According to the coreaccretion model, initially the dust particles and the interstellar gases were all combined together forming a huge cloud, that eventually gets cooled and these gases got concentrated and became gradually denser.
Due to the increasing density, this cloud got disintegrated under the action of gravity, thereby formed a proto-star. Due to the continuous fusion and gravity effects, a star formed and eventually got burst. It then formed the sun with a disk of substances and matter around it. This rotating disc in due course of time gave rise to the planets that exist today.
This is how the solar system originated according to the core accretion theory.
