Which man defined a period of a planet?

The man who defined a period of a planet is Johannes Kepler. He formulated Kepler's laws of planetary motion in the early 17th century, which describe the orbits of planets around the Sun. Specifically, his third law, known as the Law of Harmonies, relates the square of the orbital period of a planet to the cube of the semi-major axis of its orbit, thus providing a mathematical relationship that defines the periods of planets.