Here's why:

* Kepler's Laws of Planetary Motion: Kepler's Third Law states that the square of a planet's orbital period (the time it takes to complete one orbit) is proportional to the cube of its average distance from the Sun.

* Distance: Neptune is the farthest planet from the Sun in our solar system.

* Longer Orbital Period: Because Neptune is so far away, it takes a much longer time to complete one orbit around the Sun. Neptune's orbital period is about 165 Earth years.