Planetary systems primarily orbit stars because of the fundamental gravitational interaction between celestial bodies. This interaction is governed by Isaac Newton's law of universal gravitation, which states that every mass attracts every other mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Because stars are usually significantly more massive than other objects in their vicinity, they have a stronger gravitational pull and can dominate the gravitational dynamics of their surrounding space.
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