The fundamental force that keeps planets in orbit around the sun is gravity. Gravity is a natural phenomenon that attracts two bodies with mass towards each other. In the context of the solar system, the sun's massive gravitational pull draws planets into elliptical orbits around it. This gravitational interaction is described by Sir Isaac Newton's laws of motion and later refined by Albert Einstein's theory of general relativity. The balance between the gravitational force and the inertia of the planets creates stable orbits, allowing the solar system to maintain its structure and dynamics over vast time scales.