Here's why:
* Kepler's Laws of Planetary Motion: Kepler's Third Law states that the square of a planet's orbital period is proportional to the cube of its average distance from the sun.
* Outer Planets: Planets further away from the sun have larger orbits and therefore take longer to complete one revolution. This means their orbital speed is slower.
* Neptune's Distance: Neptune is the farthest planet from the sun in our solar system, making it the slowest-moving planet.
