I think one theory is that the closer to the sun, the more dense materials are; thus you have primarily 'hard' planets closest to the sun - mercury, venus, earth, mars - and beyond is the gas giants, jupiter, saturn, neptune.... and that overgrown asteroid, Pluto.
As an aside, Jupiter is not a friendly neighbour. It has some aspects of a miniature sun; some fission/fusion is going on inside it creating massive radiation. Jupiter's moons are not a good place to be.. possibly, one of the worst in the solar system.