From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.science
Subject: Re: Orbital planes of planets
Date: Wed, 5 Jan 2000 00:21:28 GMT

In article <3871B95A.39EC2C98@hotmail.com>,
Arshad Tanveer Mohammed  <atanveer@hotmail.com> wrote:
>I have read about the creation of our solar system. There is one thing I
>don't understand. How come planets and asteroids orbit the sun in one
>plane, rather than haphazardly in three dimensions...

Well, things aren't quite exactly in one plane.  But admittedly, they are
pretty close, with the exception of a few of the smaller objects.

As for why that happens...  In the very early days of the solar nebula,
when everything is colliding with everything else, if an object with (say)
a 45deg inclination collides with another that's at -45deg (so to speak),
then most of the collision debris will be in orbits with rather lower
inclinations, somewhere in between.  This sort of averaging process
operates very rapidly on a geological time scale, and it tends to flatten
the pre-solar nebula out into a pancake pretty quickly.

The only major exception is the Oort Cloud comets, which are so far out
that they simply never experience very many collisions.  And they are
indeed spread out over the whole sphere, pretty much at random.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.science
Subject: Re: Orbital planes of planets
Date: Wed, 5 Jan 2000 20:45:55 GMT

I wrote:
>...This sort of averaging process
>operates very rapidly on a geological time scale, and it tends to flatten
>the pre-solar nebula out into a pancake pretty quickly.

Forgot to mention an especially spectacular example of this, on a smaller
scale:  Saturn's rings.  The main rings are over 270,000km across (minor
rings, much less visible, extend still further) and less than 0.1km thick.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)