"As Kolch theorizes, it is to do with gravity. As matter fell towards the center of the cloud that formed the solar system, the Sun formed first. As it grew, it began to spin and the gravitational field generated, began rotate with it. This had the effect of packing the dust cloud into an "accretion disk". Such disks have been observed in other systems, by various means. Over time, of course, the materials in the disk formed planets."
-Kim (Some guy from America)
*This is a theory made by scientists, not by me.* :)