Original subtitles

In the outer solar system there lurks a monster.

Everything about it is on steroids.

Its radiation is the worst, its magnetic field is the strongest,

it's spinning the fastest.

Jupiter.

We had to build an armoured tank to go there.

Thanks to Nasa's Juno mission,

scientists can explore what's inside the largest planet

in the solar system for the first time.

A few years ago all we had was theories.

Now we get to look inside.

Enabling them to peel back the layers

and reveal Jupiter's close-held secrets.

We need to know what's going on in the interior,

and that's not something we can get

by just taking pictures of the outside.

And inside they are finding scientific wonders.

I look at an image like that and I say, "Wow". I just love it.

And some scientists are even trying to create a piece of Jupiter...

All personnel are required to exit the target bay at this time.

..here on Earth.

The power that we put in the lasers is about 500 times the

power that is used for the entire United States at a given moment.

This is the story of an audacious journey

to the most extreme world we have.

To send a spacecraft there is a little bit insane.

But this insane mission could answer questions that have long puzzled

planetary explorers.

It had a key role in the formation of the solar system.

It holds the secrets to how the whole solar system formed,

how it got started, how planets are made.

What's the point of building a spacecraft

if you're not going to send it somewhere cool and interesting?

To the Romans, Jupiter was the king of the gods,

the lord of thunder and lightning.

It ruled the sky.

And today Jupiter still holds us in awe with its incredible beauty.

But despite centuries of studying the swirls and stripes

we see with our telescopes, it holds one fundamental secret.

What lies beneath these cloud tops?

What is Jupiter made of?

What sits at its very core?

Today a group of scientists are achieving something

that would once have been, well, completely impossible.

They are taking us on a trip beneath the clouds, into the depths of this

alien world.

The clouds are hiding what's inside the planet.

But they're not driven by idle curiosity.

They believe Jupiter, the king of the gods,

is the most important planet in the solar system.

If we can make a good measurement of

what's at the very centre of Jupiter,

that's going to let us figure out what was going on

right at the start of our solar system.

Which is when all of the planet formation gets started.

Jupiter is big.

Very big.

And its size gives a clue.

A clue to why understanding the planet is so important.

But for Professor Kaitlin Kratter

that word, big, doesn't do it justice.

It's really hard to get a sense of how big it is

in comparison to piddly little Earth.

So what I want to do is I want to

give you a frame of reference and show

you how big Jupiter is compared to all the rest of the planets in the

solar system.

Kaitlin's going to measure out the planets' masses with rocks.

This is going to be our representation of Mercury,

the smallest planet in our solar system.

And you can see we're starting out with something that's pretty light.

I can throw it up and down. It's only about one kilogram.

Next up, Mars.

And you can see Mars is a bit bigger than Mercury.

It's about twice as massive, maybe two kilograms.

It's still pretty light, no big difference.

But it's a large jump to Venus at 14kg.

And our own lump of rock, the Earth, is bigger still.

You can tell it's getting a little heavy.

It's about 17kg, so I've got to work a little bit now.

But in terms of the masses in the solar system

we are just getting started.

For the outer worlds, Kaitlin needs a little help.

Even the smallest, Uranus, is more than ten times the Earth's mass.

That's 250kg worth of rock there.

I'm pretty strong but I'm not quite that strong.

Add 50 more kilograms and you've got Uranus's twin, Neptune.

And then we reach Saturn, the second most massive planet.

And you can see it's a whole lot bigger

than everything else we've seen so far.

Remember, Mercury, our tiny planet was just one kilogram.

This is 1,700 kg.

But what about the heavyweight, Jupiter?

It's in another league entirely.

Because on our scale Jupiter isn't a pile of rocks,

it's the truck itself.

And you can clearly see Jupiter out-masses them all.

So we've got 7,000 kg here for Jupiter on our scale.

If you take all the other planets in the solar system and you add them

together, Jupiter is still two and a half times bigger than that.

You can see why Jupiter was God of the skies.

And the planet's gargantuan size

reveals something even more fundamental.

Because when it comes to planets, big means old.

We think that because Jupiter's so big, it had to form

really early in the history of the solar system.

Jupiter isn't just a giant.

It's a gas giant, almost entirely made up of hydrogen and helium.

And this means the planet must have formed

when the solar system was in its infancy.

Because whilst there was a lot of gas in the early solar system,

it didn't stick around for long.

The gas goes away on timescales of maybe

a million to 10 million years at most.

So what that means is that Jupiter has to form quickly

to be there in time to suck up all that stuff.

Scientists now think that Jupiter is the oldest world we have,

forged at the very dawn of the solar system.

And inside, cloaked in swirling clouds,

should be remnants of this earliest time.

If we want to understand where it all started,

how all of planet formation began,

we need to know what's going on in the interior,

and that's not something we can get

by just taking pictures of the outside.

But Jupiter guards its secrets jealously.

It's an inhospitable and dangerous world.

And delving into its depths requires a unique machine.

Jupiter's the most extreme environment Nasa's ever visited.

To send a spacecraft there is a little bit insane.

We had to, you know, build an armoured tank to go there.

Scott Bolton heads up the Juno mission.

And this is his armoured tank.

Behind me is the Juno spacecraft, basically life-size.

It's an enormous spacecraft.

Close to 70 feet in diameter,

as it spins around and cartwheels through space.

This is one of the largest spacecraft ever made.

In Roman mythology, Juno was Jupiter's wife,

and only she could see the King's true nature.

Juno had the special powers to see through

a veil of clouds that Jupiter surrounded himself with,

to hide his mischief.

Juno the spacecraft has all these magical instruments

to see inside of Jupiter.

Juno is kitted out with 29-foot-long solar panels to harvest the sun's

energy from half a billion miles away.

200kg of titanium armour protects its fragile electronics.

It also boasts a unique array of scientific instruments.

So, almost all of the science instruments on Juno

are situated between the solar rays,

so, they're on this main deck of the spacecraft.

Here you have particle instruments, an ultraviolet camera.

That's the microwave radiometer that looks in different wavelengths into

Jupiter's atmosphere.

There are so many firsts associated with it,

to be able to explore Jupiter and get that close.

Makes me very proud to be part of the team

that designed and built that thing.

But there was almost a notable absence from Juno's arsenal.

So, as we come around, you're seeing JunoCam right here.

JunoCam is the only camera that captures visible light

in colour on the spacecraft,

but at first, Nasa told them to leave it behind.

And I just said, "I just can't imagine going

"all that way and not having a camera",

so the team ignored Nasa's direction and we kept the camera on anyway.

A regular camera was almost deemed an unnecessary luxury.

Because Juno's main mission is to peer through the atmosphere,

not just take pretty pictures of the cloud tops.

What Juno's doing is taking us all inside of Jupiter to see what it's

like. We're journeying right to the centre and passing by all these

strange phenomena on the way down.

On the 5th of August 2011, the armoured tank was sent on its way.

It had a journey of 1.7 billion miles ahead of it.

But it was to get a dramatic introduction to Jupiter's might...

..whilst still 5 million miles from its destination.

Because five years after launch,

Juno felt the power of the gas giant.

A power that could have ended the mission before it had really begun.

Heidi Becker is a senior scientist at Nasa's Jet Propulsion Laboratory.

She was tasked with ensuring the spacecraft survived

this first encounter with Jupiter's power.

My job on Juno has always been to be afraid of Jupiter.

I think of Jupiter as something that's attacking us and throwing its

weapons at us, really.

Ten days before reaching the planet,

Juno's plasma wave detector recorded this.

DEEP STATIC HISS

The sound that we're hearing is the moment when

Juno is crossing over into Jupiter's territory.

It's a moment called crossing the bow shock.

You hear this very different roar.

Juno was slamming into charged particles,

trapped in the huge magnetic field that surrounds the planet.

You're hearing the radio waves converted into sound.

This was Jupiter greeting Juno with the roar

of its massive magnetosphere.

It extends all the way back to Saturn, perhaps further.

And it's the biggest structure in the solar system

next to the Sun's magnetosphere.

It's kind of a Halloween kind of sound,

and it's appropriate given the strength

of the magnetosphere at Jupiter.

The first layer of Jupiter actually sits outside the planet itself.

It's an enormous magnetic field

extending out millions of miles above the cloud tops.

And intense magnetic fields mean intense radiation.

Other missions have experienced the fury of Jupiter's radiation

environment.

Four, three, two, one.

We have ignition and liftoff of Atlantis

and the Galileo spacecraft bound for Jupiter.

Almost three decades ago, Galileo,

the only other spacecraft to orbit Jupiter, blasted off.

It, too, journeyed across the void.

But once at the gas giant things got a little odd.

The spacecraft sent back some images that were almost totally white.

And Galileo would randomly switch into safe mode.

It didn't take long to realise that this was radiation around Jupiter,

pummelling the spacecraft's electronics.

Jupiter's radiation environment is the most dangerous environment for a

spacecraft to go into.

Juno was built to be bigger and stronger than Galileo.

An armoured tank in space.

Because it would orbit the planet far closer than its predecessor.

And the closer it got,

the more powerful and dangerous the radiation.

It's a region we were very afraid of because no-one had ever been there

before.

It feels to us like sending our spacecraft into a shooting gallery.

With bullets that are moving at almost the speed of light.

To make things more difficult, Juno was entering this extreme radiation

environment at 130,000 mph.

...minimum burn timer.

Almost there.

On July 4th 2016, Juno arrived at Jupiter.

It required a perfectly timed burst of thrusters

to get the craft into orbit.

If that doesn't fire just right you fly right past Jupiter.

That's it. You don't go into orbit, you don't get any science,

you go around the solar system again.

By the time you come back to Jupiter it's decades later.

Probably nothing's working.

Juno dived around the planet and inside the radiation belt.

You're really just sitting there waiting,

hoping that you'll come out all right at the end and be in orbit.

It's a very uncomfortable situation.

The thrusters fired on cue.

All stations... We have the tone for burn cut-off on Delta B.

Welcome, Jupiter.

And despite the shooting gallery of particles,

the spacecraft was fully operational.

We just did the hardest thing Nasa's ever done.

After a decade worth of planning,

and hundreds of thousands of hours of meticulous work...

..Juno was in orbit.

The journey to Jupiter's heart could begin.

Every 53 days, the spacecraft swoops

north to south just 3,000 miles from the cloud tops.

And that means Juno is rewarded with a close-up look

at a dazzling display.

A display that hints at just how strange the gas giant's interior is.

Because Jupiter is home to the most powerful aurora

in the entire solar system.

Professor Fran Bagenal is studying how the gas giant

creates these astonishing light shows.

Here, we're looking at the aurora on Jupiter.

This is spectacular.

But the planet doesn't produce these aurora alone.

It gets some help from its own moons.

Here, imaged by Juno.

So, we see Jupiter in the middle but around Jupiter we see the moons, Io,

Europa, Ganymede, Callisto.

The innermost moon, Io, is key.

This is a moon like no other.

This footage shows a plume of material being blasted out

from the moon's surface, 200 miles into space.

Io is a volcanic world, and material from its eruptions

goes straight into Jupiter's huge magnetosphere.

The gases that come from Io become trapped in the magnetic field.

And they become accelerated to very high energies.

Particles are accelerated along the field lines towards the poles with

spectacular results.

Energetic charged particles come in,

bombard the atmosphere, make it glow.

And then we're seeing this glow in the atmosphere of Jupiter.

Look closely and we can even see the movement of the moons

in the aurora itself.

Those dots, you can see them there.

Io and Europa.

And those are because there are a million amp electrical currents

flowing along the magnetic field into the atmosphere,

bombarding the atmosphere and making it glow.

But there is a mystery in these displays.

On Earth, the aurora is caused by the iron core at the centre of our

planet, acting like a giant bar magnet,

pulling charged particles from the Sun and giving us wonderful light

displays in the sky.

But Jupiter is almost entirely hydrogen and helium.

On Earth these are gases, not magnets.

Something strange is going on inside the planet,

and Juno's unique orbit is designed to help the team get answers.

We're looking down and seeing these aurora glows.

We're flying through the magnetic field

and measuring at the same time the charged particles

and the fields that cause these auroral particles to

bombard the atmosphere.

Something inside the planet is creating this brilliant light show.

And Juno is going to take us through the cloud tops to reveal what it is.

The outer layer of the planet is at once familiar and yet totally alien.

Jupiter's atmosphere.

Juno beamed back abstract masterpieces of spiralling storms.

Swirling bands of colourful wind.

It's almost as if the king of the solar system

had wrapped himself in a majestic cloak to guard his secrets.

Professor Andy Ingersoll has been trying to decipher

this enigmatic layer of clouds for five decades.

You have storms that last for hundreds of years.

You have winds that are three times hurricane force on Earth.

Juno gets much closer than any spacecraft ever has been.

So we have details at a scale we never had before.

And one of Juno's most stunning discoveries

was hiding in plain sight.

Because the spacecraft revealed a whole new side to the gas giant.

Quite literally.

We've never had a spacecraft over the poles.

So it's pretty exciting.

It's new territory.

You send good instruments into a new place

and there's stuff there to be discovered.

JunoCam, the instrument almost left on Earth,

revealed the polar regions of Jupiter in glorious detail.

What we find is vortices kind of like hurricanes.

They're all packed together.

And we've been watching them now for a year,

and they haven't all merged into one.

And these storms became even more astonishing when viewed with Juno's

infrared camera.

They're packed in in this pentagonal pattern.

There's actually six of them if you count the one in the centre.

And they just sort of sit there.

And this is remarkable.

This is really the first time that anyone has seen

such a geometric pattern of cyclones on any planet or anywhere.

I look at an image like that and I say, "Wow".

The idea that nature can make such a

geometrical structure is fascinating.

The images being beamed back from all over Jupiter are stunning.

But they also create more questions.

What produces the iconic stripes of wind we see in the atmosphere?

How deep do those winds go?

Professor Tapio Schneider and Dr Junjun Liu are planet builders.

With a little help from the supercomputer in Caltech's basement,

they can simulate entire worlds,

and when building a planet,

the first question to ask is - what powers it?

On Earth, the Sun powers the entire climate system.

It sets the atmosphere in motion,

the atmosphere sets the ocean in motion.

Everything in the climate system starts with the Sun.

But if you want to think about winds and climate on Jupiter there's an

additional heat source we need to think about.

That additional heat source is inside the planet.

Because when the gas giant formed,

an enormous amount of energy was trapped in its centre.

For a long time people have been puzzling,

what is the different roles played by those two different heat sources?

You know, this is... Like, our model is actually

the first general circulation model,

it's a global model,

taking into account both the solar radiation

and the internal heat flux.

The pair believed the two heat sources,

our sun and the planet's hot interior,

worked together to power Jupiter's atmosphere.

And armed with this information they could create

a virtual Jupiter, winds and all.

This was the first time the heating by the sunlight and the

internal heat flux have been combined in a model.

After a month of number-crunching on some of the most powerful computers

on Earth, their simulation eventually churned out a planet.

This is the result of our general circulation model,

which actually is a very difficult simulation

because Jupiter's radii is very large.

The colour shows the direction of the wind.

I was really happy about this result.

It looks at Jupiter in many essential ways,

it shows one strong eastward jet at the equator,

and alternating eastward westward, eastward westward jets.

Their simulation reproduced Jupiter's visible winds

almost perfectly.

It was very satisfying. It was perhaps one of the most

satisfying moments in a scientific career.

That you put together an image of how a whole planet works,

you try to put on a computer what you can of it,

and it comes out the way you thought it should come out.

If their simulation is right,

the instantly recognisable stripes of wind we see on Jupiter's surface

are powered from above by the Sun, and from below

by heat leaking from the planet's interior.

And their model even had a prediction

about how deep these wind patterns go.

We think that the equatorial region

these jets go all the way through, deep inside Jupiter.

They believe that the visible winds extend well below the cloud tops.

And thanks to Juno,

using its array of instruments to peer through the atmosphere,

we can finally see if that prediction is right.

It appears that the winds that we measure

at the tops of the clouds have roots that go down,

kind of parallel to the polar axis north to south,

so that's as deep as they possibly could go.

As predicted, Jupiter's huge bands of wind mark just the top of great

rivers of atmosphere flowing deep within the planet.

We're talking winds thousands of kilometres below the cloud tops.

Even the Great Red Spot, a storm that could swallow the Earth whole,

was found to have a root as deep as Juno's instruments could measure.

It could go down thousands of kilometres.

That pretty much answers the question that we had.

"Is the red spot shallow or deep?"

And the answer is, it's deep.

This upper layer of Jupiter is proving to be more fascinating

and dynamic than scientists dared hope.

And the atmosphere holds one more mystery,

one that could have important implications for our own planet.

It is the mystery of Jupiter's missing water.

The search for water beyond Earth is one of the most important quests for

planetary explorers.

Jupiter, being almost entirely made up of hydrogen and helium,

might not seem an obvious place to look.

But, for Scott,

the hunt for water is one of Juno's most important objectives.

Water is important throughout the whole universe.

Everywhere we look on the Earth that there's water, there's life.

It's thought that the water on Earth

may not have been here from the beginning.

It's believed to have been deposited on the young planet

by icy balls of rock.

If that were the case, Jupiter, with its enormous gravity,

should have pulled in the lion's share of these icy snowballs.

It should be rich in water.

Of course, few believe the giant planet will be teeming with life.

But if we can figure out whether there is water on Jupiter,

and how much, we can get a much better grasp

on where our own water came from.

So, when we're seeking to understand how much water's in Jupiter,

it's also telling us about the history of water,

and maybe the origin of life.

But two decades ago,

this quest for water was dealt a blow

by the only other spacecraft we've flown around Jupiter.

Galileo will expand our knowledge of Jupiter and its satellites.

Like Juno, Nasa's Galileo spacecraft

tried to find water in the giant planet.

It had a probe on it.

And the probe actually went into the atmosphere and measured directly how

much of each element there was.

Jupiter was expected to be relatively abundant in water.

Galileo found almost none.

In the 30 minutes that it took the Galileo probe

to make its measurements, it brought into question every single theory

of solar system formation.

If Jupiter really is a dry world,

the origin of our own water is a total mystery.

But could the result have been misleading?

So, one of the weaknesses of the Galileo probe was

it was only sampling one spot.

And if that spot didn't represent everywhere on Jupiter,

then the measurement really couldn't be extended to the whole planet.

We were searching for another way to do it,

and that's when Juno came along.

The Juno team had to find a whole new approach

to look for water on Jupiter.

It was Scott's brainwave to use the heat trapped in the planet's centre.

Because that heat is slowly leaking out in the form of microwaves.

Water is opaque to the microwave part of the radio spectrum.

That's why your microwave oven heats up wet food.

Juno is measuring how much of the heat leaking from the planet's hot

interior is absorbed by water in the atmosphere.

And, crucially, it doesn't just measure this at one point.

So, we actually look all over the whole planet

at all different angles,

measuring water at every spot.

It could be before the get a final answer on just how wet Jupiter is.

It's a big planet.

And Juno needs to scan it all.

But its microwave detector has made a discovery.

And it's not one the team were expecting.

This is the signal from the microwave radiometer.

And it is measuring the heat of the planet, and

every now and then we get a spike.

Bam! And then it goes back to normal

in measuring this heat of the planet.

It was a little puzzle at first, one of these spikes.

And it was possible that

there was some cosmic ray hitting the instrument.

But it became very clear very soon that this was radio signals from the

lightning in Jupiter's atmosphere.

Maybe 1,000 times brighter than the lightning on earth.

Jupiter, lord of thunder and lightning, didn't disappoint.

The planet's atmosphere is alive with

flashes of lightning of unimaginable power.

And though these signals were a surprise,

it could be evidence for the huge quantities of water

the theories had predicted.

To have lightning, the theory is you need all three phases of water.

Liquid, gas and solid.

So, inside Jupiter's atmosphere there's a layer

where the pressure and temperature are just right to form clouds.

The lightning strikes Juno has detected

suggest the Galileo probe must have fallen into a dry spot.

Jupiter is a water world, after all.

And some of the water might even be visible.

We see these little white clouds.

Little compared to Jupiter,

but they're actually 50km going across.

And we see them at the very top of the atmosphere,

you can tell it at the top, because it's casting shadows.

These, we believe, are ammonia water ice thunderstorms.

It's hailing or snowing on Jupiter.

These thunderstorms are just the tip of a whole layer of water,

hidden beneath the cloud tops.

Perhaps just 100 miles down.

And once Juno has figured out exactly how much is inside Jupiter,

scientists can better understand

how water was delivered to our own planet.

But Juno's instruments have allowed

scientists to travel even further into Jupiter.

And beneath the upper atmosphere and the water clouds...

..is almost pure hydrogen.

The most abundant element in the universe.

And it's this hydrogen that holds the key to the intense aurora

and vast magnetic fields Juno encountered

on arrival to the planet.

Because of the intense pressure inside Jupiter,

something strange starts to happen.

At the National Ignition Facility in Northern California,

Marius Millot is finding out what happens to hydrogen

when it finds itself in Jupiter's extreme interior.

So, here we're trying to replicate

the extreme conditions inside Jupiter.

Before, it was only possible with theory or simulations.

But now we are able to assess, actually, those conditions

in the laboratory, and do the real experiment.

So, basically, we are creating a

little planet interior inside our laboratory.

The National Ignition Facility is home to

192 of the most powerful lasers on the planet.

And they're all focused on a point in space

just a few millimetres wide.

So, this is the target that we put inside this target chamber.

We fill a little portion inside its cone here with liquid hydrogen,

about 20,000,000th of a gram.

And then we shine 80 laser beams from the top,

and 80 laser beams from the bottom.

The lasers create a shock wave,

crushing the hydrogen to the same intense pressure

we'd find 10,000 miles inside Jupiter.

But it takes a lot of power to get there.

So, the power that we've put in the lasers when we shine the laser onto

the target is about 500 times the power

that is used for the entire United States at a given moment.

May I have your attention,

all personnel are required to exit the Target Bay at this time.

It is believed that when hydrogen is crushed to such extremes,

it takes on a form that has almost mythical status here on Earth.

The hydrogen becomes a metal.

It's an extraordinary experiment.

-170 and counting.

PCU's charge complete.

There could be a lot of extra radiation,

high-power voltage,

a lot of laser light could leak if there was a problem.

So, you don't want to be there during the experiment.

Precision is everything.

The hydrogen sample is carefully manoeuvred into position.

-60 and counting.

It takes an army of scientists and engineers

to ensure the laser shot goes off without a hitch.

-30 and counting.

With everything set, it's time to fire the lasers.

T minus five, four, three, two, one, shot...

What's happening? The shot just happened.

We create the condition inside Jupiter

for a few billionths of a second.

After each blast of the laser, the sample is completely destroyed.

So, to find out what happened to the hydrogen,

the team shone light at the sample throughout the experiment.

Hydrogen is normally a clear gas.

But, by replicating the pressure inside Jupiter,

something astonishing happens to it.

A few nanoseconds after the beginning of the experiment,

hydrogen starts to get compressed, but it's still transparent.

And as time evolves, we increase the pressure.

And this time, the pressure got so high

that the light is now reflecting off the shiny surface of the hydrogen.

Hydrogen transformed from being transparent to opaque,

and then becomes a metal, like a shiny metal.

And, because of the blistering temperatures inside Jupiter...

..this metal isn't solid.

It's a liquid.

If we were to go deep inside Jupiter,

we would see a deep ocean of fluid, shiny metal.

This is the first time that we can actually do this kind of experiment

and recreate the exact condition inside a planet.

This is the next layer of Jupiter.

A vast ball of liquid, metallic hydrogen.

In fact, it could make up as much of 50% of the planet's huge mass.

Jupiter is not so much a gas giant as a liquid metal giant.

And this strange substance explains

the incredible electrical phenomena

we saw outside Jupiter's clouds.

Because, as the metal flows, it acts like a huge magnet,

creating the enormous magnetic field.

And a blinding aurora.

It's time to descend to the final layer.

Juno was designed to tell us once and for all

what lies at Jupiter's heart.

Is there a core?

And if there is, how big is it?

And what is it made of?

At Nice Observatory,

Prof Tristan Guillot is using Jupiter's unique orbit

to get answers to these questions.

OK, so, I'm going to use these three chalks to represent Juno.

Pretty accurate? Juno has this special orbit,

it's coming very close to the planet where,

depending on what's inside Jupiter,

Juno will move in slightly different ways.

It's doing something like that.

So, that's how we will be able to measure what's inside Jupiter.

They're using gravity to probe Jupiter.

Because, if there is a dense core at the planet's centre,

Juno will find it by measuring its gravitational pull.

We're able to measure the gravity shield of Jupiter

100 times better than what has been done before.

We were very excited to get the results back.

And we're already starting to get answers.

The answers the team are finding

take us back to the early days of the solar system.

Because at Jupiter's ancient heart

could be the blueprint for building a planet.

Finding out what's inside would tell us a lot about not only how planets

like Jupiter form,

but maybe about the early history of our own solar system,

and potentially about a lot of other extrasolar planetary systems that

we're starting to discover now.

We know what the planets were formed from.

A giant cloud of gas,

and perhaps dust left over from earlier generations of star systems.

But the mechanism is still a mystery.

For a long time, it was thought that

each world formed by a gradual process.

You start with dust grains, those dust grains somehow smush together,

they get bigger and bigger and bigger

until you get pebbles and rocks and boulders.

And we sort of keep walking up the size ladder.

And then those big bodies smash together

to make the cores of planets.

But it's not clear that that process is going to work fast enough to

explain something like Jupiter.

Because one of the big problems with that model is that it takes a long

time to go from little tiny stuff like this,

to a big honking planet like Jupiter.

Scientists believe that Jupiter formed

perhaps just a few million years after the Sun.

So, there needs to be a mechanism that works quickly.

On top of that, it's really hard to make rocks stick together.

You know, what happens if you throw two rocks together?

Nothing, right? They didn't grow, they don't stick together.

The same kind of thing happens in the early solar nebula.

So, how do you go from a dust cloud

to a giant planet like Jupiter in the cosmological blink of an eye?

There are two opposing theories, and, crucially,

they give two different predictions

as to what to expect at the planet's centre.

The first suggests that Jupiter ultimately

started life as a cloud of pebbles.

You take these little rocks,

and instead of trying to squish them together

to make slightly bigger rocks...

..you take a whole bunch of them,

and you let them interact with the gas

that we know is there in the early solar nebula.

And just like when you throw a ball on a windy day,

it slows down faster,

those little pebbles, they feel the gas drag

from the gas that's also going around the star.

And because of a combination of gas drag and gravity,

we can actually get them to clump together for little things like this

directly to 100km bodies.

If this theory is correct,

before Jupiter became the giant we see today,

it was a small, rocky world,

created by a cloud of pebbles.

And Juno could find the evidence at its centre,

because all those pebbles should still be there.

But there is an opposing theory.

One that suggests not even pebbles are needed.

Instead of worrying about pebbles or rocks or planetesimals,

you just take the gas in the early solar nebula

and you collapse it down under its own gravity

to form one big object.

So, it's kind of more like the way we think stars might form,

rather than thinking about growing up from rocky things.

In this theory,

in which Jupiter formed straight from

the gas of the early solar system,

there would be no core at all.

You don't really need rocks, they're not really part of the picture.

Two theories with two predictions.

The truth has been concealed beneath these clouds for 4.5 billion years.

But, thanks to Juno, we finally have an answer.

By analysing small changes in Juno's trajectory,

Tristan and the team have been able to map

what they think sits at Jupiter's heart.

This is the outline of Jupiter.

And, basically, what we're seeing there are just the cloud tops.

But what we want to know is what's inside Jupiter.

And what we're finding is that around here...

..there's something denser.

It's probably like 5-10 Earth masses.

What Juno is telling them is that there seems to be a mass of dense

material, too massive to be just hydrogen or helium.

So, this is the core of Jupiter.

It's mostly made of rocks at very high pressures and temperatures.

So, that shows that Jupiter started its life as a rocky, icy world,

and that it captured the gas that was around, the hydrogen and helium.

And that formed the giant planet that is Jupiter now.

It's now thought that inside Jupiter is a vast sphere of rocky material,

big enough to be a planet in its own right,

perhaps ten times the mass of Earth.

It seems that Jupiter was once a primordial, rocky world.

The vast cloak of gas came later.

But Juno found something else.

To the team's surprise,

there appeared to be an extra layer around the core,

neither rock nor hydrogen.

So, this is a fuzzy core, made of rocks, ices,

mixed with metallic hydrogen.

And that's a really important thing.

Because it really tells us about the formation of the planet,

and this is something we did not consider before.

Tristan believes that this fuzzy core

is evidence of an astonishing and unexpected stage

in planet formation.

A clue to what he thinks happened can be found

in shooting stars.

Shooting stars are little grains that float in space and are

captured by the Earth.

And when it burns, this creates light.

Most of the material is actually deposited in the atmosphere itself.

So, it doesn't reach the ground.

The unexpected fuzzy layer suggests to Tristan that there was a dramatic

period in the planet's formation,

when Jupiter was a rocky world with a growing atmosphere.

And from the heavens, the rocks fell like rain.

There must have been millions of shooting stars.

It must have been very bright, but also very hot.

And as the shooting stars and fireballs burnt up on the planet's

atmosphere, that's where this rocky material stayed.

That's how they formed a mixture of hydrogen and rocks.

That's probably what led to this fuzzy core that we see today.

This final layer of Jupiter is a surprise.

A fuzzy mixture of rock and liquid metallic hydrogen

enveloping the core.

And it marks the end of our journey into this giant.

But the mission goes on.

Juno will continue to send back insights

into the wonders inside Jupiter.

It's always better if we can really change

the direction of the field with something like Juno.

Eventually, the radiation will become too much.

The spacecraft will be damaged beyond use,

and plunge into Jupiter's atmosphere to safely burn up.

It will be a sentimental moment.

The longer you work with a machine,

and with the people, the more you get attached to it.

But scientists will long pore over its results,

perhaps for decades...

Juno has been a great success.

We've made a lot of discoveries.

Jupiter is quite unlike what we thought it was before.

..continuing to explore the heart of this vast and wondrous world.

We've discovered so many things already,

and it's an adventure when you get to go discover

things that you don't know.

Juno, mythical wife of the god Jupiter,

saw through the veil of clouds he used to hide his mischief.

And, thanks to Juno the spacecraft,

we're finally making the king of the solar system lift his veil, too,

revealing his secrets to us all.