It is one of the questions that I field the most: what is a planet? It seems like a simple enough question, but in reality the waters are a bit muddy. And there is certainly enough controversy to go around. To answer the question "what is a planet" we first need to think back to 2006 and look at the tiny orb that started the whole debate: Pluto.
The Demotion of Pluto
In 2006 the International Astronomical Union, in a move that has resonated around the globe, demoted a small ball of rock and ice out in the outer throws of the outer solar system from planet status. Relegating the tiny orb to merely a dwarf planet existence.
The outcry, both from within and outside of the scientific community, has reached deafening levels. So much so that even Neil deGrasse Tyson, director of the Hayden Planetarium in New York and one of the first to begin casting Pluto to the side in public exhibition, has received considerable hate mail. From little kids.
Why Define "Planet" At All?
The argument of course, is that our current clumping of all the round, non-star, non-moon (i'll come back to that one) objects in our solar system was not ideal. Clearly Mercury and Jupiter don't share much in common, yet we classified them the same way.
So Dr. Tyson and his colleagues at Hayden, after a renovation in 2000, attempted to group the planets by similar features, making them easier to study and understand. This also created a more coherent display with greater educational potential. However, this meant that Pluto found itself as the odd man out.
Then of course the 2006 decision by the IAU has been steeped in controversy as well, with many of the scientists working in the field of planetary astronomy having not attended the meeting felt that their opinion should have been heard. But perhaps that greater point of contention is that the definition as arrived at by the IAU committee frankly doesn't even make sense.
What is the Definition of a Planet?
Let's look at what the IAU definition mandates: the object must have sufficient gravity to become round, but not so massive that it ignites nuclear burning like a star (in effect placing a mass limit of about 13 times the mass of Jupiter) and finally that it has cleared its orbital path of debris. This last one is a problem for Pluto. But planetary astronomers have pointed out that it would be for virtually any of the terrestrial planets if it orbited the Sun at Pluto's radius.
Even if the Earth orbited that far from the Sun it would not be able to clear its path of debris (which is a considerable amount of stuff out in the Kuiper belt). However, no one is arguing with the classification of Earth as a planet. Effectively the IAU was placing a distance cap on how far a planet can be from its host star. And this just doesn't make sense.
So What Should the Definition Be?
O.k., so I don't like the IAU's definition, but I do think that we need one. It is important to classify objects, it's simply part of the scientific endeavour. Biologists classify life, while chemists classify compounds and so on. But the means by which you classify the objects in a system needs to be cohesive and non-conditional.
So what about the planets, and Pluto specifically? What if we just took the first two conditions laid forth by the IAU and let it go at that: massive enough to be round, but not so much that it ignites nuclear burning? That would leave the eight objects that we already consider planets and add in the ones that we currently call dwarf worlds.
But there is a problem. Pluto did not form in the same manner as the other planets, and finds itself in a completely different environment as the eight planets we are familiar with.
Specifically, Pluto is probably more aptly described as a planetary remnant. That is because it likely was likely part of a larger planet that was forming out beyond the orbit of Neptune. But due to gravitational interference from the nearby blue planet, the proto-Pluto, if you will, never formed properly, causing the forming world to be ripped apart.
Pluto is simply the largest remaining piece of a planet that never was. It just so happens that Pluto is large enough that it can formed itself into a sphere under the pressure of its own gravity.
And this fact is at the heart of the IAU's third condition for planet-hood. So, while I don't prefer the wording of the IAU definition, ultimately the spirit of the definition makes sense. So, Pluto can't really be seen as a planet. But that isn't the end of the debate either. There is one more issue to work out.
Is Pluto Part of a Binary System?
There is another problem with Pluto, or more specifically its "moon". I put quotes around that term to designate that I think it is an undeserved title for Pluto's companion Charon. Charon meets all the same requirements of a Dwarf Planet that Pluto does, so why is it not included.
You may believe that the distinguishing characteristic that makes it a moon is that it orbits Pluto. But that is where you would be wrong.
While there is no official IAU definition of a moon, conventionally it is designated as any object orbiting a planetary object. This terminology implies that the barycenter (effectively the center of mass) of the binary system must lie within the planetary object.
Take the Earth/Moon system for instance. Our Moon is massive enough that it gravitationally effects the Earth, pulling it slightly towards it as it revolves around us. This means that the Earth actually moves a little bit in an orbital pattern. We don't really notice because the Earth, being much more massive than our Moon, moves so slightly. Specifically, the barycenter lies within Earth's radius.
In the case of Pluto and Charon though, things are a bit different. Charon, being about half the size of Pluto, is massive enough that the barycenter of the system actually lies outside Pluto's radius. This means that both objects orbit a point between them, rather than one object orbiting the other.
For this reason, it is more correct to designate them as a Binary Dwarf Planetary system. While slightly confusing it is, strictly speaking, the correct classification.
