Here's why:
* Kepler's Laws of Planetary Motion: Johannes Kepler's laws of planetary motion describe how planets move around the Sun. The third law states that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun.
* Longer Distance, Longer Orbit: Neptune is significantly farther from the Sun than Mercury. Its average distance is about 30 times greater. Because of this vast distance, Neptune has to travel a much larger path around the Sun.
* Gravitational Influence: The Sun's gravitational pull weakens with distance. While Mercury feels a stronger pull from the Sun, Neptune experiences a weaker pull. This weaker pull allows Neptune to travel at a slower speed, taking much longer to complete one orbit.
In essence, the farther a planet is from the Sun, the weaker the Sun's gravitational pull and the longer it takes to complete an orbit. This is why Neptune takes 164 years while Mercury completes its orbit in a mere 0.24 years.
