Why is Pluto being the last planet out of the nine (9) planets referred to as DWARF PLANET?
That's an easy one: "Because it has not cleared the neighborhood around its orbit, Pluto is considered a dwarf planet. It orbits in a disc-like zone beyond the orbit of Neptune called the Kuiper belt, a distant region populated with frozen bodies left over from the solar system's formation."
From: www.nationalgeographic.com/science/space/solar-system/dwarf-planets-pluto-ceres/
Pluto is referred to as a "dwarf planet" because it no longer meets the criteria set by the International Astronomical Union (IAU) to be classified as a full planet. In 2006, the IAU redefined the criteria for a celestial body to be considered a planet, and Pluto fell short of meeting these criteria.
To understand why Pluto is now considered a dwarf planet, let's look at the criteria established by the IAU. According to the IAU's definition, a celestial body is considered a planet if it:
1. Orbits the Sun: Pluto does orbit the Sun, so it meets this requirement.
2. Has sufficient mass for its self-gravity to overcome rigid body forces, so it assumes a nearly round shape: Pluto is spherical and meets this criterion.
3. Has cleared its orbit: This is where Pluto falls short. It shares its orbit with other objects in the Kuiper Belt, a region of the outer solar system populated by numerous small bodies. This means that Pluto has not cleared its orbit of other debris, which is a defining characteristic for a planet.
Due to Pluto's failure to fulfill the third criterion, it was reclassified as a "dwarf planet" rather than being considered as one of the traditional nine planets.
However, it is worth noting that the term "dwarf planet" does not diminish the importance or scientific interest in Pluto. It is still a fascinating celestial body and worthy of study and exploration.