The term "Planet Nine" is a placeholder name given to a hypothetical planet that has not yet been directly observed but is inferred to exist based on indirect observations and mathematical models. The name "Planet Nine" is not meant to indicate that it would be the ninth planet in our solar system after Pluto's reclassification.
The reason for the discrepancy in numbering is due to the reclassification of Pluto by the International Astronomical Union (IAU) in 2006. Pluto was redefined as a "dwarf planet" rather than a full-fledged planet, as it did not meet the newly established criteria for planethood. According to the IAU definition, a planet must orbit the Sun, be spherical in shape, and have cleared its orbit of other debris. Since Pluto shares its orbit with other objects in the Kuiper Belt, it was deemed not to have cleared its orbit and was reclassified.
Following Pluto's reclassification, the eight planets in our solar system are commonly referred to as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. However, the search for additional objects and the understanding of the dynamics of the outer solar system led to the hypothesis of an additional, as-yet-unseen planet beyond the orbit of Neptune.
The term "Planet Nine" emerged in 2014 when astronomers Konstantin Batygin and Michael E. Brown presented evidence for the existence of a large, distant planet in the outer regions of the solar system. They proposed that the gravitational interactions of this hypothetical planet with other objects in the Kuiper Belt could explain certain observed orbital patterns. While its existence has not been directly confirmed, ongoing research and observations are aimed at detecting and characterizing this potential ninth planet.
In summary, the designation "Planet Nine" is unrelated to Pluto's reclassification and instead refers to a hypothetical planet beyond Neptune that has not yet been observed.