Which is interesting since that would mean that "the nine planets" is a totally valid, understood concept, just like "the seven seas" - which is actually since a more interesting concept since that's a much more vague term since arguably the actual number of "seas" is anywhere between 1 (since basically all the worlds water is interconnected, bar a few endorheic basins) and "many", if you start counting large inland bodies like the great lakes that see substantial ship traffic.