Why Isn’t Pluto a Planet?

Pluto was originally classified as a planet because when it was first discovered in 1930, it was thought to be a large, distant object orbiting the Sun. At the time, astronomers did not have the technology to accurately measure the size or composition of Pluto, so they made their classification based on its orbit and perceived size. Pluto was initially considered the ninth planet in our solar system, and it remained classified as such for over seven decades. However, as technology improved and more objects were discovered in the outer solar system, astronomers began to re-evaluate the definition of a planet. In 2006, the International Astronomical Union (IAU) redefined the criteria for what constitutes a planet. According to the new definition, a planet must meet three criteria: it must orbit the Sun, it must be spherical in shape, and it must have cleared its orbit of other debris. Pluto did not meet the third criterion, as it is part of a group of objects in the outer solar system known as the Kuiper Belt. Therefore, the IAU reclassified Pluto as a "dwarf planet," a new category of celestial bodies that includes other objects in the solar system that share similar characteristics with Pluto. For planetarium educator Talia Sepersky, Pluto remains a unique little world close to our hearts, no matter its designation.