THE SOUTHWORTH PLANETARIUM
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Founded January 1970
              "And, I wonder about that!"



THE DAILY ASTRONOMER
Monday, March 7, 2016
If a Black Hole Entered the Solar System


and it didn't make a sound, would we know it was here?    Or, more
sensibly, what would happen to the bodies within the solar system?
Most importantly, of course, is the matter of what would happen to
Earth,  named in a recent poll as Sol's most significant planet.
Though the eventuality is as remote as a dust mote in a fern patch on
an alien world in Andromeda, discussing it leavens our neurons with
some rough kneading.

We'll pretend that a stray black hole ventures into the solar system.
According to stats issued by the cosmic cogniscenti, our galaxy alone
might harbor 100 million of these space ripping monsters.  So, having
one of this stampede wander onto our pasture doesn't seem too far
fetched.   It is far fetched, of course, because our galaxy is so
unfathomably huge that you can comfortably store 100 million around
without tripping.

However, like every other single piece of paper ever scribbled on,
this is a contained Universe and we can do what we wish within it.
So, if we wish to imagine that a black hole happens by, well, a black
hole will happen by..and there it is:  roaring Viking-like onto our
defenseless shores.      Since this is our own little cosmos, we can
even pretend that black holes behave like frothing Visigoths that
wander where their moods take them, as opposed to moving in response
to the galaxy's matter distribution: the actual governing force.

First issue is the shore, itself.   What does it mean to "enter" the
solar system?    Well, fortunately for those who prefer aggravation to
simplicity, the solar system's boundary is not a sharp demarcation
line, such as the surreal border separating July flowers and the Magna
Carta.       We'll state that the Oort Cloud defines our outer
perimeter: this "cloud," for lack of a sensible term, is the spherical
distribution of cometary nuclei enclosing the solar system.   Even
though we picture it as a gossamer shell of negligible thickness, it
is a rarefied region between 2,000 to possibly 100,000 AU* from the
Sun.   This range encompasses billions of miles of space!  In fact,
the Oort Cloud's outer edge might extend more than a light year from
the Sun, or, more than a quarter of the distance to the nearest star
system, Alpha Centauri.         Within this vast volume one could
encounter billions of comet nuclei spread far apart.   A black hole
could pass through the Oort cloud easily.


This is a lovely juncture to dispel a popular misconception pertaining
to black holes.   They are not wandering storm drains.     A black
hole would not "swallow" everything in the solar system as though it
were drawn down a tube.   Instead, the black hole gravitational
effects would disrupt bodies within its vicinity.   Remember that a
stellar sized black hole contains a star's mass within a comparatively
minuscule volume.     Consequently, the more powerful effects would be
tidal.**      Any objects close to it would be ripped apart.   Also,
the black hole could induce such perturbations into the Oort Cloud
comets that it would likely send a swarm of them toward the inner
solar system.

Now, after raging through the Oort Cloud, our menacing interloper
would pass through another region called the heliosphere, the boundary
where the interstellar gases repel the solar wind.    The heliosphere
"edge," called the heliopause, is believed to be approximately 100 AU*
from the Sun, or thereabouts.     Some might consider this region as
the solar system's circumference.

Once sliding through the heliosphere,  the visitor would encounter the
comparatively more dense Kuiper Belt: a disc of nuclei from which
short period comets (those with periods of less than 125 Earth years)
originate.      The resultant disruption will send another comet
armada toward the inner solar system.    Some close bodies would
likely be captured, but these would be few and far between.


At this point, one wonders if we'd know about the black hole.

The answer is that we could, but not by direct observation.  Instead,
astronomers could observe the motions of planets the black hole would
perturb.  We recall that every massive object attracts every other
massive object.    And, the outer planets and other bodies would react
to the black hole's proximity, by either being pushed or pulled
depending on their positions relative to the black hole.         The
outer planets would act like sentries, alerting us to the black hole's
presence.

Let's now have some real fun and assume the black hole voyages toward
the inner solar system en route to Earth.  What would we see?    Well,
if the black hole is not surrounded by an accretion disk,***  we'd
have a difficult time observing it.   A black hole is a region where
gravity is so strong not even light can escape from it.  However,  if
it were close enough, it would literally distort the starlight around
it.  We might not see the object, but we'd see a peculiarities in the
night sky.

Of course, we might not pay much attention to the sky, because a black
hole close enough to cause such visual distortions would soon make
short work of our planet.  Earth would NOT be sucked into it, but
would literally be ripped apart by it.  The black holes tidal forces
would be so strong that not only would it draw away all the gases
enveloping the planet, thereby snuffing out all life within a few
minutes, but it would also tear Earth's solid structure into pieces.
These pieces would become pieces until all the material was reduced to
its component subatomic particles.        Our home world would become
so much flotsam enclosing the voracious black hole.

In other words, we'd die.

This is a basic idea of what would transpire if a wayward black hole
frolicked through our neighborhood.    We would know about it before
it wandered too close, but, of course, if it did, we couldn't do much
to stop it.

On that ticklish note, enjoy your Monday!





*AU: astronomical unit.    Defined as the Sun and Earth's mean
separation distance  (approximately 149,597,871 kilometers, or
92,955,807 miles.)      The inner most bodies comprising the Oort
Cloud would be 2000 times farther from the Sun than Earth.   Pluto,
for comparison, is about 40 times more distant from the Sun than
Earth. (quite a bit of variation in this distance, of course.)



**A teeny tidal trouble tutorial:    the physical term for tidal
effects are "differential gravitational effects."   If our aim was to
elucidate the concept in the first sentence, we failed.    In a
physically neat world, a body's gravitational influence on another
object would be constant.   For instance, a massive object would pull
on your head with the same force it exerts on your feet.     For
Earth, these forces are essentially equal.    It's true that your
head's distance from the planet is greater than your foot's distance,
but this difference is so slight as to make the gravity differential
negligible.   However, if Earth's volume were crushed down into a
little ball, the difference would be significant: enough to rip you
asunder.



***an accertion disk consists of matter around a black hole.     This
matter spirals in toward the black hole.    Since material close to
the black hole moves faster than the stuff farther away, it develops a
differential rotation.  The closer particles move faster than the more
distant one.    They rub against each other so energetically, they
generate high energy x-rays.