What force keeps planets in orbit around the Sun?

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

Answer: (A)

Gravity is the force that keeps planets in orbit around the Sun. The Sun’s mass creates a gravitational field that pulls on each planet. A planet also has forward motion due to its inertia, so as gravity pulls it inward, the path bends into a curve. The planet essentially falls toward the Sun but keeps missing it because of its sideways speed, tracing an orbit. For a circular orbit, there’s a specific speed needed at a given distance, but in general the orbits are ellipses described by Kepler’s laws and Newton’s law of gravitation. In space there’s almost no friction to slow the motion, so gravity remains the dominant force shaping the path. Electromagnetic forces or magnetism aren’t responsible for these neutral bodies’ orbital motion, so they’re negligible on solar-system scales.