How did Newton’s laws help scientists predict the motions of planets?

Isaac Newton’s laws of motion provided the physical foundation for understanding planetary motion, transforming empirical observations into a rigorous science. While Johannes Kepler had previously described planetary paths using his three laws, he did not explain the underlying physical processes.12

Newton’s second law established that a force must act on a planet to cause its path to curve toward the Sun. By analyzing Kepler’s third law, Newton demonstrated that this centripetal force must vary inversely with the square of the distance ($1/r^2$) from the Sun. He further concluded that this force of attraction is proportional to the product of the masses of the bodies, leading to his law of universal gravitation.1

By applying these laws, scientists could mathematically predict orbital paths—including elliptical, parabolic, and hyperbolic orbits—based on a body's velocity and energy. This framework allowed astronomers to account for planetary perturbations and even predict the existence of unseen planets, such as Neptune, by identifying discrepancies in the motions of known bodies. While later refined by general relativity, Newton’s work remains a cornerstone of classical mechanics.123