Final answer:
The accretion of planetesimals is a critical phase in the formation of planets, where small solid matter grows by gravitational attraction, eventually forming protoplanets and, thereafter, planets through a process of collisions, melting, and differentiation.
Step-by-step explanation:
The accretion of planetesimals is a process fundamental to planet formation within the solar system. In the vast expanse of the solar nebula, planetesimals—which were small chunks of solid matter made up of ice and dust—began to coalesce due to their own gravitational pull. Calculations suggest that for these bodies to avoid being drawn into their host star by decay of their orbit, they must grow rapidly. When these planetesimals reached about 1 kilometer in size, they were massive enough to attract more material and not fall into the star.
Protoplanets, which can be considered 'not quite ready for prime time' planets, formed as they continued to accrete more material, increasing in mass to a level comparable to that of smaller planets like Mercury or Mars. Not all impacts contributed to growth—some were so violent they resulted in destruction or melting of the material.
This stage of accretion led to the planetary differentiation, with heavier materials such as metals sinking to the cores and lighter materials rising towards the surfaces of the protoplanets. The heat from the constant collisions and radioactive elements also contributed to this melting and differentiation, which is evident in the internal structures of planets today. As a result of these complex processes over millennia, the large-scale structure of the solar system as we know it began to take shape, with a few large planets dominating the landscape.