Kepler's Third Law

In the actual practice, the sun is not stationary. Because of action and reaction force, the planets exert the same magnitude of force on the sun as the sun exerts on them.

Hence, both the sun and planets are revolving round a common centre. Because the sun's mass contributes to 99.8% of all the mass of the solar system, the common centre is still within the sun's radius. The assumption that the sun is stationary is valid.