around 4 billion years ago.

NASA's Juno probe uncovers evidence that Jupiter collides with a massive rocky

planet around the same time.

This collision breaks up Jupiter's solid core.

Neither Earth nor Jupiter end up on their sides after their cosmic crashes.

So how does a collision knock over Uranus?

Researchers at the University of Durham in the United Kingdom simulate impacts

between Uranus and large protoplanets.

They build a virtual Uranus and fire different space objects at it from

hundreds of different angles.

These simulations reveal that a head-on collision blows apart a virtual Uranus.

But a sideswipe could generate just the right amount of force to leave Uranus

spinning on its side.

without destroying the ice giant.

We think that the impact might have been a more glancing blow than a direct hit,

because a direct hit could have blown away the atmosphere, whereas a glancing

blow might have just injected enough energy to tilt the planet on its side

while keeping it intact.

Around 4 billion years ago, icy protoplanets surround a young Uranus.

One ice ball planet twice the size of Earth wanders too close.

The ice giant's immense gravity locks the planet onto a collision course.

The impact knocks Uranus backwards.

The smaller planet disintegrates in a shower of water and methane ice.

The crash leaves Uranus tilted over permanently.

The icy debris comes together over time to form the ice giants 27 moons and a

spectacular ring system similar to the one around Saturn When you think about

this violent destructive collision that tilted the whole world on its side There

was also a creative flip side to it as well. It generated a whole new system of

moons and rings

Voyager 2 arrives at Neptune in 1989.

Here it uncovers another ice giant enigma.

The spacecraft captures this image of a mysterious cyclone in Neptune's southern

hemisphere.

It is almost 13,000 kilometers wide.

There was a beautiful surprise waiting for us when Voyager finally got out to

Neptune. There was a giant storm churning away in the atmosphere.

This storm was about the size of the Earth and we called it the Dark Spot.

The instruments on board Voyager 2 measure the wind speed inside this mega

storm to be in excess of 2,400 kilometers per hour.

The winds are moving so fast, they're moving twice the speed of a commercial

airliner.

When you look at pictures of Uranus and Neptune, they're just kind of green and

just kind of... But in fact, these atmospheres are incredibly dynamic, with

enormous storms erupting all the time.

The very workings of Neptune's atmosphere are very mysterious. How do

you form a huge Earth-sized storm and winds that blow that fast at that cold

temperature?

The Sun powers the weather on Earth.

Its radiation heats the surface of our planet.

This heat creates currents in the atmosphere that move to make wind

Neptune is almost 5 billion kilometers away

from the Sun The energy that Neptune receives from this star is too weak to

drive its weather in the same way as on Earth So where does the massive energy

that drives these storms come from?

The Hubble Space Telescope finds a clue almost 30 years after Voyager 2

discovers the Great Dark Spot.

In 2018, the telescope captures this image of a new dark spot.

This one is in the planet's northern hemisphere.

The amazing thing about Neptune is how dynamic it is. We see evidence of new

dark spots forming.

So this world is very much alive.

It may be cold, but it's not dead.

Astronomers scour other images of Neptune taken by Hubble for clues that

can unlock the genesis of this new spot.

Hubble photographs the surface of the ice giants every year.

It creates these rolled-out two-dimensional images of the surface of

Neptune.

Astronomers use these images to monitor changes in Neptune's atmosphere.

They see something unusual in this photograph from 2017.

Mysterious clouds of methane cover the area where Hubble sees the dark spot a

year later.

On Earth, storms form when warm air rises carrying water vapor with it, and

that forms bright clouds.

Well, we think a similar thing is happening on Neptune.

Methane is coming up through the atmosphere, rises up and cools and forms

these bright white clouds we see on Neptune right before the big storm

erupts.

These clouds are evidence that it is an energy source inside Neptune that drives

the dark spot storms.

What is this mysterious source of energy?

The very large telescope in Chile takes infrared images of Neptune to reveal its

temperature.

This heat map shows that the upper atmosphere of Neptune is minus 200

degrees Celsius.

But this frozen ice world has a core temperature of 7,000 degrees Celsius.

That is hotter than the surface of the Sun.

Neptune's high core temperature is the result of the planet's formation

billions of years ago.

The early solar system is a massive, fast-moving disk of gas and dust.

Gravity collapses clouds of this gas into protoplanets.

The temperature inside a planet rises as more material compresses to grow its

mass.

Neptune formed from the collapse of gas and dust under its own gravity.

And that collapse can lock in a lot of that internal energy left over from the

early days of the solar system that can last within the planet for billions of

years.

Astronomers think that this difference in temperature between Neptune's hot

core and cold atmosphere is what triggers the planet's superstorms.

The extreme heat that radiates from Neptune's core

vaporizes the frozen gas close to the planet's surface.

This forces methane gas high into the coldest parts of the atmosphere, where

it crystallizes into wispy clouds.

The hot rising gas beneath finally erupts.

It punches a hole in the upper atmosphere that appears as a giant dark

spot.

The extreme temperature difference between the gases generates mega

convection currents that whip the spot into a supersonic vortex.

Uranus and Neptune are ice giants.

Uranus is 14 times more massive than Earth.

Neptune is a whopping 17 times more massive.

These huge worlds prowl the farthest reaches of the solar system between

three and five billion kilometers from the Sun This strange combination of

super size and distant position presents a big puzzle for astronomers One of the

mysteries of Uranus and Neptune is their location in the solar system They're so

far away from the Sun that there would not have been enough material in the

early solar system to build large planets of that size

It shouldn't be possible for these guys to exist where they do.

But they do.

The early solar system is a swirling disk of gas and dust.

When the sun ignites, its heat blows away the lighter elements.

700 kilometers out, it is cold enough for these elements to condense as gas

and ice.

and settle in a band around the Sun.

This material binds together to form the beginnings of Jupiter and Saturn.

These planets grow larger as they absorb more and more gas and dust.

But where Neptune and Uranus orbit, the raw materials to build giant planets are

in much shorter supply.

So how is it possible for the ice giants to form this far out?

To build a planet, especially a giant one, you need countless tiny rocky and

icy bodies to come together to grow over time.

And we think that where we see these planets nowadays in the far outskirts of

our solar system, this material was just too scarce to grow planets that large.

astronomers arrive at an astonishing conclusion to explain this weird

anomaly.

The ice giants come from somewhere else in the solar system.

One possible solution is that Uranus and Neptune didn't form where they are now.

If they formed closer into the Sun, there would have been enough material to

create such large planets, and then somehow Uranus and Neptune moved out to

where they are today.

Why did these ice giants migrate to the farthest reaches of the solar system?

Scientists believe the answer is locked inside meteorites.

Meteorites are the parts of ancient asteroids that collide with Earth.

They contain tiny quantities of radioactive elements.

These elements allow scientists to date the meteorites to discover the exact age

of the asteroids they come from.

Meteorites that fall to Earth have elements like uranium, which is

radioactive. And this radioactive decay that uranium undergoes is like a ticking

clock that allows us to get the age of the meteorite.

The radioactive clock starts ticking when the space rock first forms at the

birth of the solar system.

That means that all meteorites should be around 4.6 billion years old But

scientists make an extraordinary discovery when they date the space rocks

that fall to Earth One that could unlock the mystery of why the ice giants moved

farther out across the solar system

When we get the ages of meteorites through radioactive decay, we find that

most of them are about the same age as the solar system itself.

However, a lot of meteorites have a significantly younger age.

The clocks of these younger asteroids start ticking around 80 million years

after the solar system forms.

Superheating and melting the minerals inside rock resets their radioactive

clocks.

Scientists believe that only a super-violent, solar-system-wide

catastrophe can generate temperatures high enough to melt down so much rock.

We're talking destruction on a truly unprecedented scale.

Entire worlds colliding together and getting obliterated.

Asteroids flying everywhere you look. If you were to actually look up in the sky

during this time, you'd have seen this destructive light show unlike any other.

Objects were streaming around, smashing into each other, and we melted.

And that's why these meteorites seem to be younger than the solar system itself.

Scientists think that this disaster impacts all the planets in the solar

system, including the ice giants.

But what triggers this mega-violent event?

And how does it banish Uranus and Neptune?

to a region of space billions of kilometers from the Sun.

Astronomers believe that the proximity of the ice giants to their gas giant

neighbors Jupiter and Saturn unlocks the mystery.

Probably all four major worlds of our solar system.

Jupiter, Saturn, Uranus and Neptune were competing for planet-building materials,

jockeying for position in the young solar system.

All of a sudden, you're kind of playing planetary pinball.

Around 4.6 billion years ago, Jupiter, Saturn, Neptune and Uranus

orbit much closer to each other than they do today.

Back then, their proximity causes gravitational chaos.

The huge gravity of the larger Jupiter and Saturn forces out the smaller ice

giants, and they begin an epic journey across the solar system.

One theory is that they even swap places along the way.

Uranus and Neptune travel billions of kilometers to end up in the sparse outer

reaches of the solar system.

The truth of the matter is that if you're jostling with Jupiter and Saturn

for real estate, even an ice giant really doesn't stand much of a chance.

It's out to the suburbs.

This gravitational battle between Jupiter, Saturn, and the ice giants

triggers an intense asteroid bombardment of the solar system.

Space rocks smash together in mega impacts in the most violent period of

the solar system's history.

This stress melts and reforms the asteroids, resetting their radioactive

clocks.

Over time, many of these rocks drift into the inner solar system and crash to

Earth as meteorites.

The migration of Uranus and Neptune explains why the ice giants orbit so far

from the Sun.

Their movement reshuffles the entire solar system and locks it into the

configuration we know today.

The ice giants are the weirdest worlds in the solar system.

From Neptune's super-fast winds and the giant geysers on its moon, Triton,

to the bizarre sideways rotation of Uranus and its strange magnetic field,

Powered by an exotic form of water, these two planets break all the rules.

Uranus and Neptune are dynamic, interesting, and above all else, weird

places.

The ice giants proved to us that there's no such thing as a normal planet. They

work in an entirely different way than any other type of planet that we know

of.

They just completely blow away our ideas of how planets are formed.

so far we've only had a glimpse of these ice giant planets and the next probes

that go there might find they're even weirder than we think now our solar

system would be a very different place if it were not for these weird oddball

worlds