To: WSFAlist@keithlynch.net
Date: Mon, 16 Jun 2003 22:56:52 -0400
Subject: [WSFA] Re: Constructing Realistic Solar Systems
From: ronkean@juno.com
Reply-To: WSFA members <WSFAlist@keithlynch.net>

On Mon, 16 Jun 2003 09:43:41 -0400 "Strong, Lee" <StrongL@MTMC.ARMY.MIL>
writes:

>         My intention is to construct a system in which the planetary
> orbits
> are tightly packed together in comparison with Sol System so that
> there are
> many habitable worlds orbiting the same star, and so that the
> various
> sentients can see continents, oceans and weather patterns on their
> neighboring worlds with minimal artificial aids.

I don't think earth-sized planets can be bunched together that closely in
independent orbits, as a stable system.  The planets of our solar system
are spaced out according to a rough formula, if the asteroid belt between
Earth and Mars is counted as taking the place of a planet.  It might be
possible to have two roughly earth-sized planets mutually orbiting each
other as a sub-system, with the subsystem orbiting the star at a suitable
distance to support an earth-like cliamte.  The Earth-Moon system works
like that, and it is quite stable.

Kepler's Third Law is that T^2 varies as R^3, where T is the orbital
period (the year), and R is the semi-major axis of the planet's orbit.
So T^2 = k R^3, where k is a constant of proportionality which depends on
the mass of the star.

Or

T^2 = ((4 pi^2)/(G M)) R^3

where M is the star mass, and G is the gravitation constant,
so for more massive stars, the planets' years are shorter, for a given
size of orbit.

Another thing to keep in mind is that only stars of about the mass of our
Sun burn both long enough and hot enough for life to have much of a
chance to evolve on the star's planet(s).

Ron Kean

.

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