For a long time (well, relatively) astronomers believed they had this whole "solar system" thing figured out. They had this theory which governed its formation, called the "core-accretion theory," which described stellar and planetary formation. Basically, as all the dust and gasses that make up a star star coalescing into a star, they also begin spinning. As they spin faster, the gasses condense into a kind of spinning disk which is thicker at the center (like spinning our a blob of pizza dough, but a lot more complicated). Finally, this central mass gets hot and dense enough to trigger fusion, and the proto-star becomes a real star. Around this time, as the star is finishing its formation, the heavier elements in the spinning disk start clumping together, with the denser elements forming the smaller, rocky planets we see as the "inner planets" in our solar system, and lighter elements and compounds forming gas giants further out.