Although I'm no fan of Holst's The Planets (1914-1916, premiered 1920), I must confess to a certain amount of glee to learn that the International Astronomical Union has revoked Pluto's planetary status, thus restoring (at least temporarily) the seven movements of the suite to something corresponding to the real solar system. And, yes, I know that Pluto hadn't been discovered when Holst composed the piece.
Does it matter that no one knew of Pluto in the 1910s? At one point, I would have answered, "No." Yet around 2000, Kent Nagano and the Hallé Orchestra commissioned Colin Matthews to compose a "Pluto" movement. The movement, entitled "Pluto, the Renewer," (a recording of which has been available on the Naxos label) is a pretty good piece, not least because Matthews wisely eschewed any attempt to emulate Holst's style.
Still, I must confess a certain amount of ambivalence about Nagano's, Hallé's, and Matthews's venture. Does "completing" The Planets dehistoricize the piece? I'm not convinced that what Matthews has done is in the same category as, for example, Luciano Berio's Rendering, his realization of Schubert's sketches for an unfinished ninth symphony, or Berio's endings for Turandot and the unfinished Contrapunctus from Kunst der Fuge. It may be that Berio's efforts force us to grapple with some of the historical problems surrounding these pieces in a way that Matthews's doesn't. I don't know. I'm going to have to give this more thought.