Final answer:
The planets are arranged on the orbital plane with denser planets closer to the Sun due to the solar nebula theory, where a rotating cloud of gas and dust led to the formation of planets with different densities. The heavy elements sank to form cores in the terrestrial planets, and gravitational interactions explain the slower orbits of distant planets. Planetary migration may have further shaped the current configuration of our solar system.
Step-by-step explanation:
The arrangement of planets in the solar system with the most dense planets near the Sun and less dense ones farther away can be understood through the theory of the solar nebula. This model suggests that the Sun and planets formed from a rotating cloud of gas and dust.
As the material coalesced into planetesimals, density differentiation occurred, with heavier elements gravitating towards the center. This supports the observation that terrestrial planets have dense metallic cores and lighter silicate surfaces, indicating they were once hot enough to allow heavy elements to sink through gravity. The evidence also indicates that the outer gas giants, like Jupiter, collected most of the material, being more massive than all other planets combined. Additionally, Kepler's laws and Newton's gravity explain how the outer planets orbit more slowly than those nearer to the Sun.
Further exploration into planetary formation suggests that migration of gas giants within the solar system, like Jupiter, could have affected the orbits of closer rocky planets, possibly even causing some to fall into the Sun. This is supported by the notion that Uranus and Neptune likely formed closer to the Sun before migrating outward.