Pluto was a body in the Sol system. From 1930 to 2006, Pluto was considered the 9th planet in the Solar System, at which point it was reclassified as a dwarf planet. Pluto's status among astronomers would shift between planet and dwarf planet several times between the 21st and 24th centuries (TNG novel: Before Dishonor). The planet was named after the Roman god of the underworld. The planet was approximately a fifth of the size of Luna, and was comprised mainly of rock and ice. It had an eccentric orbit that brought it closer to the Sun than the planet Neptune for a 20-year period during its revolution around the Sun.
On reference stardate 0/8508, an unknown disease that had an exceptionally high mortality rate spread through the Pluto Research Base stationed there and killed its 200 Human and Alpha Centaurian personnel, leaving no survivors. The "Pluto Plague" was later linked to a souvenir contaminated with mutant strain of the Omega Virus, sold by a visiting Orion trader to one of the researchers. This left tense relations between the Terrans and the Orions. (FASA RPG modules: The Federation, The Romulan War; ST references: Spaceflight Chronology, The Worlds of the Federation)
The USS Galaxy spent some time in orbit around Pluto, and sent updates on the performance of her systems to the USS Enterprise-D—which was undergoing her shakedown cruises—during this time. (TNG reference: Star Trek: The Next Generation Technical Manual)
In the early 25th century, however, Pluto was unharmed and Jake Sisko had no recollection of it ever having been consumed. Former DTI Agent Dulmer, however, recalled its destruction. This clear inconsistency was possibly a result of a temporal incursion, possibly caused by the Undine. (STO novel: The Needs of the Many)