Our theory for the origin of the Solar System is a very old one with some modern innovations called the Nebular Hypothesis. A crucial ingredient in the nebular hypothesis is the conservation of angular momentum.
Recall that objects executing motion around a point possess angular momentum.
This is an important physical quantity because all experimental
evidence indicates that angular momentum is rigorously conserved in our
Universe: it can be transferred, but it cannot be created or destroyed. For
the simple case of a small mass executing uniform
circular motion around a much larger
mass (so that we can neglect the effect of the center of mass) the amount of
angular momentum takes a simple form. As the adjacent figure illustrates the
magnitude of the angular momentum in this case is L = mvr, where
L is the
angular momentum, m is the mass of the small object,
v is the magnitude of its
velocity, and r is the separation between the
objects.