The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

Here's a breakdown:

* Orbital Period: The time it takes a planet to complete one full orbit around the Sun.

* Semi-major Axis: Half of the longest diameter of the planet's elliptical orbit.

This law essentially means that planets that are farther from the Sun take longer to orbit. It also implies that the relationship between a planet's orbital period and its distance from the Sun is not linear, but rather a more complex power relationship.