Defining Planets (Part X)
Published on Mar 10, 2009 at 3:03 pm.
5 Comments.
Filed under planets.
I’m still busy with mid-semester paperwork, so I have not had time to blog much.
This is going to be my final entry (for now) on the defining planets series. Would you have believed when I started this a month ago that it would wind up being ten postings? This is a very difficult topic. It may seem easy, but if you’ve been following the topics, you can see that there are a lot of things to consider.
So, to summarize, we’ve looked at the history of the term planet. It comes from the Greek for wanderer. The ancients looked up and saw that certain celestial objects moved around. These were the planets. Over time, the term got a bit confused. After all, the Sun and Moon appear to move around the sky, but they are not the same as the little dots that we normally call planets, which look like stars to the casual observer. When, it was realized that Earth and the rest of the dots orbit the Sun, it became apparent that they may be worlds like our own. So, the concept of planet changed. Then, the first moons found were called planets. Then, they were removed from the list. The first asteroids were planets, and then they were removed. Why remove bodies? The asteroids were clearly inferior in size to any known planet at the time, so they didn’t seem to fit the pattern. Likewise, the moons of Jupiter and Saturn orbit those worlds, not the Sun, and were clearly inferior in size to those world. At the time, astronomers did not fully appreciate how much larger Jupiter and Saturn were than Earth, so those moons were thought to be much smaller than our world, perhaps about the size of the asteroids. We now know them to be far larger than the asteroids, and in the cases of Ganymede and Titan even larger than Mercury. Perhaps if that had been known then they may not have been de-listed from the planets, and the debate of the definition of planet would have come earlier and perhaps look different than it does today, and we may have a good definition.
Why do we need a definition in the first place? Well, a definition of the term planet is needed in order to classify bodies. One of the first things that we do in science when we have a number of objects to study is to try to classify them. This is done in order to order them. That is part of the process of studying the objects: you group them according to similarity. Then, you try to explain why they are similar and what makes them different from bodies that you put into other classifications. So, the primary reason for having a class of objects that we call planets is to help organize them and to facilitate study of like bodies, thus creating greater understanding for the entire class of bodies. In modern science, this extends a bit farther to securing funding for said studies. Grants are often awarded based on the type of object studied. There is a feeling that there may be more money available to study a planet than a non-planet. Personally, I think that is pretty silly. Study of a body such as Pluto, Ceres, or Pallas should not depend upon the classification of the object. They are worthy of study due to their place and role in the Solar System alone, no matter what we call them. Textbook authors also push for a definition of planet so that they can arrange topics in their books. Again, this is all wrong. Educational pedagogy should not drive science. It should be the other way around.
So, over my series, I looked at a number of possible criteria. There are plenty of others that I didn’t get around to considering. Perhaps this is something that I should put in my list of things to write a book about. After all, there is still a lot to be said on the topic! I tried to look at each criteria and look at its pros and cons. There was not a single criterion that did not have both pluses and minuses. That makes determining the definition of a planet tough. Some possible criteria failed very quickly. Others failed only upon deeper consideration. A common planetary discriminator throughout the years has been size. Too small does not make a planet. But, when the Solar System was known only to be comprised of very large bodies and very small bodies, it was easy to say that a planet is a body larger than the size gap between the two and a non-planet is on the smaller side of that gap. In recent years, the large number of Kuiper Belt objects that seem to be filling up that size gap has made that criteria problematic. And, with the discovery of extrasolar planets, there is every reason to suspect a plethora of bodies in this in between size. A good definition of planet needs to be something that is applicable to both the Solar System and to other planetary systems.
Let me review some of the more promising planetary criteria, though. One that I like is a structural definition. Unfortunately, it is really difficult to know what the interior structure of a body is simply from looking at it. It is impossible to know what the interior structure of an extrasolar planet is using today’s technology — all we can determine about those worlds are their masses. A similar criteria would be hydrostatic equilibrium. A body in hydrostatic equilibrium is a planet, and one that is not in hydrostatic equilibrium is not a planet. However, that raises a number of questions, as well. There are degrees of hydrostatic equilibrium. Also, the size and mass of a body that is required to reach hydrostatic equilibrium depends somewhat on its composition. Thus, two bodies of the same size and mass may not both be planets under this criterion if they are composed of different materials. That bothers me. And, again, we don’t really know if a body is in hydrostatic equilibrium if we can’t observe and measure it. So, all we could do is guess about the extrasolar planets.
But, permitting any body in hydrostatic equilibrium also permits a large number of Solar System bodies typically not considered planets to be classified as such. Changing a definition to increase or decrease the number by 1 or 2 is one matter, but there is resistance to a change that doubles the number of planets (particularly if it requires including bodies previously not considered planets). Of course, emotional response to the idea is not really a valid scientific reason for accepting or rejecting the definition. The same statement, though, goes to those who reject any defintion that removes certain bodies from the planetary list that have long been on the list. The IAU accepted the idea that a planet was in hydrostatic equilibrium, but it is only a planet if it also is large enough to clear its orbital neighborhood of other planetary sized bodies.
The idea that a planet must also be large enough to clear the neighborhood of its orbit, while it sounds really nice, doesn’t do much for me in practice. You have to go into a bit of additional mathematics to decide just what is clearing the orbit. While that extra mathematics gives a pretty nice dividing line for planets versus non-planets in the Solar System, I am not convinced that it would do so for planetary systems around all other stars. We really need a fairly universal definition. After all, we don’t want to be back in the same situation years down the road where we have a number of bodies that almost clear their orbital neighborhood and we have arguments over whether they are planets or not.
There is yet another issue that I have with the idea of defining planets in this manner, and that is that the whole point of a scientific classification is to group similar bodies together. Jupiter and Saturn are very similar. Uranus and Neptune are similar, too, but there are significant differences between the Uranus-Neptune pair of worlds and the Jupiter-Saturn pair. Yet, all four of those worlds have certain similarities when compared with the small rocky worlds of Mercury, Venus, Earth, and Mars. The Moon is more like those worlds than are any of the gas giants. Pluto, Eris, and the rest of the Kuiper Belt objects are very similar to one another, yet they are all quite different from the other bodies that I’ve mentioned here. Does it really make sense to put all of these bodies together in one classification? I am beginning to think that it does not. Perhaps we need to create separate categories of these bodies. Maybe the time for a single definition of the word “planet” has passed. We probably need to define each class of object separately. Then, if the body is large enough to be in hydrostatic equilibrium, we can call it a planetary body, after Alan Stern’s method that I mentioned in an earlier post. The largest moons should be included in this methodology, too. I believe that it is a mistake to separate them from the planets when they are (or may be) of similar size, composition, structure, and method of formation.
So, I am sure that a lot of people are not going to like my ideas of ditching the definition of planet like this. Still, I think that it makes more sense than to try to continue to come up with unworkable definitions.
-Astroprof
Mang on March 11, 2009 at 4:06 am: 1
Again, this is a great series. The devil is truly in the details.
Abandoning “planet” as a scientific term is probably the better way. I’ve been thinking this for some time now. Specifically, I’d like to go back to what I said in response to Part V.
“Any specialized field has specific terminology. Beyond this there is the public or the educated public. These groups are not going to fully share the same terminology and it isn’t desirable that they do. It’s therefore a goal to have the terminology not conflict or contradict in serious ways. This allows the public to be imprecise but still generally in the same direction. It also allows the specialist to pursue high degrees of precision.”
Frankly, the IAU erred in trying to define something that they never had control of in the first place!
Indeed, the IAU should formally undefine “planet” and instead provide other more precise definitions. The term planet WILL continue to be used by the general public and educators as an introduction to the subject.
Far from being demoted, Pluto will be remembered for forcing this needed rethink. Of course, Pluto would not be in this position without the (relatively) recent advances in the study of things orbiting stars including Eris, etc. and exoplanets.
Sili on March 11, 2009 at 1:16 pm: 2
De Grasse Tyson was on the Are We Alone Podcast recently, and talked about how he had been made Public Pluto Enemy #1.
All they had done at The Hayden was to play a game of “One of these is not like the others”.
They had indeed grouped the objects of the solar system according to ‘kinds’: The terrestials, the asteroids, the gas giants, the lumps of ice on the edge.
And after a while someone noticed that Pluto was out there in the boondocks and all hell broke loose.
Indeed, we should go back to “planet” meaning wanderer - excluding Uranus and outwards. Then lets have names that describe things for what they are.
Cats, dogs and sharks are carnivores, but knowing that doesn’t tell us much.
Laurel Kornfeld on March 11, 2009 at 10:30 pm: 3
I was at the Isaac Asimov Memorial Debate last night, and the topic was planet definition and Pluto. Tyson moderated a panel of six scientists, including Stern, who had an informative and congenial discussion. There was a sense, even among those who disagreed with one another (dynamicists vs. planetary scientists) that the entire field of planetary science is in its infancy, and that this entire debate may be premature. We know very little about the dynamics and arrangments of planetary systems around other stars, but we do know they may not be similar to the layout of our solar system. Most agreed that it’s not a matter of planet vs. non planet, that there is no black and white dividing line but a spectrum of objects from the tiniest in hydrosatic equilibrium, such as Vesta and various KBOs to brown dwarfs. There was a strong sense that we may just need more data to come up with a good classification scheme. Regarding education, pretty much all the panelists agreed that teaching a list of names or facts to memorize (whether 8, 9, 13, or whatever number) is the wrong way to go and does not convey the important concept of science as a process. They also agreed that neither does a vote by the IAU.
HP on March 12, 2009 at 7:18 pm: 4
I’ve been reading the whole series with great interest, but have yet to comment.
One of the things that strikes me, as a non-scientist, is the similarity between the debate in astronomy over what is a planet, and the debate in biology over what is a species. In both cases, we’re dealing with 18th century categories that no longer work.
I liken the situation with Pluto to the recent consensus in paleontology that birds are in fact a type of dinosaur. The dinosaurs never went extinct; it was only non-avian dinosaurs that went extinct.
There is, to me, something marvelous about being able to look out my window and see little feathered dinosaurs cavorting in the trees. And yet there are still biologists (mainly ornithologists) who refuse to acknowledge that birds are dinosaurs.
I think a lot of the opposition to the “demotion” of Pluto is attributed to the same type of attitudes. Kuiper belt objects, unlike birds, are not something you can see around you all the time. I think that as our knowledge of Kuiper belt objects grows, and as they become more “real” to us, the inclusion of Pluto will be seen as less of a demotion and more of a testament to the extraordinary work of Lowell, Tombaugh, et al.
Mang on April 13, 2009 at 6:36 am: 5
Great series. It certainly helped my thoughts on this.
http://mangsbatpage.433rd.com/2009/04/last-planet.html