After the first solid grains formed in our solar system, these particles could then grow by the process of the collision and sticking of one particle with another to continue the formation of planetesimals. each of these has many atoms or molecules.

Accretion

Explanation:

Accretion is the process by which there is an accumulation of particles into a bigger object by attracting more mass by gravitational force into an accretion disk. This is one of the first steps in the formation of our solar system. There was a collapse of a gas cloud which resulted in most of the mass collecting in the center leading to the formation of the sun and the rest spread out forming the planets.

this is poiselle's equation from fluid mechanic in physics. mech eng