The planet that takes the longest time to go around the Sun once is Neptune.

Here's why:

* Orbital Period: Neptune has an orbital period of about 165 Earth years. This means it takes Neptune approximately 165 Earth years to complete one full orbit around the Sun.

* Distance from the Sun: Neptune is the farthest planet from the Sun in our solar system. The farther a planet is from the Sun, the longer its orbital path and the slower its orbital speed.