How the Universe Works

Our solar system is home to giants.

The gas giants Jupiter and Saturn seem to dominate.

But two ice giants, Neptune and Uranus, determine the fate of the rest.

Two distant planets unconnected to us.

Or so we thought.

We now know the fates of the ice giants are entwined with our own.

They've gone from being these cold, dull worlds to actually

having in them the very secret of why you and I exist at all.

Their story is of epic migration, brutal destruction.

Uranus got jabbed and then knocked on its side.

Of worlds more alive than anyone imagined.

They hold the key to the history of our solar system and perhaps to life on Earth.

Uranus and Neptune.

Mysterious giants that lurk in the cold outer reaches of our solar system.

The furthest planets from the sun.

Uranus and Neptune are sort of the sentinels of the outer solar system.

They're out past Jupiter and Saturn, well over like 2 billion, 3 billion miles from the sun.

They're out past Jupiter and Saturn, well over like 2 billion, 3 billion miles from the sun.

Their size and location are a puzzle to planetary astronomers.

Uranus and Neptune are somewhat of a mystery because in a way they shouldn't

exist, or at least they shouldn't exist where they are.

Scientists don't understand how these giant planets grew so big, so far from the sun.

The mystery began four and a half billion years ago.

With the birth of the solar system, the sun sparked into life from a disk of gas and dust.

The rocky cores of the first planets then started to grow from debris in the disk.

But these inner planets had a size limit.

To grow into a giant planet, gas is needed.

Heat from the infant sun blasted lighter gas molecules beyond the point astronomers call the frost line.

Here, it's cool enough for gas molecules like hydrogen and helium to stabilize.

Jupiter and Saturn took shape first, sweeping in the abundant gas and quickly becoming gas giants.

But Neptune and Uranus are different.

Jupiter and Saturn are about 90% hydrogen and helium, whereas Neptune and Uranus are more like 20%.

So what does this difference in gas tell us about their formation?

We suspect that Uranus and Neptune came a little bit later, when there was not as much gas to be swept up.

Uranus and Neptune had less time to scoop up as much hydrogen and helium before these gases disappeared.

But they were also forming further out, where it's cold enough for other, heavier gases to freeze.

These were swept up by the growing outer planets.

Out where Uranus and Neptune are, tons of ice, tons of frozen gases,

as we might think of them, methane, ammonia, water.

And so that's what makes up their composition predominantly.

They may be smaller than Jupiter and Saturn, but consisting of these different

forms of ice meant that they grew dense, becoming ice giants.

However, there's a problem.

They are too big.

The disk of gas and ice around the newborn star didn't last forever, and the material

in the far reaches of the solar system was spread too thin.

As you move further out in the solar system, the time scale for two bodies to

find each other and collide in a crete slows down because the periods around the sun are much longer.

It just takes a very, very long time.

Neptune and Uranus orbit the sun incredibly slow.

Too slowly to have collided with enough icy material to grow into the giants we see today.

So when we look at Neptune and at this very distant orbit, we don't have enough

time in the solar system to build a planet like Neptune.

We just don't think we could build Neptune where we find it today.

A clue to Neptune's confusing location came when astronomers started to discover

planets called exoplanets orbiting other stars.

So one of the biggest surprises from studying planets around other stars is

that Neptune mass planets are very, very common around other stars,

but not where we find our Neptune.

In fact, what we find around other stars is that Neptune mass planets are quite

common, but they're common very, very close to the star.

It would be like having a Neptune inside the orbit of Mercury in our solar system.

In our solar system, Neptune and Uranus are impossibly far away from the Sun.

In other systems, ice giant-sized planets are impossibly close to their stars.

So, what happened?

It turns out, where we see them now is probably not where they started out.

One thing we've learned about solar systems is that things are on a delicate

balance, and planets migrate, they move around, they don't form in one place and stay there forever.

So, what had enough power to move a giant planet like Neptune?

An even bigger one, Jupiter.

One way for planets to move is by gravitationally interacting with each other.

So they feel each other's gravity, they can tug, they can pull, and that

effect can lead to planets slowly migrating around in their planetary systems.

In the early days of our solar system, the giant planets were much closer together, so the effect was greater.

Furthermore, they may have orbited in a different order than we see today.

Jupiter, Saturn, Neptune and Uranus.

So what caused Neptune and Uranus to swap positions?

The answer lies with Jupiter and Saturn.

When these two giants locked into a gravitational dance, there's always this interplay between them.

Think of Capoeira dancers, balancing and moving together in a careful, orchestrated way.

Over millions of years, a rhythm slowly built.

These giants pushed and pulled each other into more elliptical orbits.

But the gravitational dance reached a climax.

The stretched orbits became unstable, and the giants moved off course.

As Saturn and Jupiter twisted out from the Sun, they flung Neptune out beyond Uranus.

As Neptune moved out through the solar system, it pushed debris ahead of it.

These were the leftover icy fragments from planet formation.

Neptune snow plowed these blocks of ice.

It pulled these bodies out and they became the Kuiper Belt, the band of thousands of

small bodies of ice and rock just beyond Neptune's orbit.

You can think of the structure of that Kuiper Belt, it's like blood splatter on the wall at a murder scene.

It's the record of this really violent event of Neptune migrating outward across the solar system.

But Neptune's movement didn't just fling these small icy bodies out into the Kuiper Belt.

It also sent some of them crashing in towards the Sun and early Earth.

It was the most violent time on our planet since the birth of the solar system itself.

500 million years after the Sun flared into life.

It's called the Late Heavy Bombardment.

During the Late Heavy Bombardment you had rocks literally falling down from the sky constantly.

This would have been a terrible time for life.

And yet this cascade of icy bodies also brought something essential for life.

One characteristic of the outer solar system bodies is that we often find organics.

Organics provided the basis for all living organisms we find today.

They're carbon-based molecules that formed on the surface of dust grains in the early solar system.

The rocky inner planets swept up these organics as they grew.

But the scorched surfaces of the young planets were too inhospitable for many of these delicate molecules to survive.

Yet organics remained intact on the small icy bodies in the

outer solar system that Neptune tossed towards the early Earth.

Neptune was the deliverer of life.

As far away as Uranus and Neptune are, the existence of ice giants in the outer

solar system may have been critical for the existence of Earth today.

And the ice giants may have done more than give life on Earth a start.

They may have prevented our planet from being completely destroyed.

Uranus and Neptune, the distant ice giants that may have delivered the elements of

life to Earth, are more important than we realized.

For without them, our planet itself might not exist at all.

These ice giants are fascinating worlds, but they may be even more important than that.

They might be the reason we're here.

Around 4 billion years ago, the young Earth was under threat from our solar system's tormentor.

Jupiter.

Positioned between the rocky inner planets and the giant outer ones, Jupiter dominates the solar system.

When you have a behemoth like Jupiter in your solar system, what it does determines in part what everything else does.

As Jupiter and Saturn locked into their gravitational dance, they migrated out, away from the Sun.

Jupiter's immense gravity should have pulled Earth and Venus along with it,

and their orbits should have stretched and overlapped with each other.

A collision was an inevitability, except.

It didn't happen.

So by the fact that we're talking about this here on Earth, suggests that Earth

and Venus didn't have an impact early in the Solar System when Jupiter and Saturn were migrating.

Something appears to have protected us.

Scientists think something ripped Jupiter into a different orbit before it had a

chance to pull Earth and Venus on a collision course.

But what caused such a large jump in Jupiter's migration?

This is where the ice giants enter the story.

Getting Jupiter to make a big jump in its migration is not easy.

And so the best way that the models have been able to actually recreate this jump

is to have Jupiter actually eject something the size of Neptune out of the solar system entirely.

Jupiter has a lot of gravity.

And if you get too close to it, you're going to be accelerated as you fall in towards Jupiter.

And it's possible that you can eject a planet completely out of the solar system this way.

It's basically slingshotting it.

But a planet the size of Neptune is heavy, even for Jupiter.

And sling shooting it out of the solar system had an impact on Jupiter.

It was knocked into a new orbit, and Earth was saved.

But which ice giant sacrificed itself for us?

Because Neptune is still in the solar system.

And so is Uranus.

If you use computer models to basically predict the behavior of the planets,

what you find is that if you start with Jupiter, Saturn, Uranus, and Neptune,

you can't save the Earth without ejecting either Uranus or Neptune.

But they're there, so we know that's not right.

However, if you add a third ice giant, a fifth giant planet out there,

then that actually makes everything work.

You can save the Earth, have the planets in their present configuration,

and that ice giant gets ejected from the solar system.

Imagine our solar system starting with three ice giants.

One then swings too close to Jupiter, and our solar system's bully throws its victim clean out of the playground.

Jupiter is pushed into a new orbit by this third ice giant's gravity.

Earth is saved from Jupiter's deadly gravitational pull, and the solar system

becomes the safe and orderly place we see today.

So we have a funny story here.

This ice giant that may have existed billions of years ago yanked Jupiter back

into the outer solar system, preventing it from destroying the Earth.

But in the meantime, it sacrificed itself for us, getting ejected from the solar system.

We have to thank it for our existence, but it's not there anymore.

We humans are really lucky.

Had the dinosaurs not gone extinct, we wouldn't be here.

Had this planet not been ejected out of our solar system, we wouldn't be here.

So where is this missing ice giant now?

The answer is pretty amazing.

It could be clear across the other side of the Milky Way galaxy.

The Sun moves around the Milky Way galaxy at about half a million miles an hour.

And in the history of the Earth, we've been around about 20 times.

We could have lost that planet anywhere across the Milky Way.

But is this third ice giant really lost?

Or just hiding?

In January 2016, astronomers at Caltech made an astonishing announcement.

They claimed to have found evidence of a mysterious ninth planet, disrupting icy bodies far out in the Kuiper Belt.

Simulations suggested that if this so-called Planet Nine exists, it is similar in size to Neptune and Uranus.

Could this be Earth's savior?