NARRATOR: For many people, the idea

that Earth has been the victim of a series

of impacts caused by an orbiting Death Star seems unlikely.

After all, when we look at the sky, we only see one sun.

But in fact, the majority of stars come in pairs.

GREG LAUGHLIN: The solar system probably

formed in a cluster of stars.

And so close encounters with nearby stars,

nearby rogue planets, even, were much more

common during the very earliest days of the solar system's

formation.

NARRATOR: Most of these solar brothers and sisters

have long since dispersed.

But according to the Nemesis hypothesis, one of them

is still out there, still circling the sun,

still causing periodic chaos amongst the comets,

and still waiting to rain death on the Earth once more.

If that is correct, future generations

will be faced with a monumental threat when Nemesis

returns in 10 million years.

GREG LAUGHLIN: The moon, Mars--

the planets would basically be unaltered

from their current condition.

On Earth, the cities might not be there.

But the continents would still be

almost in exactly the same places that they are now.

NARRATOR: But according to the Nemesis hypothesis,

a major change is underway one light year from the sun.

A dark reddish star has entered the Oort cloud.

Nemesis has returned.

If humans still exist on Earth, they'll face a slow building

but imminent cosmic threat.

ALEX FILIPPENKO: The Oort cloud contains perhaps 10 trillion

comets, maybe even more.

But remember, it's really big.

So if you were in the Oort cloud,

it's not like you would be pelted by comets all the time.

The spaces between them would be pretty big.

NARRATOR: Like a bowling ball in a juggler's hands,

Nemesis simply shuffles some comets out of its way.

The Oort cloud has an empty region in the middle.

It's been cleaned out by Jupiter and by the sun.

We live in that region.

NARRATOR: But as Nemesis approaches,

the inner solar system becomes a shooting gallery.

And so there start to be a few more comets than usual.

And then suddenly, they're just comets,

comets coming all the time.

And at the peak, there might be 1,000 to 10,000

comets per year in the sky.

NARRATOR: According to the projections,

Earth would be in the crosshairs of this comet storm

for a million years.

RICHARD MULLER: Over a period of a million years,

there would be visits by about a billion comets.

But after you do the calculation,

it turns out one of them will hit the Earth, one or two.

GREG LAUGHLIN: It would be difficult to predict exactly

when the impact would happen.

But there would be this phase, this sort of danger zone,

in which the chances for a catastrophic impact,

something of a comet size, would be much, much higher

than they are now.

NARRATOR: Will the impact spark another catastrophic extinction

on Earth?

Perhaps.

But only if we let it happen.

RICHARD MULLER: I like to joke that Nemesis

planned this one poorly.

In between the last extinction and the next one,

there's plenty of time for intelligent life

to get its act together to make sure that next time

Nemesis comes back and the sky is filled with comets,

we make sure that none of them hit us.

NARRATOR: Is this future inevitable?

Until scientists either discover Nemesis or definitively rule it

out, no one can know for sure.

CLIFFORD JOHNSON: It's important to realize this is not

just an abstract discussion.

Clearly, these objects colliding with Earth happen very rarely.

But on the other hand, when they do collide,

they're extremely cataclysmic.

So what we need to be able to do is

get better understanding of our solar system

and keep an eye on the sky.

Because it may be crucial for our survival.

NARRATOR: If the future sky one day does fill with comets,

perhaps humanity will have found a way

to save itself, rather than end up one more

victim of the sun's evil twin.

[music playing]