Final answer:
The gravitational force between two planets would decrease if their distance increased, as defined by Newton's universal law of gravitation, which states the force is inversely proportional to the square of the distance.
Step-by-step explanation:
If the distance between two planets increased, the gravitational force between them would decrease. This behavior is outlined by Newton's universal law of gravitation, which states that the gravitational force is inversely proportional to the square of the distance between two objects. Therefore, if the distance is doubled, the gravitational force is reduced to a quarter of its original value, and if the distance is tripled, the force is reduced to a ninth.
The exact reasoning behind this can be seen in the history of physics where Newton invented calculus to describe these complex interactions. His work defined how the force of gravity must decrease with increasing distance to both comply with Kepler's laws of planetary motion and Galileo's observations of falling bodies on Earth.