Here's why:

* Kepler's Third Law: This law of planetary motion 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.

* Gravity's Influence: The Sun's gravity pulls on the planets. The closer a planet is, the stronger the pull, and the faster it needs to move to stay in orbit. Planets further away experience a weaker gravitational force, so they move slower and take longer to complete an orbit.

Example:

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

* Neptune, the furthest planet from the Sun, takes nearly 165 Earth years!