When it is actually found and they decide to name it, the new name needs to start with P so all the mnemonics that used to work with Pluto as #9 will work with this new planet.
"My Very Educated Mother Just Served Us Nine Pizzas"
"My Very Easy Method Just Shows Us Nine Planets"
"My Very Efficient Memory Just Stores Up Nine Planets"
"Mary's violet eyes make Johnny stay up nights, pondering"
Persephone might work.