Long study of the orbits of comets shows that the gravitational disturbances from passing stars and clouds of gas only rarely nudge a comet out of the cold attic, so the original comet inventory must rise to many trillions. That comets come in trillions verifies the planetesimal idea, for comets are but a species of planetesimal from uncrowded icy orbits. The densely pockmarked faces of the moon and the airless planets, moreover, record the waning barrage that closed the curtain on planet formation, a process that is now all but extinct. Accept that trillions of mile-size chunks formed in plenty, growing from gas and grains through sticky collisions, at best tentatively understood. The chemistry of contact as binding agent gradually gave way to gravitation as the objects grew.
Planetesimals can interact without contact.
Slowly their orbits simplify as countless gravitational energy transfers space out, circularize and flatten the initial thick-braided cloud of orbits. Eventually, large composites aggregate out of the pell-mell collisions, until at last the biggest runaway clumps sweep up most residual surrounding orbiters unto themselves. Although gravity is indifferent to the chemical composition of matter, chemical bonds determine many features of the planets. Close in there is less room, and collisions will be fast and furious. Far away, space is ampler, and slower orbital collisions will be less frequent.