Re: Planets and Moons

Density as such doesn't really determine that - it's mass that matters.
Planetary orbits (any planetary orbit) are determined by Kepler's laws, and
when you're talking about one extremely massive body (i.e., a star) and a
much much less masive one - even a gas giant like Jupiter - then even though
technically the barycentre (common centre of gravity) is not at the centre
of the star, for most purposes it can be taken as such. A lot of the
extrasolar
planets that have been discovered so far have turned out to be large,
massive, usually gas-giant types. But that's almost certainly because with
our
current technology, these are the easiest to find. I'm sure there are lots
of Earthlike worlds out there!
Mike
>
>  many of the exosolar planets being found these days  fall into two large
> categories (though there have been some that fit in neither group)...
>
>  * large molten rocky worlds, Jupiter-sized & as close to its sun as
> Mercury.
>  * large gassy worlds, Jupiter-sized & as close to its sun as Mercury.