What are the kilometer-sized bodies formed by colliding dust grains called before they become protoplanets?
Answer
Planetesimals
The process of building planets initiates on a microscopic level when fine dust grains collide and adhere through gentle electrostatic forces. This clumping process continues, gradually building larger aggregates: first pebble-sized objects, and eventually reaching sizes on the order of kilometers across. These kilometer-sized bodies are specifically designated as planetesimals. Once they reach this size, their own gravitational influence becomes significant enough to actively attract and sweep up other nearby materials through direct gravitational interaction, leading to more energetic collisions and the runaway growth necessary to form the final, larger protoplanets.
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