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From: (Henry Spencer)
Subject: Re: Why do all the planets orbit on the same plane
Date: Sun, 4 Jun 2000 02:37:37 GMT

In article <>, Don Lee  <> wrote:
>Correct me if I am wrong but don't all the planets orbiting
>around our Sun all orbit on the same plane except for one.

Close.  Mercury and Pluto are oddballs, with substantial orbital
inclinations.  The other seven planets are all *almost* exactly in the
same plane.  The asteroids show a fair bit of random scatter, and the
orbital inclinations of long-period comets are almost random.

>This has to be more than a coincidence, what is the reason for this...

The solar system began as a rotating nebula of dust and gas with a dense
spot at the center.  Particle collisions and gas drag in such a system
have a *very* strong tendency to make everything settle into a disk
quickly, by strongly damping out-of-plane motions.  (The most spectacular
example of this is Saturn's rings, over 200,000 kilometers across and a
few tens of *meters* thick.)

>...and do we know if planets around other Stars
>orbit on the same plane as the other planets around their Star.

The damping process is thought to be fairly universal, and several dust
disks around nearby young stars -- possibly containing, or someday to
contain, planetary systems -- seem to be quite flat.  Data is scarce about
the actual planetary systems around other stars; in most cases only a
single large planet is known (because those are the ones that are easily
detected from afar), and the plane of that planet's orbit is not well
known, never mind the orbital planes of possible smaller planets.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

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