The long-term decrease in the period of the system is likely caused mainly by magnetic braking.
(The long-term decrease in the period of the system; is; likely caused mainly by magnetic braking) (The long-term decrease in the period of the system; caused; mainly by magnetic braking)
The long-term decrease in the period of the system The long-term decrease in the period of the system
is caused
likely caused mainly by magnetic braking
mainly by magnetic braking