Pluto

Pluto (or Sol IX) was the ninth planet in the Sol system located in Sector 001.

Pluto was depicted as the ninth planet of the Sol system in a poster in the office where Rain Robinson worked, at the Griffith Observatory in 1996. 

In 2152, Subcommander T'Pol compared the threat posed by the Andorians on Paan Mokar to Vulcan, 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. 

Pluto was seen on a map depicting the Sol system. This map was scanned by Nomad. 

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
Only 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 International Astronomical Union (IAU) reclassified the body as a dwarf or minor planet, and then later as a Plutoid as well. These reclassifications have, to date, never been mentioned in Star Trek. 

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 (page 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.