Final answer:
Isaac Newton determined that planets stay in orbit due to the combined effects of gravity, which attracts the planet toward the Sun, and inertia, which causes the planet to resist changes to its motion, resulting in a stable orbital path. The correct answer is C) Gravity and inertia.
Step-by-step explanation:
Isaac Newton concluded that two factors combined to keep planets in orbit: gravity and inertia. So, the correct answer to the question is option C) Gravity and inertia.
Gravity is the attractive force between all masses, and it plays a crucial role in maintaining the orbital motion of planets around the Sun. According to Newton's universal law of gravitation, the gravitational force is related to the mass of the objects and the distance between them. This principle forms the basis for understanding why planets do not simply drift off into space.
Meanwhile, inertia is a concept stemming from Newton's First Law of Motion, describing the tendency of an object to resist changes in its motion. This means that a planet will continue in its path around the Sun unless acted upon by another force. The combination of gravity pulling the planet toward the Sun and the planet's inertia, trying to move in a straight line, results in the circular or elliptical orbits observed in the solar system.