Final answer:
The solar system used to have nine recognized planets, but Pluto was reclassified to a dwarf planet because it did not clear its orbital zone of other debris. This led to the official count of planets in the solar system being eight.
Step-by-step explanation:
The solar system used to be known for having nine planets, but that changed when Pluto was reclassified. According to the International Astronomical Union (IAU), which is the authority responsible for naming and classifying celestial objects, a planet must meet three criteria: it must orbit the Sun, it must be spherical due to its own gravity, and it must have cleared its orbital zone of debris. Pluto meets the first two criteria but not the third, as it shares its orbit with other objects in the Kuiper Belt, a region of space beyond Neptune filled with icy bodies. In 2006, this led to the reclassification of Pluto as a dwarf planet. As a result of this definition, the solar system officially contains only eight planets.