The solar system formed approximately 4.6 billion years ago from a dense region of a molecular cloud. Under the effect of gravity, matter contracted. Even a weak initial rotation becomes significant as the radius decreases, through conservation of angular momentum.
Matter therefore cannot fall entirely toward the center. Collisions dissipate disordered motions while preserving the overall rotation, which flattens the cloud into a disk around the protostar. This step explains why the major planetary orbits are approximately coplanar and oriented in the same direction.
Within the disk, dust grains collide and can aggregate. The transition from small grains to kilometer-sized bodies is complex, because collisions can also fragment particles or cause them to drift toward the star. Concentrations produced by turbulence and collective instabilities probably help to overcome this barrier.
Once planetesimals have formed, their gravity accelerates accretion. Planetary embryos appear, interact and migrate through the gas. Giant planets must accumulate a core and capture an envelope before the disk dissipates. Late collisions shape the terrestrial planets and can produce satellites.
Disks observed around young stars show rings, gaps, arcs and sometimes spiral arms. Some structures may be sculpted by nascent planets; others arise from instabilities, dust variations or radiative transfer effects. A spiral image is therefore not automatic proof of a planet.
The composition of the disk varies with temperature. Beyond certain condensation lines, ices can survive, increasing the amount of solid material available. The subsequent movements of the planets have, however, mixed the material, so that current positions do not tell the whole story.
The spiral appears mainly in density waves and in the trajectories of matter within the disk. The disk itself is a result of rotation and dissipation. Planet formation is a process of assembly, migration and reorganization, not a simple orderly march outward.