The solar system is thought to have formed out of a solar nebula, an extensive cloud of dust and gas that served as a nursery for star formation. A hot star in this nursery quickly burned its fuel, which exploded into supernova. The supernova created the ingredients for our solar system, stirring the elements up in a cloud of gas that was more destabilized than before. This cloud shrank into a flat disk due to gravity and the angular momentum caused by its rotation. Gravity brought many of these particles together in a central location, in what eventually became the sun. The heat from the sun is thought to have created "clumps of dust not much larger than a grain of wheat, flash-heated into droplets of molten rock, called chondrules" (TDC). These chondrules accreted away from the sun, forming a vast swarm of asteroids that orbited it. Most of these asteroids accreted into larger bodies due to the force of gravity. The largest of them were rocky spheres called protoplanets, which kept building from the collisions of smaller asteroids. It took a long time for their cores to develop, through a process called differentiation. Differentiation is when the denser material of a protoplanet sinks to the center while the lighter material rises to the surface, resulting in layers like those on Earth (core, mantle, crust, atmosphere). Because there was so much material that needed to be differentiated, the protoplanets were originally much larger than the planets we see today (Levin, 2013, p. 219).
This video below demonstrates the formation of the solar system. It also features Bennu, one of the many asteroids that survived the chaos of this early period. Asteroids like Bennu are helping scientists learn more about the formation of the solar system and its planets.
Sources:
TDC. November 23, 2014. The Formation of the Solar System. https://www.youtube.com/watch?v=x1QTc5YeO6w
Levin, Harold. 2013. The Earth Through Time. p. 219
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