The scientist who determined the nature of the forces that keep planets in their orbits was Isaac Newton. He formulated the Law of Universal Gravitation, which states that every particle in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

This law explained why planets orbit the Sun in elliptical paths, and it also explained the tides, the motion of the Moon, and other celestial phenomena.