Defining Planets (Part III)
Published on Feb 7, 2009 at 2:19 pm.
10 Comments.
Filed under planets.
Pluto was an oddity among the planets for much of the Twentieth Century. As I said in my last post, it just didn’t seem to fit with the other eight planets. There was debate for decades on whether or not Pluto should really have been on the list. In the end, most astronomers were content to simply talk. After all, Pluto was already on the list, and it was simpler to leave it on the list than to remove it. And, for some reason Pluto seems to have become one of the more popular planets — perhaps because it was discovered in modern times. But, as I said last time, once other objects very much like Pluto started to be discovered in the 1990s, the debate on Pluto’s status as a planet began to heat up. After all, how could you say that one object is a planet and another, nearly identical object, is not a planet? Some criteria for planetary status was needed. Though Pluto’s planetary status was beginning to seriously be called into question, there was no urgency in the matter, since Pluto remained a bit larger than any of the other bodies beyond Neptune. That changed early in the 2000s as bodies nearly the size of Pluto started to be discovered. Then, it became silly to call Pluto a planet while something 80% of Pluto’s size was not a planet. The real problem, though, came with 2003 UB313 was discovered. This body was larger than Pluto. So, if Pluto is a planet, it makes no sense to have something very nearly identical, and even larger, be a non-planet. One option open to astronomers was to declare all of these bodies sub-planets with Pluto remaining on the list of planets for historical and cultural reasons. That idea, though, flies in the face of the whole concept of taxonomy. It would be ridiculous to have only one of a number of nearly identical bodies classified differently than the rest. Whatever one of these bodies is, so are the rest. And, we should not be held to keeping Pluto on the list of planets simply because it was already on the list. After all, as I posted earlier, there have been several bodies that have been initially classified as planets only to be later removed from the list decades later when we learned more. Science should follow consistent rules, not bend to public pressure.
So, this leads to the central point of this series of posts. Just how do you go about defining the term planet? We need to pick a reasonable definition of planet, and then go back to the current list of planets and see which bodies actually meet the definition, not look at the list and create a definition to fit the list. The definition of planet needs to be well thought out. It needs to be something that is workable. The criteria for planetary status needs to be something that is as clear cut as possible. A definition that leaves a large number of bodies as “maybe planets” is not very good.
Most important of all (and, I think forgotten by most people involved) perhaps the term planet is obsolete and needs to be replaced altogether by new terms describing the bodies to be studies. After all, the term planet already was being used to encompass a surprising variety of bodies: small rocky planets, large gas-rich planets, and an icy body. In other sciences, such diverse descriptors would long ago have been assigned separate labels.
Why do we want to assign a label of “planet” to different bodies? Is there really a scientific reason for this? Or, is the label simply something so that textbook authors can know how to list the bodies? Do astronomers need to know the descriptor in order to study the objects? Do we need a label so that we can differentiate bodies? Just why do we need a label, anyway? Does labeling certain objects planets and other objects non-planets in any way help our understanding of these bodies? Does the label even help with students learning about these bodies? Or, does the label hinder our understanding and learning? Are we somehow limiting our studies to certain objects because they are planets and ignoring other, virtually identical objects, because they simply do not have that named label attached to them? We should not have a definition of planet that is designed solely for the sake of textbook authors’ ease of giving textbook chapter titles. The definition should not be so that school children have an easy mnemonic with which to remember the names and order of the planets. The definition should not be such that it is designed to increase or to limit the funding of research for certain objects (the presumption being that there may be more funding to study planets than to study things that are just like planets, but simply not on the list). And, most importantly, the decision of a definition should be made solely for scientific purposes. The designation “planet” should signify that the body with that designation is in some important way different from a similar body without that designation. These are the things that must go into defining the term planet.
So, shortly after the discovery of a body nearly identical to Pluto,astronomers got together to propose a formal definition of planet. This group failed to come to an agreement on the definition of planet. So, in 2005, the IAU formed a committee composed of a variety of people from the astronomical community to develop a solid definition of planet. They worked for about a year on a proposal and produced on in August of 2006. I blogged about the proposal at the time. When I first heard about it, I did not like it. The proposal fell short of the suggested requirements of a definition that I set forth in the previous paragraph. My thought at the time was, “They worked for a whole year on this, and this is the best that they could come up with?” The IAU general assembly apparently had a similar reaction to what I had. They rejected the proposal, and I think that they were right to do so. However, they then went and made matters worse with a new definition that many consider to be even worse than what the committee had come up with. They were in a hurry to come up with a new definition and did not give it the year’s worth of discussion that the committee did. As much as I disliked the committee’s proposal, it may have been better than what we eventually got. I firmly believe that the current official definition is flawed. Many other astronomers feel that it is flawed. And, I firmly believe that one day a new definition will have to be devised.
For one thing, there are bodies orbiting other stars besides the Sun. It makes sense to categorize these bodies as planets if they meet the same guidelines as planets for the Solar System. However, the definition that the IAU came up with after scant hours of discussion does not address the matter of planets around other stars. It also does not decide what to do with a body that had been a planet of a star and is ejected through gravitational interactions of some sort (with another planet or with a star). Yet, there seems to be indications that such bodies may exist.
So, how do we go about defining just what we mean by the term planet? First off, let me say that I am not a planetary astronomer, but I am a pretty good generalist in the field. So, I do believe that I have something to offer.
First of all, we need to know what makes a planet. There are several ways that planets can be defined:
1 ) Planets can be defined by orbit. This would differentiate planets from moons, at least.
2 ) Planets can be defined by mass. Big things are planets, small things are not.
3 ) Planets can be defined by size. Similar to number 2.
4 ) Planets can be defined by composition. What makes it up determines if it is a planet or not.
5 ) Planets can be defined by structure. Planets have some different physical characteristic than non-planets.
6 ) Planets can be defined by method of formation. Planets form slightly differently than non-planets.
7 ) Planets can be defined by circumstances of discovery. This would be a rather descriptive definition.
8 ) Planets can be defined by circumstances of observation. If you can see them, they are planets.
9 ) Planets can be defined by their effect on their surroundings. If they influence other bodies, they are planets.
10 ) Planets can be defined by a combination of these criteria. One characteristic might not be sufficient to create an unambiguous definition.
Unfortunately, none of these criteria gives rise to a perfect definition of planet, and I will go into that in my next installment of this series. I’ll also tackle each of these guidelines one by one and see how they fail to give a proper definition. Many yield ambiguities, or they exclude bodies from the list that most would believe should be kept on the list. Clearly, defining planet is tough.
So, stay tuned. I’ve got more to write about.
-Astroprof
Sili on February 7, 2009 at 6:56 pm: 1
I think you’re right that an allencompassing “planet” label is prolly impossible. But just as we have “dwarf planets” that are not planets, I guess it’ll make sense to look into definitions for “gas planets” and “rocky planets” separately.
To be fair - I think the designation of humans as Homo rather than Pan has been argued to be similarly arbitrary. I’m sure David Marjanovi? could set me straight.
erika on February 8, 2009 at 12:09 am: 2
wow. i never realized that it would be this complicated.
Boxorox on February 9, 2009 at 7:55 am: 3
The classification of celestial bodies as planets, asteroids, and such should be approached with a clinical objectivity that seeks to make a definitive outline for all the objects of a planetary system. We can look perhaps to the biological sciences for guidance in this endeavor. Determining what is a planet could be similar to delineating what animals can be described as mammals. Furry and live-birth were sufficient criteria for making this classification stick. Mammals range in degree of furriness from orangutans to whales and the platypuss clearly imitates birds and amphibians in their reproductive manners.
As we work carefully to compose a concise, but not necessarily simple, definition of a planet, we should do so turning a blind eye to individual members of our own Solar System. With a more universal approach, we should come up with a sweeping specification for planets that may even be applied to members of exo-solar systems as well. The time is rapidly approach–if it it not already here–when we have to consider planets in orbits around other stars and test our definition against those cases.
Lets ignore Pluto for the moment and concentrate on the bigger picture. There is nothing wrong with our solar system comprising just 8 planets. Likewise, I think our community should be satisied if the planetary membership of our neighborhood might grow to 20 or more. Tradition is not a good criterion for allowing Pluto to retain its planetary classification, unless it qualifies by technical merits.
Astronomy Link List on February 10, 2009 at 3:44 am: 4
Added to the Astronomy Link List
Laurel Kornfeld on February 10, 2009 at 6:20 pm: 5
There is something wrong with viewing our solar system as comprising only eight planets–it is not correct; it is arbitrary, and it is a gross oversimplification. Objects that behave just like the eight biggest planets are left out for no clear reason other than convenience and fear of having “too many planets.” With more and more exoplanets being found, what is wrong with recognizing the need to add on new subcategories of planets to encompass the new discoveries? Our solar system does not have only terrestrial and gas giant planets any more. Dwarf planets are an additional subcategory that likely will someday be found in other solar systems as well.
Astroprof on February 10, 2009 at 8:03 pm: 6
Hang on. I’m not done with my series yet! I have several more points to make.
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