  This paper uses an inductive method to investigate the factors responsible
for variations in planetary-rotation periods. I began by showing the presence
of a correlation between the masses of planets and their rotation periods. Then
I tested the impact of planetary radius, acceleration, velocity, and torque on
rotation periods. I found that velocity, acceleration, and radius are the most
important factors in explaining rotation periods. The effect of mass may be
rather on influencing the size of the radii of planets. That is, the larger the
mass of a planet, the larger its radius. Moreover, mass does also influence the
strength of the rotational force, torque, which may have played a major role in
setting the initial constant speeds of planetary rotation.
