Here's why:

* Distance from the Sun: Neptune is the farthest planet from the Sun, receiving very little sunlight.

* Atmosphere: Neptune's atmosphere is composed mainly of hydrogen, helium, and methane, which are poor absorbers of solar radiation.

* Internal Heat: While Neptune does have some internal heat generated from gravitational pressure, it's not enough to offset the extremely low temperatures caused by its distance from the Sun.

The average temperature on Neptune is a frigid -214°C (-353°F).