The term "nine planets" refers to a former classification system used in astronomy, which characterized our solar system as consisting of nine major celestial bodies orbiting the Sun. Historically, these nine planets included Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
The definition of a "nine planet" is based on the belief that these celestial objects possess sufficient mass to maintain a nearly round shape, they orbit the Sun, and they have cleared the neighborhood around their orbit. This implies that a planet is a dominant object in its vicinity and does not share its orbit with other significant celestial bodies.
This classification system, however, has evolved over time due to advancements in scientific understanding. In 2006, the International Astronomical Union (IAU) redefined the term "planet," resulting in Pluto being reclassified as a dwarf planet. Hence, the current understanding holds that our solar system contains eight major planets.
The concept of the nine planets contributes to our comprehension of our cosmic neighborhood and helps to define the characteristics of celestial bodies within our solar system. While the specific number of planets may have changed, the understanding of their orbits, characteristics, and influence on the greater system remains crucial to our understanding of astronomy and the universe as a whole.
