While others like Johannes Kepler had previously observed and described planetary motion mathematically, Newton was the one who formulated the law of universal gravitation, which explained the force responsible for these motions.

Here's a breakdown:

* Kepler's Laws: Johannes Kepler described the elliptical orbits of planets, the speed of planets in their orbits, and the relationship between the orbital period and the distance from the sun. However, he didn't explain *why* these laws held true.

* Newton's Law of Universal Gravitation: Newton's law explained the force of gravity as a universal force that attracts any two objects with mass. This force depends on the masses of the objects and the distance between them. This law explained Kepler's observations and extended the understanding of gravity beyond planetary motion.

So, while Kepler laid the groundwork with his observations, Newton's theory is what ultimately explained the underlying physics of planetary motion.