The End of the Solar System Horizon

Our sun.

The heart of the solar system.

The giver of light, heat...

..and, of course, life.

But what does its future hold?

Scientists are looking to the stars to find out.

Between these two stars is what's going to happen to our sun.

Scientists today are almost like modern-day prophets.

They foresee an apocalyptic future.

Imagine the ball is Andromeda Galaxy

on a head-on collision with the Milky Way Galaxy.

The fate of the Earth hangs in the balance.

Wow! Look at this!

The temperature at the surface of the Earth

will be enough to melt rock.

Enough to melt the whole surface of the Earth.

Unfortunately, nobody will be around to see it, which is a pity.

This is the story of how our sun

will transform the solar system it binds together.

Before bringing it to a spectacular end.

Peoria, Illinois.

An average city in Midwest America.

But it has one claim to fame that's out of this world.

In the middle of the town, there's a 46-foot-wide mosaic of the sun.

The centrepiece of a huge scale model of our solar system,

created by local astronomer, Sheldon Schafer.

And here we are at the sun. And, boy, is it hot!

It's about 10,000 degrees here at the surface.

And over a million Earths could fit inside of the sun.

Peoria's solar system,

99 million times smaller than the real thing,

accurately reveals the relative sizes of our sun and its planets.

OK, we're all together?

And the distances between them.

My job title is curator of the solar system.

And we just went 33 million miles

until we got to this tiny little two-inch Mercury.

All right, so we're headed off to Venus!

From Mercury, the inner planets are strung along

a picturesque riverside park, all the way to Mars.

These planets are relatively close together.

The outer planets are much further away,

in some bizarre locations.

Five miles from the image of the sun,

above the local airport's check-in desks,

is five-foot-wide Jupiter.

If you're going to have a planet, you may as well have the biggest!

So it's fun to have Jupiter.

Occasionally, we have birds that decorate,

so we've had to clean it. But not very often.

While the children's section of a neighbouring town's library

is home to Saturn.

Uranus is in Princeville, Illinois.

From there, it's a 10-mile drive along Route 91,

or almost a billion miles in cosmic terms,

to the old railroad depot in Wyoming, Illinois, and Neptune.

And finally, in a furniture store

40 Earth miles away from the centre of the sun

in Kewanee, Illinois is distant Pluto.

Peoria's models are a perfect likeness of the solar system today.

But it won't always be this way.

Scientists know that one day, the sun will fundamentally change.

And transform the planets.

Imagine fast-forwarding through the next seven billion years

to watch the end of the solar system.

Dr Eva Villaver can predict this future.

Because everything that will happen to our sun

is already happening to countless other stars.

Some, known as solar twins,

are remarkably similar to our own.

The studies we are doing is because they are very important

to understand not only the sun,

but they tell us how the future of our own solar system will be.

In 2013, a solar twin called CoRoT Sol 1 was discovered.

CoRoT is over there, in the constellation of Monoceros.

It's a star, like the sun,

and has the same mass. Exactly the same mass.

But astronomers found one particularly significant difference.

It had a lower concentration of the element lithium,

which helped them to accurately calculate its age.

It's a star that is a little bit older than the sun.

A few billion years older.

And if we observe a star that is older than our sun,

we know what will happen to the sun.

This older version of our sun was giving out more radiation.

So it helped us put the pieces together.

As the sun will get older,

it will become brighter. Much brighter.

Our sun's luminosity is slowly increasing

because of a change deep inside the sun.

Where two opposing forces are in constant battle.

Similar forces to those that act on a hot-air balloon.

Pushing up and out is the immense pressure of hot gas.

In the sun, this is created by nuclear fusion.

The sun has been burning hydrogen into helium

for thousands of millions of years now.

This is like the propane bottles here.

It's like generating heat that warms up the air

that keeps the balloon going.

That's what happens in the sun, too.

But pulling down into the core of the sun

is an equally powerful force - gravity.

The life of the sun is nothing but a battle against gravity.

We have the gravitational force trying to pull the stars,

crush the stars together.

I mean, like pushing it in, and then we have

the thermal pressure of the gas pushing outwards.

So the balance between the two forces is what keeps the sun stable.

For 4.5 billion years,

the two forces have been in perfect balance.

But as time passes, this balance is shifting.

As the sun fuses hydrogen,

it produces around 600 million tonnes of helium every second,

which is a denser gas.

This change in density

has a profound effect on the nuclear reactions.

As the core gets denser, hydrogen is burned at a higher rate.

It's like turning the burners up.

I mean, we are increasing the energy

that is coming out of the core at that point.

As a result, our sun is getting 10% brighter every billion years.

So the older it gets, the more it heats up the solar system.

And scientists know that will one day have serious consequences...

..for Walter Kinsman's favourite planet.

The Earth is my favourite planet to paint.

I never get my fill of it.

He's painted all the planets in the Peoria solar system model,

except Jupiter, which was too big to fit in his house.

He's now painting a spare Earth for the local museum.

I'm in the process of painting a storm system

in the southern Indian Ocean.

The beautiful white clouds

up against the blue oceans is breathtaking.

The Earth only has its oceans and clouds

because it orbits in a band around the sun called the habitable zone.

Which means it's just the right temperature for liquid water.

And that makes it the only planet in the solar system

where we know life can thrive.

But as the sun becomes more powerful,

the habitable zone will move.

For a vision of the Earth in two billion years' time,

astrobiologist Professor Lynn Rothschild

believes we should look to Venus.

Venus is up in the sky there.

It's the brightest object after the sun and the moon.

It's right near Jupiter this morning.

It's just an absolutely spectacular day to see it.

Venus and the Earth formed out of the same materials.

They're roughly the same size.

The difference is that Venus is closer to the sun.

The surface of Venus is the most hellish planetary surface

in our entire solar system.

The winds are ridiculous. They're 350 miles per hour.

And then the temperature is unbelievably hot,

about 900 degrees or so Fahrenheit.

So this is not a place that you'd want to be.

It's no surprise Venus is warmer than Earth,

but strangely, Venus is even hotter than Mercury,

despite being further from the sun.

In 2006, the Venus Express probe

launched towards our nearest planet

to analyse the Venusian atmosphere in unprecedented detail.

It found a vital clue among the clouds

to how Venus became so hot.

Venus Express allowed us to see that

there was a lot of deuterium, which is a heavy form of hydrogen, left.

And that's indicative of the fact that there was once water here.

It soon became clear that in the past,

Venus was a very different world.

So here was this beautiful water world,

not too dissimilar to maybe what the Earth is like today.

There was liquid water and reasonable atmospheric pressure

and organic compounds.

There's no reason that there shouldn't have been life.

The evidence suggests that Venus was once in the habitable zone.

But, as the sun grew brighter three billion years ago,

it would have had a dramatic effect on the planet's water.

As the sun started to get hotter,

the surface of Venus started to get hotter.

And therefore, the water turns into steam.

And steam is a greenhouse gas,

so that means it traps the solar radiation.

And therefore, just like a greenhouse,

it starts to get hotter and hotter.

It seems a runaway greenhouse effect

caused Venus to become the hottest planet in the solar system.

Mercury, although closer to the sun, has no atmosphere and no water.

Earth has both.

And as the brighter sun evaporates our oceans,

the effect is likely to be far more intense

than the man-made global warming we see today.

Over the next two billion years, temperatures on Earth will rocket.

Life here must adapt...or die.

Yellowstone National Park in North America

is a natural laboratory for Lynn to study

how life can survive in extreme conditions.

The reason it's so great is that we have the whole range,

from the top predators, things like wolves and bears and so on,

all the way down to the beavers and the herbivores

and down to the very tiny organisms

and even some incredible microbes.

Life here is used to dealing with extremes.

But in about half a billion years' time,

these extremes will go in the opposite direction

as temperatures could climb by up to 20 degrees in some places.

By then, life as we know it will have evolved to be very different.

But just as some of today's animals

have adapted to survive harsh winters,

in the future, they may use similar strategies

to cope with scorching summers.

As the sun gets hotter, you could imagine the winter

as being the very pleasant season

and the summers become unbearably hot.

So if you're thinking about a bear that lives in an area like this

that would normally hibernate in the winter,

if you turn the thermostat on the Earth high enough,

it might be the reverse.

So that now, animals would be hibernating in the summer

and be active in the winter.

And grasses would be setting seed now, in the spring,

the seeds would be what would carry the plant through this harsh summer,

and then, as the rains started again in the autumn,

they would germinate and you would get the lush green in the winter.

In less than a billion years' time,

the greenhouse effect is expected to take off.

Sending temperatures soaring.

As it gets hotter and hotter on the land,

eventually, even the winters will be too hot

for most organisms, certainly, to live.

So if you have a large animal, like, say, a bison,

that's also warm-blooded, as it gets hotter and hotter,

it won't be able to cool down and it will eventually die.

And so ultimately, large animals like that will go extinct.

In just over a billion years from now,

the land could be nothing but a parched desert, devoid of life.

The air is going to heat up much more quickly than water will.

And so I predict that, just like the ancestors of whales

and dolphins and so on moved from the land to the water,

so will the descendants of bison, if they want to survive.

But models suggest that in two billion years' time,

even the water will have gone.

As it boils away, the Earth would increasingly resemble Venus today.

For those of us who are interested in the future of planet Earth,

Venus is a really good model system.

As the sun heats up and the oceans turn into steam,

we will have a world that's not too dissimilar

from what you see behind me in Yellowstone,

where you see the hot water coming up to the surface

and then turning into steam and going away.

In less than three billion years' time,

it's thought that the searing sun and a runaway greenhouse effect

will have wiped out virtually all life on Earth.

But intelligent life may just find a way out.

We have something that the other organisms out there don't have.

And that is we have technology.

And we're going to have the option of going to other planets.

As it gets too hot for the Earth, Mars will start to warm up.

And so that means that it's just possible

Mars will become a better place for life.

Who knows? I have great faith in our descendants.

By then, Mars is expected to be in the habitable zone.

So it could provide a refuge.

But not for ever.

Because the next threat will be to the entire solar system.

From 100 billion stars racing towards us.

The Andromeda Galaxy.

Scientists have long suspected

it will one day crash into our galaxy, the Milky Way.

But until recently, no-one had been able to say for sure.

In 2012, Dr Tony Sohn stepped up to the plate.

He and his team set out to precisely measure Andromeda's path

and discover if it would be a near miss,

a glancing blow...

..or a head-on hit.

To predict the outcome, he used a technique

familiar to baseball players.

I ran a experiment that can help explain how we measure

the motion of Andromeda.

Imagine a game of baseball.

The batter is waiting for the ball thrown by the pitcher.

To work out if the ball is on target,

the batter needs to see whether it's drifting to the side or not.

So they instinctively compare the motion of the ball

against the background.

Tony needed to apply the same principle

to discover if Andromeda was heading towards us.

But in order to measure the galaxy's motion,

he had to find fixed points behind Andromeda to compare it to.

A daunting task.

Most of the stars we see in the sky are in our galaxy,

so they cannot be used as background objects.

Instead, Tony had to search for distant galaxies

hundreds of millions of light-years away.

Only one telescope was up to the job.

We used the Hubble Space Telescope to do this project

because we needed a very stable instrument

and we needed to be above the Earth's atmosphere

to get very high resolution of the image.

With data from Hubble,

Tony painstakingly tracked stars in Andromeda

against distant galaxies.

Just like a batter tracks a ball.

Imagine the ball is Andromeda Galaxy

and the fence behind that are background galaxies.

And what we did was we compared the position

of the Andromeda galaxies against the background galaxies over time.

And that's how we measure the sideways motion.

The results were conclusive.

The sideways speed of Andromeda we measured was effectively zero.

So we can say with certainty that Andromeda

is on a head-on collision with the Milky Way Galaxy.

Tony's team confirmed that over 100 billion stars

are on course for a strike

at 2,000 times the speed of a fastball.

But since it's so far away, the galaxies won't collide

until nearly four billion years from now.

Tony's precise measurements allow him to predict

how this clash of the titans will look.

To anyone on Earth, it would be a spectacular sight.

We'll see the Andromeda Galaxy getting bigger and bigger on the sky

and then eventually, in about four billion years from now,

we'll see the collision of the two galaxies.

On impact, clouds of dust will be crushed together.

With sensational results.

What we'll see is a lot of stars getting formed,

and this will look something like stellar fireworks on the sky.

Tony can even calculate the odds that our solar system

will crash into one of Andromeda's billions of stars

during the collision.

Perhaps surprisingly, the prognosis is good.

Galaxies are essentially empty space.

So the chance of stars colliding with another star is very slim

because this distance between the stars is vast.

So when the collision happens, the solar system

will pass through an empty space between the stars.

After passing like ghosts in the night,

the irresistible pull of gravity will draw them back together

over the next two billion years.

To finally settle as a new super-galaxy, nicknamed Milkomeda.

Our galaxy will no longer exist.

Yet calculations suggest the solar system will survive.

It will merge into one big galaxy

and it will look like a giant ball on the sky.

Sadly, it's unlikely anyone will be on Earth

to witness this colossal galactic collision.

But there's a slim chance an extreme form of life

could be clinging on as the two galaxies meet,

despite the searing heat from the ageing sun.

In Yellowstone, Professor Lynn Rothschild has found evidence

of what those last remaining Earthlings might be like.

This area of Yellowstone is extremely acidic, and it's also hot.

You can see the steam rising.

So in other words, it's sort of like boiling battery acid.

Very few living things can actually live at this high temperature.

But there are a couple of organisms that are very well adapted for it

and you can see the beautiful colours behind me.

The kaleidoscopic colours of Yellowstone springs

are caused by heat-loving microbes.

We can pretty much use these as a thermometer.

Anything that is green means

that it's got chlorophyll, just like plants.

And once they get to a temperature above about 73 degrees or so,

their chlorophyll breaks down.

And so when you start getting warmer than that,

you start to move into other sorts of organisms.

Organisms that, for example, eat iron.

And then you see these beautiful orange colours.

Once all the water on Earth has turned to steam,

it's possible that heat-loving microbes could continue to live.

In the clouds.

We know some of the earliest organisms on the Earth were thermophiles.

Organisms that lived at high temperature.

And so at some point, it may be organisms like this

that once again inherit the Earth.

The microbes will have their day.

But their reign will inevitably be cut short.

Because when the sun is twice the age it is now,

astronomers foresee a turbulent new phase...written in the stars.

On a clear night, many of the stars

you can see with your naked eye today

are going through this phase.

You can tell which ones they are because of their colour.

They're known as red giants.

It's very easy to see red giant stars because they are very bright.

They are giant and they are bright.

So they are everywhere in the sky.

Some red giants are so large,

you could fit our own sun inside them - millions of times over.

Yet astronomers are confident our sun will one day grow

to become one itself.

So these stars are a glimpse of our future.

If we study the stars that grow in size, we can tell the fate

of the planetary systems that are orbiting in them.

Stars like that give us already clues

about what will be the future fate of our own solar system.

The transformation of our sun into a red giant

will begin deep below its surface,

where all the heat is generated.

The burning core is the only place hot enough for hydrogen to fuse.

And yet it makes up less than 2% of the sun's total volume.

For the next five billion years,

it's thought the core will be stable,

finely balanced between two phenomenal opposing forces.

The crushing pull of gravity...

..and the explosive push of nuclear-heated gas.

But, like a hot-air balloon,

the core will eventually run out of fuel.

Just as gravity pulls the spent balloon down,

in the sun, gravity will pull on the core, unopposed.

When the balance is broken,

because the hydrogen runs out in the core,

the dominant force will be gravity.

It will try to squeeze the core.

But the sun will be far from spent.

As gravity crushes the core,

it will trigger a transformation in the rest of the sun.

For the first time, the hydrogen gas surrounding the core

will begin to fuse,

giving the sun access to far more fuel than it's already burnt.

We ran out already of one bottle of propane,

but we have three more.

So...it's like the sun.

The burning shell of hydrogen releases so much heat

that gravity is overwhelmed.

Tipping the balance in favour of rapid expansion.

Gravity is not winning the battle,

so the star expands as a red giant.

Astronomers predict that in about five billion years,

the sun will start to grow into a vast, seething ball of fire.

A red giant.

Sending temperatures soaring across the solar system.

The inner planets will become far too hot to support any kind of life.

But the distant outer planets

will bask in the warm glow of the sun for the first time.

The habitable zone, where life can exist, will sweep out.

In Peoria's solar system model, it would mean the habitable zone

would leave town and head for the outskirts.

Here, at the airport, is Jupiter.

You got your bag sheet?

You're all good, you're going to go to gate number ten.

-And it is delayed until 1:30?

-Yes, ma'am.

My favourite planet, I would say, is Earth,

but Jupiter's second, for sure.

It's very cool. It's very cool.

When the sun grows, Jupiter will come in from the cold.

And although life as we know it

could never survive on gassy Jupiter,

the solar system's biggest planet has several icy moons.

These are likely to melt and become cosmic watering holes

for any refugees fleeing the parched inner solar system.

Astronomers have speculated that Jupiter could change colour.

As clouds of ammonia vaporise,

it might turn a deep shade of blue.

After Jupiter, astronomers expect the habitable zone

to move swiftly towards Saturn.

On Saturdays, we'll have families making that interplanetary trip

from one planet to the other in our area.

I think Saturn is more interesting because of the rings.

If Saturn still has its icy rings by then,

they're forecast to vaporise and disappear.

But, like Jupiter, Saturn's icy moons could melt

and be safe havens for life.

Then, models predict the habitable zone

will sweep out faster and faster,

past the solar system's most distant planets and their moons.

First, Uranus.

Then, deep-blue Neptune.

Astronomers think they, too, will be transformed.

But exactly how they'll look in the future is still a mystery.

Eventually, the habitable zone is forecast to pass beyond

all the planets and their moons.

But although Neptune's the final planet,

the solar system doesn't finish there.

At the Good's furniture store in Kewanee, Illinois...

..adjacent to a wide selection of cabinets and coffee tables,

is ex-planet Pluto

and its large moon, Charon.

In 2006, Pluto was downgraded to a dwarf planet.

They say planet Pluto is no longer a planet,

but to us, it will always be a planet.

People are so amazed at how small planet Pluto is.

They get up real close with their camera,

just a couple of inches away to snap a really close shot.

Astronomers have tried to predict what will happen

as this distant outpost of the solar system warms.

But because it's so small and remote,

this world was shrouded in mystery...

..until recently.

In July 2015, the New Horizons' mission

finally revealed Pluto's secrets.

The first clear images ever captured of the dwarf planet

revealed some startling terrain.

Strange troughs, cliffs...

and even dunes.

Professor Lynn Rothschild is fascinated

by this tiny world and its potential for life.

With the New Horizons' mission,

we really knew almost nothing about Pluto.

And our knowledge of Pluto has just blossomed enormously.

In fact, it's not blossomed, it's exploded.

One of the most unexpected features is a towering series of peaks.

Much, I think, to everyone's surprise,

there were huge mountains that were found on Pluto.

These things are as high as 11,000 feet.

Sort of like the mountains behind me, here in the Rockies in Montana.

Here on Earth, the chemical bonds that bind rock

are strong enough to defy gravity by holding up mountains.

Yet Pluto's crust is not made of rock, but ice.

The -220 degree temperatures there

alter the chemical bonds in Pluto's ice.

And make it as strong as rock is here.

Strong enough to hold up ice mountains as high as the Rockies.

But the arrival of the habitable zone would change this.

Pluto's frosty peaks could be destroyed.

As the sun becomes hotter and hotter,

the ice mountains will start to collapse, I would imagine,

under their own weight because at that point,

the ice won't be as hard as it is today.

And at some point, it may in fact be warm enough

for all this ice on Pluto to melt.

Amid the destruction, something remarkable could emerge.

A water world at the edge of our solar system.

Once you have liquid water and little energy,

that's very good news for life.

At that point, it'll be warm enough

that even, even Pluto will be in a habitable zone.

It will finally have its moment in the sun.

After a 12 billion year long winter,

the expanding sun may bring spring to Pluto.

But while the red giant nurtures Pluto...

..it poses a grave threat to the planets of the inner solar system.

They face total annihilation.

In 2012, Dr Eva Villaver stumbled across grisly evidence

of what red giants can do to their inner planets.

A search for distant worlds had led to the constellation of Perseus,

where a star called BD+48740

caught her attention for two reasons.

There we have a star, a red giant,

that was very peculiar

because the star itself has a very high content of lithium.

And that's very unusual for this type of star.

So that was one of the pieces of the puzzle and the other one was

that it has a Jupiter-like planet orbiting the star

that has an orbit that is very unusual.

Eva thought the two strange features must be somehow connected.

Something had happened that had affected

both the planet and the star itself.

The team analysed the possible causes...

..and concluded there was only one event that could explain both.

The most simple explanation is that something very violent happened.

We think that the star had a multiple planetary system

and what we see is just the leftover planet,

but there was another planet that was eaten by the star.

As one planet was engulfed by the star, it destabilised the other.

Then it triggered lithium production by stirring the hot gases.

So this star has eaten one of its planets

as the star became a red giant.

Eva had found compelling evidence

that ageing stars can grow so large,

they devour their inner planets.

In around 5.5 billion years,

our own sun will enter this extraordinary phase of its life.

Evidence suggests its surface will reach out towards Mercury,

Venus and Earth,

threatening their very existence.

Local astronomer Sheldon Schafer

is leading his weekly inner-planetary bicycle tour.

With the sun's surface hot on his heels.

So right now, we're going at about four miles an hour.

That's about half the speed of light.

This peaceful Midwestern town is about to go on the ride of its life.

And here we are, approaching Mercury.

You can see it's easily a stunt double for the Earth's moon.

It's a heavily-cratered world

without an atmosphere, hot in the sun

and cold in the darkness.

But the solar system's smallest planet will get hotter still.

Off to Venus!

Because astronomers predict

that less than a billion years into the red giant phase,

the sun's surface will reach Mercury.

After more than ten billion years of relative calm,

the solar system will lose a planet.

And the sun will continue to expand.

Growing ever closer to Venus.

OK, so here we are. We've come about 66 million miles

and, er...Venus, you might notice,

is almost exactly the same size as the Earth.

And for that reason alone, it's been called the Earth's sister planet.

But Earth will probably lose its sibling.

Because most models of the sun's evolution

show it easily enveloping Venus.

The next planet...is Earth itself.

You can see from wherever you're standing

that the Earth is blue, with lots and lots of liquid water.

By the time the sun engulfs Venus,

the Earth's oceans are expected to have boiled away.

The ultimate fate of our world appears to be on a knife edge.

-A fortune cookie.

-SHE CHUCKLES

For years, scientists have been unsure

what fortunes await the Earth.

Uh-oh!

Will it be swallowed by the sun?

"The world will end in fire."

Or will it outlive the sun, to face a frozen eternity in space?

"The world will end in ice."

Maybe!

In 2001, astronomer Dr Robert Smith decided to investigate.

His first calculations had ominous results.

What we found, to our disappointment,

was that the sun will expand to something like

250 times its present size

and the Earth's orbit is only about 215 times

the present size of the sun.

So it will certainly go beyond the present orbit of the Earth.

But Robert foresaw that there was still hope for our planet.

Another factor that could potentially save the Earth

from the sun's clutches.

He realised the Earth's destiny

hangs on something called the solar wind.

Highly-charged particles that stream out from the sun

as its hot surface evaporates.

Like the wind on Earth, this stream of particles is invisible.

But you can see its effects.

You can see the tail is always downwind of the kite.

And you get the same kind of phenomenon with comets, for example.

You can see that the tail of a comet,

it's not always behind the direction of the comet,

it's streaming away, always away from the sun.

The solar wind also affects the sun itself.

Solar wind is carrying away particles, so it does reduce the mass.

As the sun loses mass,

so the gravitational field of the sun gets weaker.

It pulls less strongly on the planets

and so the planets tend to move out, the orbits get bigger.

As a red giant, the sun will lose a lot of mass through the solar wind.

Robert wanted to know if it would be enough

for the Earth to escape the advancing sun.

So initially, it was just sheer curiosity.

What happens to the Earth when the sun becomes a red giant?

Robert and his colleague calculated how the sun would evolve.

And, in particular, how much mass it would lose

after it becomes a red giant.

There was an amazing amount of interest in this.

We found that the mass of the sun itself

would go down by something like 20%

at the end of the red giant stage.

As the sun loses mass, the planets will shift further from its centre.

Robert predicts that the Earth will move out millions of miles...

..as the sun expands.

So here we are at Mars, the last of the terrestrial planets.

But in seven billion years' time,

calculations show that Mars will no longer be here.

Instead, the red planet is forecast to have moved all the way out

to where the asteroid belt is today.

With the Earth in its place.

According to Robert Smith's calculations in 2001,

the sun would then stop growing

when it's still ten million miles away from the Earth.

And our world would survive.

We were quite pleased when we found that the Earth would escape.

Unfortunately, nobody will be around to see it, which is a pity.

But within a few years, scientists began to realise

that there was another effect they hadn't considered...

..which could potentially draw the Earth back towards the sun.

Dr Eva Villaver has analysed these so-called tidal interactions

that exist between all red giants and their planets.

She foresees that the same forces

will one day act between the sun...and the Earth.

I have a third experiment that maybe can help understanding

how the Earth and the sun will interact

as the sun becomes a red giant.

Imagine that you have a carousel, which is the sun,

and you have a bicycle orbiting it,

going around it, and the bike is the Earth.

As the sun expands, the rate it spins will slow down.

So by the time it reaches its maximum size,

the Earth will be going around the sun

much faster than the sun itself is turning,

which has a critical effect.

The rotation of both is going to be connected.

So imagine that you have a rope tied on the bike.

If the carousel rotates more slowly than the bike,

it will pull whatever is rotating around it.

And as a consequence of that, the planet will be forced to slow down.

So that's basically the tidal force,

this connection between the carousel and the bike.

As a result, the Earth would lose speed.

The planet will be moving more slowly

and as a consequence of that,

the Earth would get closer to the surface of the sun.

Our world would be drawn towards the sun.

Could the Earth be doomed after all?

Dr Robert Smith went back to work

to calculate whether the tidal force pulling the Earth in

could counteract the solar wind

reducing the sun's grip on our planet.

Unfortunately, we found that

the tidal effect was really quite important.

And it caused the Earth to spiral in towards the sun.

And the overall effect was that

the Earth actually was swallowed by the sun.

Well, that was a very disappointing result

because we had hoped that the Earth would still nonetheless escape,

but unfortunately, that's the way things are.

And the Earth, by that stage, wouldn't have been liveable on,

so perhaps it doesn't matter too much.

For a vision of those final days on Earth,

Dr Eva Villaver has come to a unique facility in Odeillo, France.

The world's largest solar furnace.

As the sun becomes a red giant,

we will have a red star occupying most of the sky.

And the energy that every single inch of the Earth

will receive will increase.

And here, this is exactly what these mirrors are doing.

Around 10,000 mirrors focus the sun's rays,

like a giant magnifying glass.

Which allows them to replicate the conditions the Earth will face

when the sun becomes a red giant.

Eva calculates that the radiation shining on the Earth's surface

will be nearly 3,000 times more intense than today.

So, to simulate our future,

the solar furnace has magnified the sun's power by 3,000 times.

We are focusing the light of the sun in a bin

and trying to see what will be the effect on a rock.

Because the Earth is a rock floating around the sun.

Wow, look at this!

There it goes.

The temperature at the surface of the Earth at that point

will be of the order of 1,400 degrees.

Enough to melt rock.

Enough to melt the whole surface of the Earth.

It's thought the planet will be covered in a vast ocean of molten lava.

But even after the Earth's surface has melted,

the heat is expected to increase further as the planet is engulfed.

The maximum intensity of the solar furnace

is 16,000 times the sun's power today.

Still only a fraction of what the Earth would encounter

inside the red giant.

The rock would be stripped away,

leaving just the planet's iron core.

Wow, look at this!

Just the sun's radiation.

That's iron being melted by the radiation of the sun.

This is how the last moments of our world would be.

So everything, the whole material of the Earth,

will melt all the way down through the core.

Even the iron core will melt.

The whole material of the Earth will be part of the material of the sun.

Everything will be mixed together.

According to the latest calculations,

the world will end in fire.

But our solar system's story is not quite over yet.

Because the final phase of the sun's life

will be the most spectacular of all.

-There's Seven Sisters...

-It's like an upside-down owl.

Nick, you wanted to see the Andromeda?

-It's really cool.

-Wow!

In Peoria, every Saturday night,

the Astronomy Society meets

by the Northmoor Observatory at the edge of town.

And between these two stars is the remnant

of what's going to happen to our sun.

So we're going to move the telescope

and, Brian, do you want to move the dome?

Tonight, Sheldon is searching for a distant, dying star.

The Ring Nebula.

OK, that's good, Brian.

Ha! I think it's there.

OK. So come on over and take a look.

Look through the eyepiece and you should see a lot of stars

and then right in the middle, do you see that little smoke ring?

-Yes.

-It's just barely there, right?

-Yeah.

-Wow!

-So this is a star that, after the red giant stage,

it puffs off shells of itself.

It expels most of its matter into, like, bubbles of gas.

The planetary nebulae produced by dying stars

are some of the most spectacular celestial objects in the night sky.

When our sun dies, it, too, could make a nebula.

Astronomers have calculated that up to half of the sun's mass

would be thrown off into space as gas and dust.

Including much of the material that came from the Earth.

And then the star itself shrinks from the red giant

down to a white dwarf,

which is a star about the size of the Earth.

Very, very hot, but extremely tiny.

And, er...then the shells of gas

are really the only thing that's left to see.

The vaporised remains of half the solar system

would glow brilliantly for around 10,000 years.

Then, as it spreads into space,

the light would slowly fade.

And our solar system will end.

But in a sense, it's just a new beginning.

The materials that make up our bodies

may well ultimately get spat out into the cosmos

and be the raw materials for another generation of stars,

planets and maybe even life forms.

We're all famously made of star stuff.

One day, we may return to a star.

Our sun.

But then, in an extraordinary process of cosmic rebirth,

the sun would return our atoms to interstellar space.

To form new worlds...

..and perhaps new life.