Formation of the solar system: a nebula condenses into a swirling disc with a central ball surrounded by rings. The ball at the center grows dense and hot and is ready for fusion. The ball becomes the sun, in the rings, dust particles collide and join forming planetismals, which then grow into proto-earth. Gravity re-shapes proto-earth into a sphere as the interior differentiates, with dense matter sinking to the center and binding together. After earth forms, a small planet collides, leaving debris forming the moon. Inner core dense, solid iron alloy, 1216 km thick. 5 methods used to understand the structure of earth are: experiments the mimic lower mantle conditions, computer models, meteorites, kimberlite pipes, seismic waves. Earth is 4. 57 billion years old this is determined from dating rocks, minerals, and meteorites recovered which consist of protodust (carbonaceous chondrite) P waves are pressure/primary waves that are compressional and move 40% faster.