Re: How delicate is planetary gravitational balance?

Well, although the mass of a planet affects how the sun pulls it, their 
specific other characteristics are not what have given them their exact 
place in the solar system.  Indeed, every planet is slowly moving towards 
the sun, and their orbits are decreasing in length (of time). Don't worry 
though, they aren't coming in quickly, inches per year or less.  We are not 
really sure how every planet got to where it was exactly.  There are two 
theories. One is that the planets were formed from residual matter that was 
part of the sun's starting mass.  Another theory is that the sun was the 
only part of the original Sol mass, but other stellar objects (the planets) 
were caught in its pull and achieved orbit.  If you have any more 
questions, feel free to email me at arrmatey@email.com.  Hope this clears 
things up for you.

[Moderator's note: The mass of a planets doesn't make any different in where it
orbits the Sun, and in fact there are a group of asteroids, called the Trojans, 
that share Jupiter's orbit. And, as Max said, the planets' orbits
aren't fixed - they change slowly over time because of the small gravitational
effects of the other planets. A good book about this is "Newton's Clock".]