by ANNIE FURMAN
There are three qualifying factors an astronomical object must have before it is considered a planet. According to the International Astronomical Union, the three different aspects defining a “planet” are: a planet must be an object in orbit around the Sun with a diameter greater than 2000km, a planet must have a stable shape due to its own gravity and lastly, a planet must be dominant in its immediate neighborhood due to its own gravity. When examining Pluto, the small planet of our solar system comes up short.
Most of us remember in elementary school learning the phrase – “My Very Eager Mother Just Served Us Nine Pizzas” –where Pluto equaled Pizza; partly fulfilling the first qualification of a planet by orbiting the Sun. Pluto orbiting the Sun with a diameter greater than 2000km settles the first requirement. We intuitively know Pluto also has a stable shape due to its own gravity. The problem with Pluto being a planet is derived with the third requirement; dominance. The third requirement alone classifies Pluto to dwarf planet status.
According to the site Universe Today, a dwarf planet can be defined as any planet beyond the orbit of Neptune that has a size comparable to Pluto. There are currently five recognized dwarf planets in our solar system and 6 more waiting for approval for recognition. In the future, there are up to 500 objects that are located in the Kuiper Belt that could also possibly be considered dwarf planets. According to spaceplace.nasa.gov, the Kuiper Belt is a circumstellar disc outside of the orbit of Neptune that spans 50 astronomical units (distance from Earth to the Sun) away from the Sun. The Kuiper Belt is very similar to the asteroid belt (which is located outside the orbit of Mars) but much, much larger.
Pluto is in quite good company as a dwarf planet, however many people are still confused as to why the Planet was reclassified. To better understand why Pluto falls into this new category, we need to further examine the third classification of a planet; clearing its own neighborhood.
The third classification of a planet is being able to clear its own immediate neighborhood due to the planet’s own gravity. What this means is a planet itself needs to be massive enough to have enough gravity to either have bodies orbit it, a moon, or be big enough to push everything else away. Pluto simply doesn’t not have enough gravity to “clear its neighborhood” so it has a good amount of other bodies surrounding it in its orbit.
When Pluto was classified as a dwarf planet, many people were really upset because it seemed like we were “demoting” Pluto for no reason. Plus, the mnemonic phrase we learned as kids stopped making sense – our mother served us nine what? We needed Pluto to be “pizza”. However, we can clearly see that we are just moving Pluto to a new and exciting classification where it really belongs.