[music playing]
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]
