Neptune

Neptune (or Sol VIII) was the eighth planet of the Sol system.

History
Neptune was seen on a map depicting the Sol system scanned by Nomad. 

At warp 4.5, it would take six minutes to go to Neptune and back from Earth. 

According to Arex, the cosmic cloud encountered by the USS Enterprise (NCC-1701) in 2269 was twice the diameters of Jupiter, Saturn and Neptune in size. 

A painting depicting Neptune as seen from Triton hung above Geordi La Forge's bed in his quarters aboard the USS Enterprise (NCC-1701-D). 

Background information
Neptune was named after the Roman god of water. The Earth Starfleet starships might have been named after this planet.

According to Star Trek: Star Charts ("United Federation of Planets I"), Neptune was a charter member of the United Federation of Planets in 2161.

Apocrypha
In, the Mego toy company produced a reptilian action figure doll called a "Neptunian," although such a creature never appeared at any time during the Star Trek franchise. Mego may have intended to create a Skorr or Aquan action doll, but it looks nothing like the animated species.