Question

What causes the small planetesimals to form larger protoplanets?

Answer 1

Gravity, the ultimate form of greed.

Explanation:

When a planetary disk is formed around a young star, it contains rocks of various masses and sizes. The bigger, heavier ones will exert more gravity and thus capture more other rocks than their smaller, lighter competitors.

That makes the bigger, heavier rocks heavier still, giving them even more of an advantage in capturing other material and continuing to grow. Like the Once-Ler in the Lorax movie, it's a runaway process that won't be stopped until nothing is left.

And once you have a planet, it's gravity will continue to "clear" anything that gets close, ultimately affecting the structure of its orbital lane across hundreds of thousands of kilometers. It's a major reason astronomers now distinguish such neighborhood-clearing "planets" from their less mass-endowed, "dwarf" counterparts.