With the vastness of the Universe,
everything in it seems so spread out.
But some objects are closer
than you might realize.
Every now and then,
a random interstellar asteroid
passes through the Oort cloud,
the wall of icy debris at the very edge
of our Solar System.
But what if a rogue star
unceremoniously rampaged
right through the Oort cloud?
How bad would it be, if it brought
other planets along for the ride?
What if I told you that a rogue star
has already invaded our Sun's territory?
A dim red dwarf, dubbed Scholz’s star,
crossed the Oort cloud
some 70,000 years ago.
It passed just 0.8 light-years
away from the Sun.
And then it made a turn
in the opposite direction,
without causing much trouble.
Right now, another rogue star
is moving in our direction.
Gliese 710 has about 60%
of the mass of our Sun,
and it's traveling across the galaxy
at 52,000 km/h (32,000 mph).
If another star rampaged
through the Solar System,
the extent of the chaos it would cause
would depend on the size of that star,
and its trajectory.
When Scholz's star passed
through the Oort cloud,
it came five times closer
than Proxima Centauri,
the closest star to our Solar System.
Scholz's star didn't have much effect on Earth,
although around the same time,
early humans almost got wiped out
by a massive volcanic eruption.
But that was pure coincidence, right?
In any case, before Scholz's star
turned away minding its own business,
it did change the orbits of about 10%
of comets and asteroids in the Solar System.
But, what if it was a bigger incoming star,
like Gliese 710, that's currently
making its way towards us?
Gliese 710 isn't scheduled to make
its first contact with the Solar System
for another 1.29 million years.
But when it does, it could
shake the Earth quite a bit.
At first, the rogue star would
make its way into the Oort cloud.
At this point, it wouldn't affect us directly.
But it would send massive
chunks of space rock
showering the Solar System. 
About 170 meteors, comets and asteroids
would hit the Earth every day.
That's ten times more
than what's bombarding our planet right now.
Comets and meteors might
not seem like a big deal,
since most of them are small,
and they often fall in
unpopulated regions. But...
Back in 1908, in Siberia,
it only took one asteroid
to obliterate 80,000 trees,
and blow out windows 60 km (37 mi) away.
If the same asteroid were to hit New York,
the entire city would be in the impact zone.
And that's if just a single
star smaller than the Sun
slightly made it just inside our Solar System.
If a star larger than our Sun
entered the Oort cloud,
it would disrupt the orbital cycle
for every planet it passed.
Since there are such large
distances within the Solar System,
this disruption would happen
over the span of millions of years.
It would be an almost slow-motion
chaos of debris waves.
It might even set some
planets on a collision course.
And that's not the worst it could get.
If the rogue star had other
planets and moons following it,
our Solar System would
turn into a galactic soup,
with stars and planets
being pulled out of their orbits.
Massive collisions would
create a rippling effect,
disrupting planetary orbits even more.
Eventually, the Earth would be
knocked out of its orbit too,
if it wasn't already destroyed by meteor storms
and the remnants of other planets.
Not to sound too dramatic, but
there are estimates that
40,000 stars have entered the Oort cloud,
at some point in the history
of the Solar System. But...
They all came here on "just visiting" trips.
Chances that any of those rogue stars
will ever make it past the Oort cloud
are close to zero.
Let's just hope that we'll never have to deal
with anything like a black hole
coming too close to the Solar System.
But that's a story for another WHAT IF.
