Final answer:
Pluto's reclassification as a 'dwarf planet' is based on its failure to meet the criterion of clearing its orbit. Additionally, Pluto's composition and distant location from the Sun are major differences from the terrestrial planets.
Step-by-step explanation:
In 2006, the International Astronomical Union (IAU) revised the definition of what constitutes a "true" planet and set up three criteria that a world must meet to be called a planet. Pluto was determined to not meet the third criterion, which states that a planet must clear the neighborhood around its orbit. This means that it has not gravitationally dominated its orbital zone, unlike the eight major planets. As a result, Pluto was reclassified as a 'dwarf planet'.
Another reason scientists agree that Pluto should no longer be considered a planet is because it is very different from the terrestrial planets. Unlike the rocky, solid surface of the terrestrial planets, Pluto is primarily composed of ice and has a much smaller size. It is also located much further from the Sun compared to the terrestrial planets.