Answer;
The distance from the sun to Neptune is about the same as the distance from Neptune to the next closest star, Proxima Centauri
Explanation;
When a planet like earth is closer to the sun the heat of the sunlight warms the surface of the planet hence,a planet like Neptune is far away from sun, the sunlight can't reach the planet and it's surface becomes cold so,the distance of the sun and planet is the main factor effecting the temperature.