Clumps grow into planetesimals by a. gravitationally pulling in other clumps. b. colliding with other clumps. c. attracting other clumps with opposite charge. d. conserving angular momentum.

Planetary formation is the process by which celestial bodies come together to form planets. It begins in a protoplanetary disk, which is a rotating disk of gas and dust surrounding a new star. Over time, small particles within the disk start to stick together due to electric forces and form bigger particles called clumps. These clumps grow into larger bodies known as planetesimals, which are the building blocks of planets.
To understand this process fully, let's examine two key mechanisms: gravitational attraction and collisions.

Gravitational attraction is one of the primary forces that leads to the formation of planetesimals. Any object with mass exerts a gravitational pull on other objects. In space, this means that clumps of particles can attract each other and merge.
When these clumps become large enough, their gravitational pull increases, allowing them to pull in even more material from their surroundings.
This process leads to the gradual growth of the clumps into larger and more defined planetesimals, setting the stage for eventual planet formation.

Collisions in space play a significant role in the growth of planetesimals. When clumps of matter collide, the impact can cause them to stick together, forming a larger body. This is different from collisions we think of on Earth because in the vacuum of space, there's no air resistance to slow down the particles.
Repeated collisions and mergers cause these bodies to grow larger and more massive. Sometimes, these collisions also result in fragmentation, but over long periods, the dominant effect is the accumulation of mass into larger structures.
This collision-driven growth, combined with gravitational attraction, leads to the creation of substantial planetesimals, which can eventually form planets.