Planetesimals are thought to form when dust particles, debris and rock that are in close proximity in space in a young planetary system are attracted to each other through gravitational forces and compress to form a solid object. Planetesimals represent the first step in planet formation.
Planetesimals can be small objects that are only a few meters in diameter or they can be as large as several kilometers in diameter. The birth of planets is thought to start off with a protoplanetary disk, which is a disk consisting of small particles such as dust and debris revolving around a central protostar.
Gravity is the major force that drives the formation of planets from planetesimals. Gravity is present between any two objects in the universe, and the force of gravity depends on how big the objects are and how far they are from each other.
With a protoplanetary disk, gravity acts on the particles that are in close proximity to each other and pulls them together to collide and form small objects. These objects grow in size due to the continued addition of particles to them by gravity and are called planetesimals. As more objects collide with the planetesimals, they either break apart or grow bigger and bigger until they form either planets or moons, according to Universe Today.