Neptune is a blue gas giant in Earth's Solar System. It was the most outlying planet in the Solar System, if dwarf planets are not included.

By the time Dave Lister left the Solar System on the Red Dwarf, Neptune's moon Triton had been heavily colonised by the Space Corps. Crapola Inc. - the same company which produced Talkie Toaster - also had an outlet on Neptune. (Infinity Welcomes Careful Drivers)

Neptune is mentioned in a song that Lister sings in the Series I episode "Me²".

Neptune had "famous planetary architecture" and "had been terraformed to the highest specifications." (Better Than Life)