Written in late-1915, and strangely enough, not the last of the compositions to be written (“Mercury” was written in early 1916). At the time of The Planets‘ composition, Neptune was the eighth and last planet in the Solar System; Pluto was to yet be found. (It was eventually discovered by Clyde Tombaugh in 1930, albeit it was downgraded to 'dwarf planet’ status in 2006) Discovered in 1846, Neptune’s status as an ice giant on the outer limits of the Solar System is reflected in the name ‘the mystic’; little was known about it in Holst’s lifetime, as Earth-based telescopes struggled to make observations due to the distance between the two planets. According to Classic FM:

When Holst scored this work as a piece for [a] piano duet, he used an organ to represent this planet – the piano, he thought, couldn’t portray a planet as mysterious as Neptune. Beautiful harp and string melodies slide over each other, until Holst brings out the crowning glory: a mystical choir, which gives the music an other-worldly quality.

A wordless female choir enters around two-thirds of the way into the compostion, entering on a high G. The singing continues, without words, embedded in a diaphanous veil of orchestral sound. This then this dies away, with the voices left alone to intone a cadence over and over again with ever-diminishing tone, until it is consumed in silence. (link)