Answer:

The formation of planetesimals, the building blocks of planets, was a complex process that occurred over millions of years. There were several factors that contributed to the accretion of dust and condensing material into planetesimals, including collisions and gravitational attraction.

Collisions:
Collisions between particles in the early solar system played a significant role in the formation of planetesimals. As particles collided, they stuck together due to the forces of attraction between them. Over time, these collisions led to the formation of larger and larger bodies, eventually resulting in the formation of planetesimals.

Gravitational Attraction:
As planetesimals continued to grow in size, they began to exert gravitational forces on each other. This gravitational attraction caused them to pull together and merge into even larger bodies, eventually forming protoplanets and, ultimately, planets.

Nuclear Fusion:
Nuclear fusion is the process by which stars generate energy and is not directly related to the formation of planetesimals.

Rotation of the Proto-Sun:
The rotation of the proto-Sun may have played a role in the formation of planetesimals, as it could have caused the dust and gas in the early solar system to become concentrated in certain areas. However, the primary factors that led to the formation of planetesimals were collisions and gravitational attraction.

In summary, the accretion of dust and condensing material into planetesimals was primarily caused by collisions between particles and gravitational attraction between bodies. These processes led to the formation of larger and larger bodies, eventually resulting in the formation of planets.