Many learned the 9 planets via a mnemonic such as “My very efficient mother just served us nine pizzas” or some variation; was there a popular similar mnemonic before the discovery of Pluto?
I never heard the mnemonic. I learned the plantes from a song called The Wandering Stars.
Mercury
Venus
Earth and Mars
Jupiter, Saturn, Uranus
Neptune and Pluto
The wandering stars…
Your mnemonic is as good as any. Thinking back about this, I never had a mnemonic since it’s only nine planets. The first 4 everyone knows. After that, there are only 5 more to remember. Easy.
Mary visits every Monday (and) just stays until noon.
The (and) covers the asteroid belt.
Apparently when Pluto was discovered they modified this one by saying aloud the period at the end of the sentence. :dubious: That’s how I first learned it.
My very educated mother just served us? Nein!
My very educated mother just served us ninjas.
M VEM J SUN P - Pronounced em vem jay sun pea
Way back I was told one in french:
Mon Vieux Tu M’as Jeté Sur Une Nouvelle Planète.
Works fine for me, and it seems there’s a few more on Wikipedia. 
(None for english that I can find though).
Now we just need to figure out a way to stick an X on the end, for Xena.
Or whatever the IAU intends to call it, but they’d better announce it quick, or everyone’s just going to keep calling it Xena anyway.
Slightly off topic, but I don’t use a mnemonic to remember the names of the planets. I have no recollection of how or when I actually did memorize them. It’s like the piece of information just kind of made its way inside my brain one day, weird, huh?