What keeps planets in orbit?

• A. Magnetism
• B. Friction
• C. Gravity
• D. Inertia

Answer: (C)

Planets stay in orbit because gravity pulls them toward the Sun while their forward motion carries them ahead. In space there’s very little friction, so the planet doesn’t slow down and fall inward due to drag. The Sun’s gravity constantly pulls the planet inward, and the planet’s inertia makes it move forward. The combination bends the path from a straight line into a curved orbit around the Sun. If gravity weren’t there, the planet would travel in a straight line; if gravity were overwhelming compared to its forward speed, it would spiral inward or fall toward the Sun. The inward pull of gravity acting as the centripetal force is what keeps the planet in a stable orbit.