The force of gravity between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. This means that the closer two objects are, the greater the gravitational force between them. The sun is much more massive than any of the planets, so the gravitational force between the sun and a planet is much greater than the gravitational force between any two planets. As a planet moves closer to the sun, the distance between the planet and the sun decreases, so the gravitational force between them increases.
