What is a belief you were completely certain about that turned out to be incorrect?

As children, we are often taught that Pluto is the ninth planet in our solar system, a fact that many of us accepted without question. For a long time, Pluto was widely recognized as a planet, having been discovered in 1930 and holding that status for 76 years. However, in 2006, the International Astronomical Union (IAU) redefined the criteria for planet classification, and Pluto did not meet the new standards. Consequently, it was reclassified as a “dwarf planet.” The primary reason for this reclassification is that Pluto shares its orbital zone with other objects in the Kuiper Belt and doesn’t clear its neighboring region of other debris, which is one of the criteria for being classified as a full-fledged planet. This shift in understanding highlights how scientific knowledge evolves over time as we gain more insights and data, reminding us to remain open to new information and perspectives.