While Earth's Moon and Pluto are similar in size, they are classified differently due to their distinct characteristics and the criteria used for classification.
The classification of celestial bodies, including planets and dwarf planets, is based on the International Astronomical Union's (IAU) criteria established in 2006. According to these criteria, a celestial body must meet three conditions to be considered a planet:
- It must orbit the Sun.
- It must have sufficient mass for its self-gravity to give it a nearly round shape (hydrostatic equilibrium).
- It must have cleared its orbit of other debris.
Pluto, although once considered the ninth planet, did not meet the third criterion. Its orbit crosses that of Neptune, and it shares its region of space, called the Kuiper Belt, with numerous other objects. As a result, in 2006, the IAU redefined the term "planet" and classified Pluto as a "dwarf planet."
On the other hand, Earth's Moon is not classified as a dwarf planet because it does not orbit the Sun directly but instead orbits Earth. The Moon is considered a natural satellite or moon of our planet.
So, while both the Moon and Pluto are similar in size, their different orbital characteristics and the IAU's classification criteria account for their distinct classifications as a moon and a dwarf planet, respectively.