Why Pluto is not a planet

While I was taking my first college level astronomy course, a rare event occurred. The International Astronomy Union voted to demote Pluto from planethood and assigned it the title of dwarf planet. My professor, being the excellent educator that she is, decided to use this demotion as an opportunity to educate the class. For the following two weeks, we were required to research why this demotion took place and then debate the cogency of the decision (if you teach astronomy, I suggest you do the same). Given that my class overwhelmingly consisted of Americans that grew up in the 1990's, almost no one saw the IAU's decision as a wise move. Our debate consisted of angry statements like: "This isn't right, everyone knows there are nine planets." "Tombaugh (Pluto's discoverer) was an American and so is Pluto."

If you were also seething mad at the IAU's decision to demote Pluto, I want you to think about a couple of things to calm your nerves. First, you need to understand that Pluto is still orbiting the Sun. NASA did not nuke it out the sky or send a giant boot to kick it out of the solar system. It is still made out of ice and rock and has a disproportionately large moon named Charon. The spacecraft New Horizons is also still due to to explore and photograph it in 2015. None of this has changed. All that has changed is Pluto is now grouped with a set of icy objects that are much more similar to it than any of the classical planets. That is it.

Second, you need to understand that this is not the first time that an object has been demoted from planethood. At the dawn of the 19th century, an Italian priest and astronomer named Giuseppe Piazzi (1746-1826) discovered a planet between Jupiter and Saturn. This object, which is named Ceres, was considered a planet for 50 years. It was eventually demoted, however, when it was discovered that Ceres was one body in a vast field of objects. These objects, now know as asteroids, were not known about before because of telescope limitations. Once the existence of countless asteroids were discovered, Ceres was demoted to being an asteroid and grouped in with them. Pluto's demotion, as we will discuss in a second, is strikingly similar to Ceres'.

Now that we understand that Pluto is still part of our Solar System and that Planetary regrouping has happened before, we can consider the reasons why I took the position in my class that Pluto is not a planet. The crux of my case focused around the history of the definition of planet. While it may sound odd, there was no formal definition of planet before the IAU's 2006 decision. Before we get to their definition, however, it will help us if we consider how the term planet has been informally used throughout the ages.

The shifting planets

In the ancient and medieval world, the word "planet" simply referred to those bodies in the sky that moved (the word itself comes from aster planetes, which is Greek for wandering star). Since the Greeks were predominantly geocentrists, they believed that there were seven planets (Mercury, Venus, Mars, Jupiter, Saturn, the Moon, and the Sun). This number was modified after geocentrism came under heavy attack by Copernicus, Kepler, and Galileo at the end of the Middle Ages. These men argued that the Earth was a planet and the Sun and Moon were not. By the time that Newton laid down his ideas on mechanics (the mid-late 17th century), the solar system was accepted to be six planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn) orbiting the Sun.

The solar system started to change again when, in 1781, William Herschel discovered the first planet not known to the ancients: Uranus (pronounced your-in-is). The orbit of Uranus, however, caused astronomers many headaches. Unlike the other known planets, it did not conform to the path predicted by Newton's mechanics. To explain this anomaly, astronomers predicted that there was another planet even further out who was tugging on Uranus. Scientists finally found the culprit in 1846 when Neptune was discovered. While this solved much of the problems about Uranus' orbit, it unfortunately opened up another mystery. Much like Uranus, Neptune's orbit deviated from what it was expected to be via Newtonian mechanics. To explain this discrepancy, an even further out planet was postulated.

This planet was discovered in 1930 by the American Clyde Tombaugh. When Mr. Tombaugh discovered Pluto, it was believed to be much large than it actually is (the size of Earth)  It is also worth noting that, like Ceres, none of the objects surrounding Pluto were known at the time of its discovery. 

The case against Pluto

Now that we understand that the solar system has shifted around when new evidence emerges, my case focuses on Pluto's characteristics and the new formal definition of "planet." As I mentioned a second ago, Pluto is now known to be part of a collection of objects called TNO's (trans-Neptunian Objects). These objects are made of rock and ice and reside in a big belt past Neptune. It is helpful to think of them as a much bigger, further out asteroid belt.

the dots are the TNO's

This factor is important for two reasons: (1) like Ceres, Pluto has much more in common with these non-planetary objects than it does with any of the classical planets and (2) the IAU's drafted definition of "planet" includes a requirement that Pluto is unable to meet because of its TNO neighbors. Since we have explored what it historically meant to be a planet, we are now ready to ponder this definition:

The definition that the IAU drafted has three parts:

1. A planet must orbit its star.
2. A planet must be massive enough to be round.
3. A planet must clear its neighborhood.

Number (3) is the requirement that Pluto fails. But what does it mean to "clear its neighborhood?" This simply means that the object must contain the majority of the mass in its orbital path. For example, Earth is by far the most massive object in its path around the Sun. Pluto, on the other hand, has a minority of the mass in its orbital path. This is because of the large amount of icy and rocky debris around it due to its TNO neighbors. Since all of the 8 classical planets clear their neighborhood and Pluto does not, it is disqualified from being a planet.

There are also three more factors that weigh significantly against Pluto.

1. Pluto is not even a unique for a TNO. We now know that there are several other sizable objects in this area. The most notable is Eris, which is actually 1/5 more massive than Pluto. 
2. Pluto's orbit is tiled 17 degrees.  This cannot be said about the classical planets who orbit elliptically on a relatively flat plane.
3. Unlike any of the classical planets, Pluto and its moon both orbit a common center of gravity. This makes them a binary system. 

Simply put, Pluto just does not have that much in common with the eight planets. It also has a lot in common with the other "dwarf planets." To me, this along with the new evidence about the existence of TNO's (especially the larger Eris) means that the the IAU was correct in their decision to demote Pluto. This new label is also pedagogically sensible and makes it much easier to teach planetary science to children. Like introducing the label of "asteroid" and "comet", introducing "dwarf planet" prevented us from having too many small planets and it cleaned up our textbooks. 

If you have any doubt about my argument, I have a thought experiment for you. Imagine that Pluto was discovered today at the same time as all of the rest of the TNO's. Also imagine if, unlike Tombaugh, you knew its correct size. Would you consider it and its many friends as planets? I do not think so. 

In closing, I have one final remark. Many people love to anthropomorphize Pluto and pretend that its feelings were hurt when it was demoted. If you must make Pluto feel, I advise a small change. Instead of imagining it sad, I think you should imagine it as being much happier with its new set of friends than being the "odd man out" and a planet. This is because Pluto is now among its own kind.