The Planets

Lift off of Messenger on Nasa's mission to Mercury.

Beyond the inner solar system...

past the asteroid belt...

lies Jupiter.

The largest of the planets is a giant swirling ball of gas...

its marbled appearance generated by violent winds

that rage through its clouds.

Jupiter is a world as strange as it is remote.

If you fly upwards, only about 100km or so,

you reach the edge of our atmosphere and the blackness of space.

It's really not that far.

But once you're in space,

the distance scales are sort of incomprehensible.

I mean, Venus, the closest planet, is over 40 million kilometres away.

Jupiter around 650 million kilometres.

It takes our fastest spacecraft years to get there.

So, the idea that those planets could have any influence on us

here on Earth seems fanciful.

And, indeed, if you're talking about

your chances of meeting a handsome stranger

or your life plans requiring a moderate reconfiguration

about a week on Wednesday, then you would be right.

However, if you ask more profound questions, deeper questions like,

"Why is the Earth the way that it is?

"Why is there life on Earth?

"Why do we exist at all?"

then it turns out that the planets, and Jupiter in particular,

have had a profound effect.

Early in its life, the young Jupiter went on the rampage.

The giant planet embarked on a voyage of destruction

across the solar system...

that transformed the destiny of the planets,

and the course of life on Earth.

Jupiter is the godfather of the planets.

Understand it and you understand how the solar system came to be.

On Earth, we can see glimpses of Jupiter's power...

in the scars it's left...

on the face of the planet.

This is the famous Barringer Meteor Crater in Arizona.

It is about a kilometre across,

170 metres from the rim to the crater floor

and it was created about 50,000 years ago

when a meteorite, 50 metres across, entered the Earth's atmosphere

travelling at about 13km per second.

That means it would have taken around, what, seven seconds

to go from the top of the atmosphere to the ground

and it hit with such force that the energy released

was equivalent to ten megatons of TNT.

And that is a very large nuclear bomb.

The meteorite that hit the Earth here

is thought to have come from the asteroid belt...

the ring of rocky debris that orbits at the edge

of the inner solar system.

And it was deflected from its orbit

most likely by the gravitational influence of Jupiter.

Jupiter is a completely different class of planet to the Earth.

A gas giant.

A swirling ball of hydrogen and helium so large

you could fit 1,300 Earths inside.

Jupiter can exert such a powerful influence on the asteroids

because it's so massive.

It's about two-and-a-half times the mass of all the other planets,

moons and asteroids in the solar system combined.

And, therefore, it has a strong gravitational pull.

And this means that Jupiter exerts an influence

across the entire solar system that's second only to the sun.

And that influenced us far more than create the odd crater.

It can change the history of worlds.

To understand how Jupiter rose to such dominance,

you have to go back to a time before the solar system even existed.

Nearly five billion years ago,

a distant exploding star sent a shock wave across the galaxy...

causing the cloud of gas and dust

that would become our solar system to collapse...

as the sun began to form.

But further out, something else was growing.

After 50 million years...

the sun's nuclear furnace ignited.

The light of its first dawn...

revealing Jupiter...

the solar system's first world.

A planet born so early, it swept up most of the material

left over from the formation of the sun.

Because of its dominance, Jupiter had a profound effect

on the newly forming solar system

and paved the way for our living world to form.

This is a model of our solar system.

So, here's the sun, and then we have the four inner rocky planets,

Mercury, Venus, Earth and Mars.

Then comes the asteroid belt,

the ring of millions of smaller pieces of rock

orbiting around the sun.

And then the gas giants.

Jupiter, Saturn, Uranus and Neptune.

Now, this has a pleasing regular structure about it.

All the rocky planets close to the star

and the gas giants further out.

And we thought that this must be the natural order of things,

the way that solar systems have to form,

until we started to detect planets around distant stars

and then we found that this arrangement is not typical at all.

Our sun is just one of around 300 billion stars in our galaxy...

and almost every one of those stars

is home to its own system of planets.

In most other systems, the region where our planet orbits is empty.

Instead, closer to the star, we see super-Earths,

vast rocky planets many times bigger than our own.

But, crucially, these worlds are thought

to have thick toxic atmospheres,

making them completely inhospitable to life.

And that raises a very interesting question.

Why, then, is our solar system the way that it is?

To answer that question,

we've sent spacecraft to every one of the planets...

and they've uncovered strange anomalies

that reveal our solar system's past.

Our missions to Venus found that it once had far more water

than you'd expect for a planet so close to the sun...

just as Earth does today.

The spacecraft we've sent to Mars

touched down on a world that is curiously just half the size

of her sister planets Earth and Venus.

And in the asteroid belt,

we've uncovered a graveyard of failed worlds...

their growth mysteriously cut short.

To cause all these anomalies,

it's thought that something catastrophic must have happened

very early in the solar system's history.

All the evidence suggests that around 4.5 billion years ago,

Jupiter's orbit began to change,

triggering a period of unprecedented violence

which completely transformed the face of the young solar system.

And we know this because the marauding planet

left a trail of destruction in its wake.

Early in its life, Jupiter began to move inwards towards the sun...

and the effects of that journey can be seen to this day.

In between the orbits of Mars and Jupiter...

lies the asteroid belt...

a desolate wasteland of debris

left over from the beginning of the solar system.

Rocky bodies too small to be planets in their own right.

In science fiction films, the asteroid belt is often depicted

as an un-navigable mass of tumbling rubble.

But it's not like that at all.

So much so, in fact, that modern spacecraft engineers

can fly a spacecraft through without making any course corrections.

The reason for that is there's not much mass in the asteroid belt.

A sum total of all of them is only about 4% the mass of the moon

and they're spread out in a huge sort of ring,

out beyond the orbit of Mars. So, it's a vast amount of space.

That means that the average distance between any two large asteroids

is about eight times the distance between the Earth and the moon.

So, I could stand on one of those large asteroids

and look out into space

and my nearest neighbour would just be a dim star in the dark.

But scattered amongst the rubble are objects

that are far more than mere rocks.

Five, four, three,

two, main engine start, one, zero and lift off

of the Delta II Rocket.

In 2007, Nasa's Dawn mission

was sent to explore the two largest objects in the asteroid belt.

After a year in orbit around the giant asteroid Vesta...

in March 2015...

Dawn made its final approach on a body so large

it makes up almost a third of the mass of the entire asteroid belt...

Ceres.

On arrival, the first images Dawn sent back to Earth

contained something entirely unexpected...

mysterious bright patches in one of Ceres' largest craters.

Spectral analysis of the images

showed that the patches were, in fact,

the salty residue left behind by liquid water...

suggesting there must be large amounts of ice

just beneath the crust.

Dawn went on to discover over 300 bright spots all over the surface.

And this means that beneath this thin crust,

Ceres has a thick ice rich mantle surrounding its rocky core...

evidence that the dwarf planet is made up of multiple layers.

The interior of Ceres is like nothing else in the asteroid belt.

The differentiation, that separation into layers, observed by Dawn

is more characteristic of a planet than an asteroid.

So, 4.5 billion years ago, Ceres was well on its way

to becoming a fully-fledged world.

It's thought that's in its infancy Ceres was very different.

The water that now lies frozen within its interior was once liquid.

Protected beneath a thin layer of ice...

Ceres was once covered by a deep saltwater ocean.

Before Ceres had reached its full potential,

a great disturbance sent shock waves through

what would one day become the asteroid belt

and cut its development short.

As Jupiter circled the young sun,

it started to clear a path through the gas and dust

that shrouded the early solar system...

but that process caused it to do something alarming.

Gravitational interactions with the gas made the giant planets

spiral inwards towards the sun...

ploughing straight through the

that would become the asteroid belt...