What is the definition of a planet?
Astoundingly, no precise scientific definition of the word "planet" currently exists. It is rare for scientists to have to define a word that is already in common usage and that everybody from school children on up already understand. How does one then go about constructing a scientific definition of such a word after the fact?
In such cases, we believe that it is important to be both true to the historical and popular perception of the meaning of the word while being scientifically descriptive, accurate, and meaningful. We will use these points -- historically valid and scientifically meaningful -- as the criteria on which to judge potential definitions of the word "planet." We have identified 4 major ideas for the definition of the word "planet" (though the most common have never been written down to our knowledge):
* Purely historical. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are planets. Nothing else in the solar system is a planet. This definition is definitely historically valid, but fails miserably under scientific meaning. What if a new object larger than Pluto is found? What is it? Why is Pluto a planet but an object 3/4 its size, like Sedna, is not? This definition, completely lacking in scientific motivation, makes the word "planet" meaningless as a scientific description.
* Historical plus. Mercury through Pluto are planets, as is any newly discovered object larger than Pluto. This definition is, we believe, the one in most common colloquial use throughout the world, even if people don't realize that this is the definition they are using. Indeed, if Sedna had been larger than Pluto, most would have hailed it as a 10th planet. This definition -- like the previous -- is historically consistent, but -- like the previous -- still fails the scientific test. Why is Pluto the cutoff size? Is there really a big enough difference in size between Pluto and Sedna and Quaoar that one should be called a planet while the others are not? The scientific answer remains a resounding no.
* Gravitational rounding. Any object which is round due to its own gravitational pull and which directly orbits the sun is called a planet. This definition is very different! It is strictly scientific, yet historically valid, as all objects that we call planets by the historical definitions are indeed round due to their own gravitational pull. More importantly (and by a complete coincidence) the dividing line between objects which are round and those which are not round is just a few times smaller than the size of Pluto. So why not take advantage of this coincidence and simply define planets to be objects which are round? To do so means that we must admit several other bodies to the class of "planet." Sedna, Quaoar, the asteroid Ceres, and perhaps a dozen Kuiper belt objects are also likely to be round and thus, by this definition, planets. But these additions are perhaps a small price to pay for a definition which rests on solid scientific principles.
Unfortunately, this definition completely fails the historical sanity check. Historically, where does the criterion to be round come from, except for the near coincidence between the historical definition of planet and the transition size from round to not round? At no time in previous history has any discussion of whether or not an object is round been part of the discussion of whether or not it should be called a planet. Ceres was initially considered to be a planet, but not because it is round (which was unknown at the time), but because it was the only object known to exist between Mars and Jupiter. When other asteroids of similar sizes were found at nearly the same location it was decided to call them all members of the asteroid belt, rather than planets.
Roundness is an important physical property, and gravity is the dominant force in the solar system, so perhaps it is important to have a special word which describes the class of objects in the solar system which are round. But simply because all historical planets are round does not at all mean that it is good science to define all round objects to be planets. A much better idea is to use a different word to descibe these objects. Spheroids? Gravispheres? Actually, we prefer the word "planetoid" as a new word to descibe round objects orbiting the sun. All planets are planetoids. All planetoids are not planets.
* Population classification. This definition requires a little more explanation and a little more understanding of the solar system, but, in the end, leads to the most satisfactory definition of "planet". Just like the solar system very naturally divides itself between round objects and non-round objects, it also very naturally divides itself between solitary individuals and members of large populations. The best known example of a large population is the asteroid belt. We call it a population because one region of space contains objects with a continuous range of sizes from one moderately large object (Ceres) to a handful of slightly smaller objects (Vesta, Pallas, Hermione) to a huge number of extremely small objects (rocks, dust particles). The solitary individuals are much different. In their region of space there is only them (Earth, say) and then a collection of much much smaller objects (the near-earth asteroids), with no continuous population in between. A single example helps to dramatize the difference between a continuous population and a solitary individual. Ceres, the largest asteroid, has a diameter of 900 km. The next largest asteroid, Pallas, has a diameter of 520 km. After that is Vesta at 500 km, and Hygiea at 430 km, and the list continues on down. The jump in size between asteroids is never more than a factor of two. In contrast, the Earth has a diameter of about 12,000 km, while the largest other object in the Earth's vicinity, the asteroid Ganymed, has a diameter of about 41 km, a factor of 300!
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all count as solitary individuals by this definition. Pluto and Quaoar do not. Pluto is clearly a member of the Kuiper belt population, as can be seen from the fact that there are objects in the same vicinity slightly smaller than Pluto (Quaoar, 2004 DW, Varuna), and then even a larger number slightly smaller than that, and then on down.
What about Sedna? Sedna is currently the only object known in its orbital vicinity, but we strongly suspect that there will be many others found out there with time. We thus feel it is more reasonable to classify Sedna as a member of a large population (the inner Oort cloud of objects) rather than a solitary object. This classification saves us from having to go back and reclassify Sedna in a decade when we find more objects!
Since there is a clear scientific distinction between solitary individuals and members of large populations it is instructive to come up with words to describe these objects. The large populations can each be described by the particular population (asteroid belt, Kuiper belt, inner Oort cloud, Oort cloud). What about the solitary individuals? Isn't the best word to describe them "planet"?
Let's examine this definition in more details. First, it is certainly scientifically motivated and well-founded. But so was the "gravisphere" definition above. Is there any historical basis for saying that a planet is a solitary individual that is not a member of a large population? Yes! As mentioned earlier, historically Ceres and the first few asteroids were initially classified as planets. Only when it became known that there were many many asteroids in similar orbits was it decided that they should no longer be classified as planets. Historically, there is a clear distinction between planets and populations. Any definition which fails to make this distrinction is in strong trouble on historical grounds. This simple look at history shows that Pluto is completely analogous to Ceres. Pluto was initially thought to be a solitary individual. Over time we found more objects in the vicinity and realized instead that it is a member of a large population. Historically, then, Pluto, too, should no longer be considered a planet.
We are thus left with a final concept of the word planet. Every object in the solar system quite naturally can be classified as either a solitary individual or a member of a large population. The individuals are planets. The populations are not. This definition fits the historical desire to distinguish between asteroids and planets, and this definition fits all of the requirements of scientific motivation.
Even this definition is not perfect. People will always be able to imagine (and perhaps even find) pathological scenarios in which the above classification scheme fails. In contrast, the first three definitions are much more rigorous and will never need refining. We don't find this aspect of the first definitions an advantage, however. As we learn more about our solar system our language -- both popular and scientific -- should change to fit our knowledge. We think that our proposed classification scheme will suffice for everything that is found in our solar system, but we would like nothing better than to find some object which defies everything that we currently think we know and forces us to completely rethink fundamental questions like "what is a planet."