The correct option is Accretion.
Explanation
The formation of planets is primarily explained by the Nebular Hypothesis. This theory posits that planets originated from a rotating cloud of gas and dust known as the solar nebula. The transition from microscopic dust to massive planetary bodies involves specific physical processes of growth and accumulation.
- Differentiation is incorrect: Differentiation is the process that occurs after a planetary body has formed and become molten. It involves the separation of materials based on density, leading to the formation of a distinct core, mantle, and crust.
- Accretion is correct: Accretion is the process by which solids agglomerate to form larger bodies. In the early solar system, dust particles collided to form planetesimals, which then aggregated through gravity and collisions to form protoplanets and eventually planets.
- Expansion is incorrect: Expansion typically refers to the expansion of the universe (Big Bang Theory) or the physical expansion of a star (e.g., into a Red Giant). It does not describe the aggregation of matter into planets.
- Degassing is incorrect: Degassing refers to the release of gases from a planet's interior, often through volcanic activity, which contributes to the formation of an atmosphere. This process occurs after the planet has already formed via accretion.
Key Takeaway: Accretion is the fundamental mechanism of planetary growth, characterized by the gradual accumulation of mass through gravitational attraction and collisions of planetesimals.