The Nebular Theory

- Origin of Earth
Most researchers believe that Earth and the other planets formed at essentially the same time
- Nebular theory
Solar system evolved from an enormous rotating cloud called the solar nebula
Nebula was composed mostly of hydrogen and helium
About 5 billion years ago the nebula began to contract
Assumes a flat, disk shape with the protosun (pre-Sun) at the center
Inner planets begin to form from metallic and rocky clumps
Larger outer planets began forming from fragments with a high percentage of ices

- Formation of Earth’s layered structure
As Earth formed, the decay of radioactive elements and heat from high-velocity impacts caused the temperature to increase
Iron and nickel began to melt and sink toward the center
Lighter rocky components floated outward, toward the surface
Gaseous material escaped from Earth’s interior to produce the primitive atmosphere