Final answer:
Pluto was reclassified as a dwarf planet in 2006 because it did not meet the criteria set by the International Astronomical Union. This decision sparked controversy among astronomers and the public.
Step-by-step explanation:
In 2006, the International Astronomical Union (IAU) decided that Pluto should be called a dwarf planet and not a planet. They set up three criteria that a world must meet to be called a planet. The criteria are:
The world must orbit the sun.
The world must be spherical in shape.
The world must have cleared its orbit of other debris.
Pluto did not meet the third criteria, as it shares its orbital neighborhood with other objects in the Kuiper Belt. As a result, it was reclassified as a dwarf planet.
The decision to reclassify Pluto as a dwarf planet caused some controversy among astronomers and the public. Some disagreed with the IAU's classification criteria, while others felt that the reclassification was necessary to accurately categorize objects in our solar system.