Astronomy Practice Exam 2026 – Complete Preparation Guide

Pluto was officially classified as the ninth planet in our solar system for many years after its discovery in 1930. It was unique due to its size and the fact that it is located in a region known as the Kuiper Belt, populated with many similar small celestial bodies. Pluto's status as the ninth planet was well-established in popular culture and education.

However, it's essential to note that in 2006, the International Astronomical Union (IAU) redefined what constitutes a planet, leading to Pluto's reclassification as a "dwarf planet." This change has sparked ongoing debates in the scientific community and among enthusiasts regarding the nomenclature and categories of celestial objects. Despite this reclassification, Pluto remains widely recognized in context as the former ninth planet.

The other celestial bodies mentioned—Eris and Haumea—are also classified as dwarf planets, while Mercury is the closest planet to the Sun, making them not applicable in the context of identifying the ninth planet from the Sun. Pluto is, therefore, retained in popular thought as the ninth planet, especially in historical discussions or informal contexts.