In 2006 the International Astronomical Union (IAU) stripped Pluto of its planet-hood, demoting it to dwarf planet status. But the debate has raged on ever since. So should Pluto be a planet, and in the end does it even matter?
Ruling By The IAU
The discovery of "trans-Neptunian" objects led some scientists to push for a ruling by the IAU to more strictly define what a planet truly is. In response the IAU designated a set of properties that all objects must possess in order to be classified as a planet.
- A body that circles the sun without being some other object's satellite (meaning it is not a Moon of some other planet).
- Is large enough to be rounded by its own gravity.
- But not so big that it ignites nuclear fusion (like a star).
- And has cleared its orbital path of other orbiting bodies.
The Reasons Pluto Should Not Be a Planet
Simply looking at the IAU definition, it is the last point that is a problem for pluto. Because of its low mass and slow orbital speed around the Sun it has not been able to clear its path of asteroids, comets and the like.
However, the main reason is deeper than that. Namely there are other objects, similar to Pluto that populate the outer reaches of the solar system. If Pluto is allowed to remain in the ranks of the planets, then there is a strong argument for the inclusion of many others.
And these other worlds, including Pluto, are considerably smaller than the smallest Terrestrial planet: Mercury.
The Reasons Pluto Should Be a Planet
Perhaps the strongest reason for Pluto's inclusion as a planet is that the arguments against it are so weak. Addressing the final point of the IAU definition, noted planetary astronomy Alan Stern notes that such a requirement does little more than place caps on distance from a host star.
Specifically, he notes that if we placed Earth out at Pluto's orbit, that it would not be able to sweep its orbit clear of debris either. And obviously no one is debating the validity of Earth's inclusion in that coveted class. Since orbital speed decreases quadratically with distance, objects at Pluto's radius are at a great disadvantage.
Even more convincing is the fact that the stumbling point for Pluto is very vague. The clearing of an object's path is not a well defined astronomical concept. And if it is meant to imply that the orbit must be swept clear of all objects, than virtually every planet in the solar system would be disqualified.
Lastly, proponents of the IAU view on planetary definition argue that without it the number of planets in the solar system could balloon into the dozens as more and more are discovered out in the Kuiper belt. But so what? Why is that a reason to not call them planets?
Does It Even Matter?
The whole debate has been said to have been raised when the Hayden Planetarium in New York decided to change their solar system exhibit to include only eight planets. The point was not to alienate Pluto, but rather to group the planets together by likeness in order to better express the nature of the objects.
While this move was met with great criticism, it nonetheless raises the critical issue behind the whole debate. The classification of "planet" is nothing more than a name. Obviously classification is important, but the methodology employed should be irrelevant as long as it efficiently and effectively groups like objects together and separates those that are dissimilar.
The action of removing Pluto from the planetary ranks successfully accomplishes this goal and provides clarity to the nature of planetary evolution. Perhaps Stern's assertion that a planet simply be a round non-star is the best one. But taking things a step further is most definitely necessary, in my opinion, in order to provide clarity.
Further subdividing the planets into Terrestrial, Jovian and Dwarf allows us to further group the objects together into more logical groups. I mean logical in that someone may mistake an image of Neptune for Uranus, but no one will confuse Mars with Saturn or Pluto.
Ultimately it does not matter. The way we classify objects has no influence on the physical nature of the Universe, it only dictates what terminology we learn in textbooks. As long as the classification is clear and concise the particulars are unimportant. And it is to that end that the debate should be motivated.