Figure 1a. A basic concept of the origin of the solar system. Scheme for the formation of the solar system, from the collapse of a molecular cloud fragment through the formation of the proto-Sun and protoplanetary disk (1,2), followed by its breakup into individual ring clumps of solid particles, eventually giving birth to planetesimals (3,4). Continuing collisional interactions of planetesimals ultimately leads to the formation of planets (5). Adapted from Wikipedia.