The Planets

The night sky is ablaze with stars...

..hundreds of billions in our galaxy alone...

..many larger, brighter and more majestic than our sun.

On the scale of galaxies and stars, the planets of our solar system

are little more than grains of sand caught momentarily

in the light of the sun.

But on those motes of dust, for over four billion years,

great stories have played out unseen.

Stories of worlds born...

..and worlds lost.

Planets forged amongst the calm...

..and the chaos.

Their destinies more entwined than we ever imagined.

We know this...

..because in the last few decades...

..we've sent spacecraft to all seven...

..of the worlds beyond our own.

These are the stories that they return to Earth,

the stories of the planets.

For the first few million years after the sun formed,

there were no planets to see it rise...

..just clouds of dust and the gas...

..the leftovers from the birth of the sun.

Over tens of millions of years, the dust began to stick together...

..and form the first rocks.

Eventually, gravity assembled the rocks

to create planetary embryos...

..that in time formed the four closest planets to the sun.

Today, Mercury is the closest of all,

enduring the sun's full glare.

Further out...

..lies Venus...

..chocked by a thick atmosphere.

Then Venus's neighbour, Earth.

And farthest of all, Mars...

..a cold desert world.

Together, they formed the only rocky so-called terrestrial planets

in the solar system.

And of the four...

..one is unique.

Just look at this...

..and listen to it.

This is what a planet looks and sounds like

after four billion years of evolution by natural selection.

There is nowhere else in the solar system

that looks and sounds like this,

which is interesting, when you think about it,

because all the planets and moons and are made out of the same stuff -

they're carbon, nitrogen, oxygen, iron.

All those atoms were present in the cloud that collapsed

to form the solar system, four-and-a-half billion years ago.

And yet Earth appears to be exceptional -

a lone living planet in an otherwise desolate solar system.

So what is it that makes this place so special?

Is it fate? Is it chance?

These are important questions

because Earth is the only place we know of

where the most complex phenomena in the universe exists,

the thing that brings meaning to the universe - life.

Earth is a special world in our solar system

and perhaps even for thousands of light years beyond.

Our world certainly has unique properties.

It's the right size and distance from the sun

to have retained an atmosphere that's protected its oceans

of life-giving water for billions of years.

But as we've left the blue planet,

and explored our sister worlds...

..we've discovered that each appears to have had a moment

when it enjoyed almost Earth-like conditions.

Every one of our rocky neighbours has a story of what might have been.

Mercury is a small, tortured world.

More than any other planet,

it's endured the unflinching glare of the sun for billions of years.

Mercury is a world of mystery and apparent contradictions.

It's in quite an elliptical orbit,

which means it can be as far away from the sun as 70 million km,

but as close as 46 million.

That means that temperatures at midday

can rise to 430 degrees Celsius on the surface,

but, at night, because it's a small planet and it's got no atmosphere,

temperatures fall to minus 170 degrees.

It's also locked into what's called the spin-orbit resonance,

which means the planet spins precisely three times on its axis

for every two orbits

and that in turn means that its day is twice as long as its year.

That means that I could walk over the surface like this

about 2mph

and keep the sun at the same point in the sky.

I could stroll in eternal twilight.

Mercury is the least explored of the inner rocky worlds...

..because getting to a planet in such a strange oval-shaped orbit,

so close to the sun...

..is a tremendous challenge.

Five, four, three,

main engine start, two, one and zero.

And lift off of Messenger on Nasa's mission to Mercury...

..a planetary enigma in our inner solar system.

Now going through the sound barrier.

A direct route to Mercury is impractical.

Now going through the period of maximum dynamic pressure.

A spacecraft would arrive with so much speed

that it would need vast amounts of fuel to slow down

and enter orbit around Mercury.

We just had spacecraft separation.

So Messenger controlled its trajectory

by stepping from one planet to the next,

using gravity to slow itself,

spiralling inwards towards its target.

Even so, Messenger approached Mercury at such high speed

that it was forced to fly past the planet three times...

..slowing on each pass...

..until, after almost seven years of flawless navigation,

it arrived safely in orbit.

Messenger set about its mission to map Mercury's surface...

..and began revealing the secrets of the most cratered planet

in the solar system in exquisite new detail.

Messenger was able to do much more

than just take images of Mercury's surface.

By tracking radio signals emitted by the spacecraft,

we're able to see very slight changes in the orbital path

around Mercury, as seen from Earth,

and that allows us to map out Mercury's gravitational field.

There are also instruments that allow us

to see how the planet wobbles around

as it spins on its axis.

And putting all these measurements together

allows us to take a cross section through the planet,

to see what it's made of.

And when we do that, we find something very strange.

Mercury's core extends out about 85%

from the centre of the planet to the surface.

It's almost entirely an exposed planetary core.

It's as if the rocks of the surface were smashed away

and removed at some point in its past.

And there was more.

The tiny probe began detecting chemical elements in concentrations

that no-one had thought possible this close to the sun.

The discovery of relatively large concentrations of elements

like sulphur and potassium on Mercury's surface

was a huge surprise.

If you think back to the time when the planets were forming,

you don't expect high concentrations of those elements close to the sun,

where Mercury orbits today, because they're so-called volatile elements.

They boil away easily,

so you only find high concentrations further out,

in the colder reaches of the solar system.

So Mercury is an enigma

and discoveries like these have forced us to completely rethink

our theories about the formation of the planet.

Just a few million years after its formation,

Mercury was still seething with the heat of its violent birth.

Slowly, it cooled and a crust formed.

Over time, the crust became enriched

in the volatile elements that were escaping Mercury's interior.

But this could only happen if Mercury started out

not in the position we see it today...

..but much further out.

We now think Mercury was born perhaps 170 million km

further away, close to the orbit of Mars...

..a place where, if it had stayed,

its destiny could have been very different.

But it wasn't to be.

The young planetary embryo was ripped from its promising position

long before it could mature.

Today, it's hard to imagine the planets

in orbits other than the ones we see in the night sky.

They feel eternal, permanent.

It's natural to think of the solar system

as a piece of celestial clockwork, almost like a Swiss watch.

So if we knew where all the planets were at some point in time,

let's say today, then we could imagine calculating

exactly where they're going to be at any point in time.

Now, that is true if there's only one planet and one star.

So imagine that's the sun and this is Mercury.

Now, we know the gravitational force between Mercury and the sun.

And, indeed, if that's all there is,

then we can calculate its orbit around the sun

with essentially infinite precision.

But add in one more planet, let's say Jupiter over there.

Now there's a gravitational force between all three of these objects

and it turns out that, even in principle,

it is not possible to calculate exactly

where they're all going to be in the future

or where they were at some point in the past.

This means that any uncertainty, even of a few metres

in our knowledge of the position of the planets,

can lead to radically different predictions.

And that's because the system itself, the orbits of the planets,

are not stable over very long timescales.

So planets don't necessarily remain in the same orbits forever.

And the evidence we've gathered

from the volatiles on Mercury's surface,

and the unusual size of its core,

suggests that this may have been what happened.

If Mercury began its life

170 million km further away from the sun...

..then it would have been in a

where the young Mars was also forming.

This region was full of scores of planetary embryos,

all fighting for position.