How do scientist theorize the earth was created?
The principle is pretty simple and applies everywhere in our universe. When stars are being formed they are drawing from pools of available hydrogen around them. The star rotates and as it grows its gravitation influence causes everything within its influence to rotate around it.
It is a simple law of physics that the heavier the atom the more likely it is that it will be drawn to the center of gravity it is closest to.
In our solar system there were four pools of rocky gravity that came into existence early on. Bits of dust, pebbles, rocks and boulders were form and were eventually drawn into the forming planet they came into contact with.
Even though all these pebbles and rocks were orbiting around the new sun, their orbits were not circular and most were quite elliptical. These elliptical orbits cause these rocks to eventually crash into one of the four inner planets.
With the inner planets claiming most of the solid matter the next 4 planets were left to gather the much lighter gases.