Eventually, we may have to leave.

What if instead of finding a potentially habitable exoplanet

light-years away,

we stayed in our Solar System

and built a habitat so enormous

we could never overpopulate it?

This is WHAT IF,

and here's what would happen

if we built a ringworld in space.

Imagine you lived on a ring

with a radius of 150 million km (93 million mi)

encircling the Sun.

A gigantic artificial world

with its own gravity,

ecosystem and atmosphere.

Big enough for trillions of humans to call home.

You'd live on an enormous landmass on the inner side of the ring.

The outer shell would protect you and all those trillions of people

from the hazards of outer space.

Problem is, assembling such a thing -

suspended out in the Solar System -

wouldn't be easy.

You couldn't just pull the Earth apart

and have an army of robots.

reassemble it into a ringworld.

Among the many problems you'd run into,

your first would be finding the material.

The International Space Station roaming the Earth's lower orbit right now

weighs about 420 tons.

Something like a ringworld

would tip the scales at no less than a million tons.

Where would we find all this material?

I know some places.

The Kuiper Belt beyond the orbit of Neptune would do just fine.

This ring of icy bodies stretches out for almost 3 billion km (1.9 billion mi).

Some astronomers think the Kuiper Belt

would have enough material for this project, but...

it's hard to tell exactly how much we'd need

to construct a thing like this.

We might have to sacrifice all the planets,

moons and asteroids in the Solar System.

Our ringworld and the Sun would be the only things left.

If we could manage to gather and transport all the material available,

then construction would begin.

It would take a lot of physical labor,

an army of robots and maybe hundreds of generations

to realize that our structure was not stable enough.

Because the megastructure would turn out so enormous,

it would break any known molecular bonds.

We'd have to find a way to make use of one of the fundamental forces of nature -

the strong nuclear force.

Of all forces, it's the grippiest.

It bonds material on the scale of atomic nucleus

so that nothing can break it apart.

Or maybe we'd come up with a new super-strong material altogether.

But until we figure that out,

every interstellar body passing through our Solar System

would be a threat to our megastructure.

The next thing we'd have to worry about is gravity.

That part is pretty easy -

we'd just have to spin the ringworld at nearly 2,000,000 km/h (1,200,000 mi/h).

I know, that's really fast.

We'd have to build up the speed over time.

Luckily, maintaining it in the frictionless environment of space

wouldn't be too hard.

Such rotation would generate centrifugal force,

and that, in turn, would create an artificial gravity

equal to the one we have here on Earth.

With gravity solved, we'd bring in the atmosphere

and start populating the ringworld.

For the inhabitants of the megastructure, it would always be daytime.

Unless we could create a day and night cycle

with extra panels inside the ring.

But for all the epicness of the world we just created,

it wouldn't be stable.

A single asteroid strike could cause the structure to drift closer to the Sun.

A hole punched through the ring

could let all our atmosphere out.

And a massive solar storm?

Don't even get me started on that one.

One failure inside the ring

could doom the entire structure together with its inhabitants.

It's just too risky to build it around the Sun.

We might have better luck

with a huge ring space station in the Earth's orbit.

But that's a story for another WHAT IF.