Final answer:
Pluto is classified as a dwarf planet because it does not clear its orbital neighborhood, sharing space with other objects in the Kuiper belt, which prompted the IAU to set criteria that Pluto could not fully satisfy.
Step-by-step explanation:
Pluto is now called a dwarf planet because it did not meet all the criteria set by the International Astronomical Union (IAU) in 2006 to be considered a full-fledged planet. The criteria for a celestial body to be classified as a planet are: it must orbit the Sun, it must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape (hydrostatic equilibrium), and it must have cleared the neighborhood around its orbit. Pluto meets the first two criteria but does not clear its orbital neighborhood; it shares its space with other objects in the Kuiper belt.
Many other objects of similar size were found in the Kuiper belt, leading astronomers to determine that Pluto was not unique, but part of a larger class of objects. This resulted in the creation of the category of dwarf planets. These discoveries have sparked ongoing debate among both astronomers and the public regarding Pluto's classification, revealing strong sentiments and differing opinions.