Planets orbit the Sun at different speeds and therefore take different amounts of time to complete one orbit. The reason for this difference is Kepler's Third Law of Planetary Motion, which states:

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

Here's what that means:

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

* Semi-major Axis: The average distance between a planet and the Sun.

In simpler terms:

* Planets farther from the Sun have longer orbits. This is because they have a larger semi-major axis, and according to Kepler's Third Law, a larger semi-major axis results in a longer orbital period.

* Planets closer to the Sun have shorter orbits. They have a smaller semi-major axis, leading to a shorter orbital period.

Example:

* Mercury, the closest planet to the Sun, takes only 88 Earth days to complete one orbit.

* Neptune, the farthest planet from the Sun, takes 165 Earth years to complete one orbit.

The reason behind Kepler's Third Law:

This law arises due to the combination of gravity and the planet's inertia. The Sun's gravitational pull is weaker at greater distances, causing planets further away to move slower and have longer orbital paths.