QUESTION:
Is it possible that Pluto is a former moon of Neptune that go hit out
of its orbit? If so, how can this be proven?

ANSWER from Marc Buie on May 26, 1996:
I find this question fascinating but probably not for the reason you think.
 
First, to answer your question.  No, Pluto is not a former moon of Neptune.
A colleague of mine, William McKinnon at Washington University studied
this question about 10 years ago.  He found getting Pluto away from
Neptune and putting it where we see things now would have required a
"dark star" to pass by our solar system.  It had to be dark (perhaps dead)
so that it wouldn't fry all the cold stuff in the solar system.  Now
it is POSSIBLE a star could pass close to us but Bill found one more
thing was required.  The star would have had to come by at a time when
Neptune was nearer the star and ALL of the other planets would have had
to been on the opposite side of our Sun so that they wouldn't get messed
up too.
 
There a principle we use in science, actually it is more like a guideline,
called Occam's Razor.  This guideline says if you have two (or more)
different explanations for the same thing, then you'll usually be right if
you pick the simpler explanation.  In this case, it COULD happen that
Pluto was ripped away from Neptune but it's a really complicated method.
We've got another, simpler, explanation for Neptune and Pluto.
 
How's this?  Pluto and Neptune formed on their own in the solar system.
This is pretty simple, the rest of the planets did this too.  There may
have even been lots and lots of Pluto's a long time ago.  Then as the
solar system grew and evolved, those other Pluto's got either gobbled up
by Neptune or flung out of the solar system.  One of those Pluto's might
have even been captured by Neptune.  That might even be Triton --- which
could explain why it orbits Neptune backward.  What we now know as Pluto
is the only one of these objects out there that survived being swallowed
up by Neptune.  This is pretty likely since we know the path that Pluto
travels NEVER gets close to Neptune.  Now, I realize that this explanation
sounds long-winded and even a little complicated instead of simple.  But
in applying Occam's Razor, this explanation wins becuase it depends on
things that we know happen while the escape theory calls for this odd
passage of a star.
 
I haven't gotten to the really interesting part yet.  I find this question
interesting not for what it tells us about the solar system but for what
it tells us about the human race and how knowledge gets passed around
and what gets remembered.  This "theory" about Pluto escaping Neptune
is the result of a casual comment made by a scientist in the 1950's.
Someone asked him (he was just about the only planet expert at the time)
where Pluto came from.  Without doing any serious thinking about the
question, he tossed out the idea that Pluto and Triton got tangled up
somehow and Pluto got thrown out and Triton's orbit got reversed.  It
sounded good but he didn't do the work required to prove this idea.
Unfortunately, people that were listening didn't know any better and what
ever a planet expert said must be right.  So, other people that write
textbooks heard of this and began to pass the idea around as if it were
fact.  You STILL see this in today's textbook and just about everyone
thinks they know this is what happened.  I've been working 10 years now
trying to undo this incorrect knowledge and it hardly seems like I've
made a dent.  Everyone still knows Pluto is escaped from Neptune and
they don't know it's WRONG!
 
So my question is: what else have we been taught in school (or elsewhere)
that is wrong like this?  I keep this story in mind all the time when
I hear an explanation for something.  If it's right, then when you check
the story from every possible direction it will still make sense.  I think
this is the essence of what it means to be a scientist.  Unfortunately,
everyone needs to have this questioning attitude, not just people that do
science.