The sun is the primary gravitational force in the solar system because

The sun is the main source of energy and is present at the center of the solar system. It is enormous, having a mass of about 1.989 × 10^30 kg, and has a strong gravitational force of attraction, whose value is nearly 274 m/s². This force of gravity of the sun allows the planets to revolve around it, forming the solar system.

The sun is the only astronomical body which has the highest gravitational force of attraction. So, it is considered that the sun is the primary gravitational force in the solar system.

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