According to the law of conservation of angular momentum, a rotating body spins more rapidly as it decreases in size. So, as a protostar shrinks, its rate of spin increases. As the protostar rotates, it is much easier for material to fall right onto the poles (which spin most slowly) than onto the equator (where material moves around most rapidly). Therefore, gas and dust falling in toward the protostar’s equator are “held back” by the rotation and form a whirling extended disk around the equator. The circumstellar disks are where planets form.