Part 12/14:
The fate of Pluto is a testament to scientific refinement. Once the ninth planet, Pluto was reclassified in 2006 as a dwarf planet by the International Astronomical Union.
Many assume size was the culprit—after all, Pluto is smaller than our Moon, and Mercury is twice its diameter—but the true reason lies in orbital dynamics. To be classified as a planet, an object must have cleared its orbit of other debris. Pluto shares its orbit with many Kuiper Belt objects and lacks the gravitational dominance needed to clean it up over billions of years.
Ironically, if size were the criterion, Earth itself wouldn't qualify during its early formation, when it was surrounded by many planetesimals. Jupiter hosts thousands of Trojan asteroids sharing its orbit, yet it remains a planet.