Re: The Astronomers Giveth and the Astronomers Taketh Away

On Sun, 31 Jul 2005 15:54:02 +0000 (UTC), Aaron Denney wrote:

> One reasonable distinction is "what body is it most gravitationally
> bound to". For the Moon, that would be the Sun, by a factor of 1000.
>
> For, Io, say, that would be Jupiter, by a factor of 2.
>
> (One could also go with something life force from sun vs force from
> primary. The Sun _still_ wins there with the Moon.)

Here's one that doesn't let the moon win:

First, any body that doesn't pull itself into a sphere by virtue of its
gravity is NOT a planet.

Presumably, we can tell what stars are, and If it's orbiting a star but
isn't a star itself. That criterion, combined with the
pulls-self-into-sphere criterion, yields candidates for planethood.

Now: in any group of candidate planets orbiting a star, where each member
of the group always stays closer to the group than to the star throughout
the time period of orbiting the star (naive notion of "orbiting each other
while group orbits the star"), the *most massive* member of that group will
be deemed the planet, and others satellites.

If you want double planets, this could be modified to say that all of the
most massive members that are within a factor of 1.5 of each other will be
called planets, the rest moons. Adjust factor of 1.5 as necessary.
.

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