It all begins with a swirling cloud of dust and debris (perhaps some old-star remnants). Gravitational attraction causes this cloud to slowly contract. As it contracts the cloud speeds up its rotation (much as an ice-skater turns faster when he/she draws her hands towards his/her body), and it heats up. The cloud becomes unstable and separates into blobs, some might be ejected due to centrifugal force, others condense into planets. The center of the cloud condenses into a big blob of matter (mainly Hydrogen since this is the most abundant element in the universe). This process takes about one billion years to complete and produces a primitive planetary system: a protostar (which is very big but too cold to produce nuclear reactions) circled by protoplanets. As time goes on, the protoplanets in their orbits will ``sweep-out'' the remaining debris from the cloud.