The planet with the slowest or longest orbit around the sun is Neptune.

Here's why:

* Kepler's Laws: Kepler's Laws of Planetary Motion state that the further a planet is from the sun, the longer its orbital period (the time it takes to complete one orbit).

* Neptune's Distance: Neptune is the furthest planet from the sun in our solar system.

* Orbital Period: It takes Neptune approximately 165 Earth years to complete one orbit around the sun.