Why Pluto is not a planet anymore
Well, it is only sort of not a planet anymore. There is still controversy surrounding the decision. Pluto is still categorized as a "dwarf planet" by the International Astronomical Union (IAU). According to them, the term dwarf planet does not mean the same thing as what we learned in school 30 years ago. We used to call Pluto a dwarf planet long before this new definition. Today it is no longer considered to be a planet at all, yet still a dwarf planet. Go figure!
At an IAU conference toward the end of August and the beginning of September 2006, a group of astronomers in committee were told to revise the recommendation to define the word "Planet". This was the same mission given to a separate task force that had already been working on it for several years.
There was a near consensus within the full conference that their original recommendation released to the public a few days prior was untenable - not because it would have kept Pluto as the ninth planet, but because so many other objects would have been added to the list. Many thought that some of these then new designations would have been very strange indeed, if approved. The IAU was facing a PR disaster and needed a quick answer. The new committee had little more than a week to find a new recommendation to be voted on and accepted in full conference. "The pressure was on."
What was the problem?
Without going into too much detail, the original recommendation would have promoted Pluto's moon Charon to status of planet too! It had something to do with the barycenter (the center of gravity) of the two bodies being completely outside of either. This aspect of the proposed definition would have ignored our own moon (which is far larger) when it will have a similar relationship with Earth within the next few billion years. Orbital dynamics of these objects change with time. How can we accept objects moving in and out of planetary status over time? How can we accept much smaller objects as planets while ignoring ones that are clearly on the order of magnitude larger?
The committee did not have time to work from scratch. They did not have time to consider such things as the ability to retain an atmosphere. Possible obscure alternative standards were assumed to have been studied and rejected for good reason by the earlier task force. They put together a compromised proposal between the two most prevalent feuding parties, the orbital dynamists against those that thought the roundness due to gravity was the most important criteria.
Here is what they decided upon:
"Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation planets'. The word planet' originally described wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A planet1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
The IAU further resolves:
Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects."
Quote end quote.
The debate is far from over. There continues to be significant opposition to this definition two years after the announcement. The New Mexico state legislature has even summarily declared Pluto to be a planet despite the IAU pronouncement. Further, there is debate upon the legitimacy of the IAU's ruling, especially when most of the astronomers in attendance (about 95%) walked out of the conference before the vote. The IAU was never given the authority to actually categorize anything, merely to make definitions. Yet in there footnotes they clearly categorized. It is not just public disgust; many scientists have clear disdain for this new definition too.
Mike Brown, the astronomer who discovered Eris and numerous other Kuiper Belt Objects, is one of the few that support the IAU decision. However given the reaction over the last couple of years, he seems to agree that a revisit of this issue in the near future is likely. This current debate is being discussed in the blog on his website.
Scientists try to make sense of the world or universe around us through their intense investigative methods. As knowledge grows, we must categorize what we learn if we want to teach others what we know. Categorization is how we humans make sense of the world around us. Apathy toward this endeavor creates a real danger to progress. The longer this debate continues, the more apathetic the average person becomes toward this issue. This may seem to be just a game in semantics, but the outcome has a direct effect upon how the general public perceives the scientific community. Scientists should care.
Yet often when one of them talks about this issue, he often seems to be just as apathetic as "Average Joe". They just want the problem to go away. Many claim that it really should not matter. These newly discovered objects in the outer solar system are what they are. They can be defined by the numbers as they are observed by high powered instruments. Some say that once the New Horizon probe reaches Pluto, we will have enough information to really decide one way or another about these distant worlds. Frankly, that is the very attitude that has allowed this conflict of words to continue for so long. People keep procrastinating.
Would anybody feel right about any decision that might have come from the IAU? Any determination should have been at least consistent with the evidence.
A strict interpretation of the criteria for "planethood" would only allow Mercury and Venus to be declared planets by the numbers (technically these are the only two objects that appear to have actually cleared their orbits). The IAU must have had some sort of unstated threshold in mind when coming up with their new definition. Basically, they simply listed the planets that supposedly met the criteria in a footnote and excluded Pluto. What this amounts to is, in fact, an exclusion by declaration regardless what they may claim. Pluto is simply no longer considered a planet because a few highly intelligent and educated people say so.
Ironically, this may change again in the future. Maybe, maybe not. Ceres was demoted from a planet to an asteroid 150 years ago, yet now it is considered to be a "dwarf planet"! Not all science is as precise as physics - certainly not astronomy. The famous physicist Richard Feynmann once stated, "But we never are right; we can only be sure that we're wrong."