Space's Deepest Secrets

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 gonna 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 of the surface of the Earth

will be enough to melt rock.

Enough to melt the whole surface of the Earth.

This is the story of how our sun

will transform the solar system

it binds together...

Before bringing it to a spectacular end.

captions paid for by Discovery communications

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 town,

there's a 46-foot-wide mosaic

of the Sun,

the centerpiece 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...

Okay, 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...

Well, if you're gonna have a planet,

you may as well have the biggest one, right?

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 neighboring 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 center 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.

We study solar twins because they are very important

to understand not only the Sun,

but to 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 it 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.

Eva and her team discovered

that 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 bottle here.

It's generating heat that warm 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 star,

crash the star 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 tons

of helium every second,

which is a denser gas.

This change in density

has a profound affect 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.

The Earth has oceans and clouds

because it orbits 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.

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 analyze 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