Before 2006, our Solar System was commonly thought to have nine planets, including Pluto. However, in August 2006, the International Astronomical Union redefined what constitutes a planet, resulting in Pluto being reclassified as a dwarf planet. This left us with the eight major planets we recognize today.
The International Astronomical Union established three specific criteria that a celestial body must meet to be classified as a planet. First, it must orbit the Sun. Second, it must have sufficient mass for its gravity to pull it into a roughly spherical shape. Third, and most importantly, it must have cleared the neighborhood around its orbit, meaning it is the dominant gravitational force in its orbital path.
Pluto successfully meets the first two criteria for being a planet. It clearly orbits the Sun, and it has enough mass to maintain a nearly spherical shape. However, Pluto fails the crucial third criterion. Unlike the eight major planets, Pluto has not cleared its orbital neighborhood. It shares its orbital space with many other objects in the Kuiper Belt, making it gravitationally non-dominant in its region.
These eight celestial bodies successfully meet all three planetary criteria. The inner planets - Mercury, Venus, Earth, and Mars - are rocky worlds that have cleared their orbital neighborhoods. The outer planets - Jupiter, Saturn, Uranus, and Neptune - are gas giants that dominate their respective orbital regions. Each of these eight planets is the gravitationally dominant object in its orbital zone, unlike Pluto and other dwarf planets.
In conclusion, our Solar System has eight major planets because of the precise scientific definition established by the International Astronomical Union in 2006. This classification system, based on orbital dominance, provides a clear framework for understanding celestial bodies. While Pluto and other objects are now classified as dwarf planets, this doesn't diminish their scientific importance. The eight-planet system reflects our improved understanding of how planetary systems form and evolve.