Here's why:

* Kepler's Laws of Planetary Motion: The farther a planet is from the Sun, the larger the circumference of its orbit.

* Gravity: The Sun's gravitational pull weakens with distance. The outer planets experience less gravitational force, resulting in a slower orbital speed.

Think of it like a merry-go-round:

* The horses on the outer edge of the merry-go-round have a longer path to travel and move slower than the horses closer to the center.

* Similarly, the outer planets have a longer path to travel and move slower due to the weaker gravitational pull.

This slower orbital speed is why the outer planets take many years to complete a single orbit around the Sun:

* Jupiter: 11.86 Earth years

* Saturn: 29.46 Earth years

* Uranus: 84 Earth years

* Neptune: 165 Earth years

This long orbital period is one of the reasons why the outer planets are less explored than the inner planets.