How I would say an object would be called a planet is if #1, if it orbits a star, #2 if it has gravity of any kind, and #3 ( This one doesn't apply to all ) but if it has an atmosphere ( Like Venus or Jupiter )
I haven't got to astronomy yet but from what I know so far is stated. Hope this helped! :D
Ca+ binds to troponin -> Troponin removes tropomyosin from G actin -> Myosin binds to actin -> Myosin generates a power stroke -> the sarcomere shortens -> ATP recharges the myosin head