The nebular hypothesis states that the entire Solar System stars as a large cloud of gas that contracted due to self-gravity. Conservation of angular momentum requires that a rotating disk form with a large concentration at the center, which stars as the protosun, while planets began forming within the disk.
While it incorporates more basic physics, the hypothesis could not account why 99% of the Solar System's mass is in the sun, but 99% of its angular momentum is in the planets. It also does not provide a mechanism to explain why the disk would turn into individual planets.