I address three questions regarding Solar System planets: How long did they take to form? What determined their number? Why are their orbits nearly circular and coplanar?
Planets grew in a disk around the Sun by accreting smaller bodies. As the planets grew, they forced the small bodies to collide with each other in increasingly violent collisions. By the time that around half of the small bodies had been accreted, they had ground themselves down to a tiny size--perhaps as small as a few centimetres. Subsequently, the protoplanets began to perturb each other, until eventually their orbits crossed. In the inner Solar System, this led to a period of giant collisions, during which a collision with the Earth formed the Moon. In the outer Solar System, beyond Jupiter, some protoplanets were ejected into interstellar space.