What you learned in school
Astronomy textbooks definitively listed nine planets in our solar system, with Pluto as the smallest and most distant. Students memorized the order using mnemonics like "My Very Educated Mother Just Served Us Nine Pizzas." Pluto's planetary status was taught as scientific fact for over 75 years.
What we know now
Every astronomy textbook and classroom poster listed nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Students memorized mnemonics like "My Very Educated Mother Just Served Us Nine Pizzas." Pluto was discovered in 1930 and immediately classified as the ninth planet. In 2006, the International Astronomical Union reclassified Pluto as a "dwarf planet" due to its small size and failure to clear its orbital neighborhood. This decision was controversial but based on more precise definitions of what constitutes a planet.