Pluto is mostly made of ice and doesn't have a regular orbit around the sun. It's too oblong for astrologists happy with defining Pluto as a planet. Unfortunately, this messes up the mnemonic device "My Very Educated Mother Just Served Us Nine Pizzas" for the first letters of each of the planets M,V,E,M,J,S,U,N,P Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. Maybe she will serve Nachos! Condolences!