Pluto

Pluto (also known as Sol IX) was the ninth planet in the Sol system located in Sector 001.

Location
In 2267, the orbit of Pluto was depicted on "Chart 14A: The Sol System". This chart was stored in the USS Enterprise (NCC-1701) library computer. The chart was scanned by the probe Nomad in Auxiliary Control. (, production art)

History
In 2152, Sub-Commander T'Pol compared the threat posed by the Andorians on Paan Mokar to, with the menace a hypothetical Klingon military base on Pluto would represent to Earth. 

When using the slingshot effect to return to the 23rd century after the USS Enterprise (NCC-1701) had been hurtled through space to Earth and back in time to the year 1969, the ship engaged full braking power shortly after passing Pluto. 

In 2293, the maiden voyage of the USS Enterprise (NCC-1701-B) was planned as a "quick run around the block", taking the ship out beyond Pluto and then returning to spacedock. However, the flight plan was abandoned when the Enterprise received a distress call from the SS Lakul. 

Background information
Discovered in the 20th century, Pluto was named for the god of the underworld in Roman and Greek mythology. Pluto was previously considered a planet until a series (2006, 2008) of controversial decisions of the reclassified the body as a dwarf or minor planet, and then later as a  as well. Neil deGrasse Tyson had actually already actively argued for the original reclassification, and his opinions weighed in on the ultimate decision. He had to answer for this in the popular Star Trek friendly sitcom The Big Bang Theory when he made his guest appearance in the season four episode "The Apology Insufficiency". These reclassifications have, to date, never been mentioned in Star Trek. 

The described Pluto as a minor planet.

In the first draft script of, Charles Tucker told a ghost story about a Tellarite transport which had crash landed on Pluto, a few years previously. In the final version of the episode, though, this was replaced with Tucker commenting, "Has anyone heard the one about the haunted comet?"

According to the Spaceflight Chronology, the first Pluto Base began operation in the year 2039 as a navigational check point for extra-solar missions. According to The Worlds of the Federation (p. 12), Pluto was the site of the Pluto Research Base, where all life was decimated by a strain of the omega virus, some time after the founding of the Federation. According to Star Trek: Star Charts (Pgs.39, 62, "United Federation of Planets I"), Pluto was classified as a C-class planet. The planet was a charter member of the United Federation of Planets in 2161.

Apocrypha
In the novel Before Dishonor, Pluto was "absorbed" by the evolved Borg, who literally devoured the planet and its associated satellites en route to attacking Earth. Characters mention the controversy about its status, mentioning that it had switched back and forth several times. Upon its destruction, one of the characters sardonically comments that at least this would settle the controversy.