The other day in the car my 6 year old asked me the question that all parents face. It's one of those questions that shapes the rest of your child's life! No, it's not the sex question. No, it's not the where do babies come from question. It's the why is Pluto not a planet question!!!
It's too bad there wasn't a camera on me when she asked this question. My face lit up in a smile and as an astrophysicist with a Ph.D., I had an answer prepared. So why is Pluto NOT a planet? Well, it boils down to 3 things. There are three criteria that must be met for an object to be classified as a planet.
1. The object must orbit the Sun. Pluto satisfies this requirement.
2. The object must have enough mass such that it is spherical in shape. Pluto satisfies this requirement.
3. The object must have "cleared" its neighborhood of debris. This is where Pluto fails.
Larger objects have enough mass to clear their area of other debris. They may have moons and asteroids, but this is a result of large enough gravitational forces to strongly influence their orbits. Pluto does not do this. There are other Pluto-like objects near Pluto and Pluto has neither pushed them away nor gravitationally attracted them. Thus Pluto is not a planet. Instead, it fits into a new category of objects called dwarf-planets.