Why did Gustav Holst not mention Pluto?

Answer 1

Holst did not mention Pluto in his suite "The Planets" because Pluto had not been discovered yet!

Holst wrote "The Planets" between 1914 and 1916. Pluto was discovered in 1930, four years before Holst's death.

From wikipedia: "Pluto was discovered in 1930, four years before Holst's death, and was hailed by astronomers as the ninth planet. Holst, however, expressed no interest in writing a movement for the new planet. He had become disillusioned by the popularity of the suite, believing that it took too much attention away from his other works.

In 2000, the Hallé Orchestra commissioned the English composer Colin Matthews, an authority on Holst, to write a new eighth movement, which he called "Pluto, the Renewer". Dedicated to the late Imogen Holst, Gustav Holst's daughter, it was first performed in Manchester on 11 May 2000, with Kent Nagano conducting the Hallé Orchestra. Matthews also changed the ending of "Neptune" slightly so that movement would lead directly into "Pluto"."