A New and Unique Perspective
There has been a considerable crescendo in the debate over whether a tiny orb in the outer reaches of the solar system should or should not be considered a planet. One could make a compelling argument as to why Pluto should be a planet. Similarly, the opposite could be contended with equal veracity. While still others argue that it doesn't even really matter. So where do we go from here?
A Look at Ceres as a Test Case
Well let's take a fresh look at our solar system through the eyes on an alien. What would we see? How would we classify the objects orbiting the yellow main sequence star in the middle?
Of course there are the obvious isolated objects -- the Terrestrial and Jovian worlds -- that comprise the canonical planetary system as defined by the IAU. But then you would notice two other regions filled with millions, or perhaps billions, or tiny bodies all in orbit around the central star.
The first (moving radially out from the Sun) region is what we call the asteroid belt. Made up of tiny fragments of rock and dust, some measuring the size of a grain of sand, this torus of material between Mars and Jupiter is what remains of a planet that never was.
During the formation of the planets it is hypothesized that Jupiter began a rapid evolution, devouring material all around it. As it pulled mass from the neighboring orbit the gravity began to rise rapidly. Eventually its gravity was so great that it began to disrupt the formation of the planet that was congealing nearby.
As the matter in the inner solar system began to cool, the remnants of the failed planet orbiting just beyond Mars could no longer grow through impacts with other matter. And the asteroid belt was born.
At least one of the planetary remnants, that we know call Ceres, is large enough to have formed a spherical shape under the influence of its own gravity. But it remains stuck in a sea of material left behind from the destructive gravitational force of Jupiter.
But just because Ceres is large enough to have collapsed under its own gravity and orbits the Sun, is that really justification for calling it a planet? In reality it is only a planetary building block.
If we were an alien race studying this solar system, we would conclude that the asteroid belt is merely the remains of a planet that was either destroyed by some cataclysmic event, or simply never formed at all; leftovers from the collapse of the solar nebula. We would never deem to call any of the tiny intermixed rocks a planet.
What About Pluto?
A similar analysis of the region beyond Neptune, the space we call the Kuiper belt, would draw a similar conclusion. During the rapid formation of the gas giant, the increased gravitational pull drew in material from the body forming in the distance. This disruption further impeded the growth of the world, ultimately leading it its demise.
The remnants of the rocky-icy body were broken apart and scattered. The Kuiper belt exists as the leftovers of a planet that was never given the chance to properly form.
But the question, like that of Ceres, is what to do with the objects that are left behind? With some of them large enough to collapse into spheroids under the pressure of their own gravity, it is tempting to label them as planets, the way that we label the other non-star spheroids in the solar system.
Conclusions
Ultimately I find myself somewhere between the views of the IAU and those that support a geophysical definition of a planet. I believe the IAU definition is too broad and difficult to accurately apply. But I do agree with the spirit of their assessment; namely that the asteroid belt and Kuiper belt are unique systems that need to be addressed separately from the other eight planetary orbits.
I'm not sure what to do with objects like Ceres, Pluto and Eris that are all remnants of planets that never formed. I don't think it is accurate to lump them in with the 8 other planets since they are actually remnants of larger planetary bodies that either never formed or were ripped apart after their formation. But since they are the largest chunks/leftovers it does make since do identify and classify them over their smaller, less significant counterparts.
It seems to me that it doesn't really matter what we call them, as long as whatever term is used is applied universally (i.e. to any such object, no matter what the size or where we find it - even extrasolar objects). And that use of such a term recognizes that they came to be in a manner different than the 8 larger planets (namely that they are remnants of a planetary object that was ripped apart or never allowed to form). And of course the definition needs to be more clear than the current one put forth by the IAU.
Of course these are just my opinions. And I respect those of whom would disagree (unlike some scientists out there), because while I see the merit in distinguishing planets from spheroidal objects that are remnants of planets, I can also see why some would argue that it shouldn't matter how the object came to be or where it is located. (A great resource for those in this camp can be found here.)
And herein lies the problem, no matter what decision is made there are those that are going to argue that there is scientific justification for the other side. So the debate is not likely to end anytime soon.
There is also the question of what to call Pluto and its cohorts. The now ubiquitous dwarf planet is a popular choice. But there are those that find this terminology misleading. I prefer planetary remnant as it more clearly identifies what the object truly is. But at this point it is just semantics.
