According to the nebular theory of planet formation, which type of force was responsible for causing particles in the protoplanetary disk to clump and bind together, growing from soot-sized specks into boulder-sized blocks, and eventually into planetesimals?

The force of gravity is the one that rules all the process.

Explanation:

A star is formed in a molecular cloud of gas and dust, mainly composed of hydrogen and helium. The Nebular Theory establishes, for the formation of the solar system, that the cloud starts to collapse under its own gravity when it receives a shock wave from a near event, for example, a supernova explosion. That results in the cloud breaking in small pieces, and those pieces constitute a possible future star.

Then it begins to accrete and rotate as a consequence of the angular momentum. In the center of that disk when it reaches the necessary temperature and pressure a protostar¹ will be born.

Around the star, fragments of dust combine as a consequence of the force of gravity between them, until they get a meaningful size (planetesimals).

Hence, the force of gravity is the one that rules all the process.

Key terms:

¹Protostar: A young star.