Planets, including Mercury, Venus, Earth, and Mars are rocky planets and are small compared to gas giants. And all these planet are near to Sun. But mass is directly proportional to force. So why don't any planets lie between rocky planets. And if gas giants lie farther always they why does Pluto lie after all the gas giants?
The planets all have differences based on where they are and their compositions. The outer gas giants are so large today because of how quick they were to accrete material. In a protoplanetary disk, once an object gets massive enough, it can attract others through gravity, making the process much quicker than granules sticking together through electromagnetic forces or random collisions. The reason the outer planets were able to get massive much quicker than the inner planets is that there is more material with larger circumferences and that material was able to stay a solid rather than become gaseous.
With a larger mass, the outer planets attracted material quicker and the process was self-perpetuating. In addition, these gas giants had the opportunity to collect more gas that the sun did not end up taking as they are farther out. Hot Jupiters can confuse scientist because of how close they are to their host star when they could not have formed there.
The Kuiper Belt and Oort cloud are also two interesting topics in planetary formation and are most likely relics of planetary formation. This would result from the material that was created around that area, but also material that was flung out by gas giants to larger orbits. The Oort Cloud could also have been formed from interactions of the sun with neighboring stars but we do not have direct observation of this cloud yet.
As far as our understanding of planetary formation goes, rocky planets tended to form closer to the Sun because the materials they're made of -- silicates and heavier gases -- 'fall' inwards towards the Sun. In the areas closer to the Sun, it's too warm for the lighter gases to condense there, and so the larger gas and ice giants tended to form further from the Sun. As for trans-Neptunian objects like Pluto...we're not exactly sure. Can anyone else shed some light on the current research on the formation of the Kuiper Belt and Oort Cloud?