Planets Some of the larger planets are assumed to be primarily gaseous, small Suns, perhaps, that didn't make the grade because they were too small, their lack of bulk preventing them from either lighting or attracting planets themselves. This concept is in the main correct, beyond the fundamental fact that suns and gaseous planets are not composed entirely of light elements. Quite the contrary, and they invariably have heavy elements at their core, though large, gaseous planets should be looked upon as no different than the small but more dense planets when contemplating their influence on a solar system.
Planets find their niche, based on how crowded the solar system is and their relative mass.
For instance, if Jupiter were not in your Solar System, the planets close in to the Sun would have essentially the same orbits, though would fan out a bit more. A planet's position is based primarily on the gravity attraction between it and its sun and the concurrent repulsion force invoked. If the niche a planet would normally assume is already taken, as was the case when the clobbered Earth wobbled out of the Asteroid Belt into her current orbit, then more than one planet may settle into the same orbit, sharing this. Small planets may fail to drift into a closer orbit due to the buffering action of larger planets closer in. Essentially a bumping occurs, where the smaller planet is repulsed outward by a larger planet.