As you are probably aware, the orchestral suite comprises seven movements, each representing a planet in the solar system (other than Earth). However, the Naxos recording includes:
… the new eight movement, Pluto … as an “appendix” to the Suite, the planet Pluto having been discovered only a few years before Holst’s death.
Of course, in 2006 the International Astronomical Union determined a definition of the term planet which excluded Pluto.
So Holst was right after all.