Science Olympiad Solar System Practice Test 2025 – Complete Study Resource

The force that keeps planets in orbit around the Sun is gravity. This fundamental force is a result of the mass of the celestial bodies involved. The Sun, which has a much larger mass than the planets, exerts a significant gravitational pull. This pull causes the planets to move in elliptical orbits rather than flying off into space.

Gravity acts as the centripetal force necessary for the planets to maintain their orbit. It continuously pulls them toward the Sun while the planets also have a forward motion due to their initial velocity. This balance between the gravitational force pulling them inward and their inertia trying to move them in a straight line results in a stable orbit.

In contrast, while centripetal force is indeed involved, it is a result of gravity in this context, representing the effect rather than the cause. Electromagnetic force and friction are not relevant to planetary motion in space, as gravitational forces dominate in this scale and environment. Hence, gravity is the correct answer as the fundamental force responsible for the orbits of planets around the Sun.