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"."

Gustav Holst has written:

'The planets' -- subject- s -: Suites - Orchestra -, Scores

In the original suite by Holst, there are seven movements: Mars, the Bringer of War; Venus, the Bringer of Peace; Mercury, the Winged Messenger; Jupiter, the Bringer of Jollity; Saturn, the Bringer of Old Age; Uranus, the Magician; and Neptune, the Mystic. The movements are ordered in the planets' increasing distances from Earth. When Pluto was discovered in 1930, Holst did not write a movement for it. However, in 2000, Collin Matthews was hired to write a movement for Pluto. He named it "Pluto, the Renewer". Now that Pluto is no longer a planet, the suite is once again complete.

No. Doing so will render Gustav Holst's orchestral masterwork "The Planets" incomplete. This great work is fine as it is. In addition, Holst is dead and cannot write any additional movements

The concept of the work is astrological rather than astronomical (which is why Earth is not included)because earth is here, which would not be studying and interpreting the universe throughout his musical pieces. Also Pluto was not discovered until 1930.