Why isn't Pluto a planet anymore? 

That's the question I'm going to answer for you today because, until recently, I was pissed off and in search of an answer myself. See, I grew up learning an acrostic (My Very Excited Mother Just Sent Us Nine Pizzas) that drilled the names and locations of all nine planets into my brain. Then, suddenly, on August 24th 2006 I woke up to headlines claiming that one of my beloved nine had been demoted and, as a highschool senior, no one around me could give me a good reason why. I've done a ton of independent research and today I'm going to share my findings with you to answer the question of "Why Isn't Pluto A Planet Anymore?" in a way that makes sense.

It's simple really.

Pluto lost its planetary status because for the first time in history the International Astronomical Union (IAU) agreed on what defines a planet and Pluto didn't meet the requirements. In  fact, if we'd had this definition sooner Pluto might not have ever been considered a planet at all.

According to the IAU, in order to be considered a planet, the object in question must orbit a star. Pluto does this by orbiting our Sun. The second requirement for planethood says that the object in question must have enough self-gravity to maintain hydrostatic equilibrium despite outside forces working on it. This basically means that the object needs enough self gravity to maintain a spherical shape- and Pluto does that too. Where Pluto fails to meet the requirements is in the third part of the definition; the object in question must have cleared out its orbital path.

Pluto has not cleared out its orbital path. It is a member of what is known as the Kuiper Belt. We now know, according to NASA and EarthSky.org, that the Kuiper Belt is similar to the asteroid beltbetween Mars and Jupiter except that the objects are icy and lie on the outer rim of the Solar System. According to Earth Sky, Astronomers were willing to ignore the presence of these icy bodies so long as they didn’t find one larger than Pluto. However in 2006 their hand was forced by the discovery of Eris, whose mass is 27% larger than Pluto’s. Now, not only was Pluto incapable of clearing its orbital path, it wasn’t even the largest object inside of its orbital path.  

That's it folks, because of the simple fact that Pluto is a member of a community it is no longer considered a planet but, instead, a "Dwarf Planet". Bill Nye the science guy commented on Pluto’s size and composition by saying“If you brought Pluto close to the sun, like where Mercury is, it would have a tail like a comet and in a few millennia, it would volatize. It would just disappear into space. … I mean, is that worthy of a planet -- a planet that just evaporates?”  While he is obviously happy about the change, I know that a lot of people are still having a hard time coming to terms with it. At this point, though, we know that there are many moons in our Solar System (including Earth's) that are bigger than Pluto and if we were to include everything Pluto-sized in the Planet Club then the numbers would skyrocket as we continue to discover new bodies in the universe. 

All we can do now is suggest a new acrostic for kids to learn to help them remember the 8 planets. My vote goes to: My Very Eerie Mother Just Saw Us Naked

Click Here To Download The Full IAU Resolution On Planet Requirements.doc (85.0K)