Planet: Hard to define
Published on Aug 19, 2006 at 1:42 am.
3 Comments.
Filed under planets.
Well, apparently, I’m not the only one who believes that the proposed definition of planet isn’t all that it could be. According to this story from Space.com, an IAU sub-group met to consider the committee’s proposed definition of planet, and less than half favored it!
I’ve been going on for the last several posts about my feelings as to why this proposed definition is screwed up. I had planned another couple posts, but I am sure that by now you are tired of it! So, this will be the last one (unless some other news about this topic catches my attention, or until they finally vote).
The problem is that Pluto shouldn’t have been on the list to start with. It was at first believed to be far larger than it really is. There are two reasons for that. First, around the beginning of the Twentieth Century, astronomers thought that they had noticed an irregularitiy in Neptune’s orbit. An irregularity in Uranus’ orbit is what led to the discovery of Neptune. So, a mad hunt was on for the next planet past Neptune. Pluto was found in the general area that they were looking. That was purely by accident. Pluto isn’t nearly big enough to have any effect at all on Neptune. As it turns out, Neptune’s orbit is perfectly well behaved. The supposed irregularity was due to errors in data analysis. Secondly, when Pluto was found, astronomers knew of only two types of planetary bodies: rocky planets and gas giants. Clearly, Pluto was no gas giant. Even in the best telescopes of the day, it was just a dot. To be as bright as it is, it was assumed that Pluto had to be somewhere between the sizes of Mars and Mercury. Well, that assumed that Pluto was made of dark rock. But, its surface is coated in ice, which is very reflective. So, Pluto can be much smaller than a rocky planet, and still be just as bright. That is the situation.
So, for years, in my astronomy classes, we have talked about the four terrestrial planets, the four gas giants, and Pluto, which doesn’t fit. We’ve talked about asteroids, comets, and Kuiper Belt Objects, with which Pluto fits just fine. It was understood that Pluto was on the list purely by mistake. That was fine. This is how most other astronomy professors that I know have dealt with Pluto. The problem, though, has come in the last five years or so, when quite large Kuiper Belt Objects have been found. Some were nearly half the size of Pluto. Then, 60% the size of Pluto, and finally 85% the size of Pluto. Oh, dear. It was clear that Pluto wasn’t really much bigger than the other things out there, and Pluto shares many features, including composition and orbital characteristics, with these Kuiper Belt Objects. It became clear that Pluto was simply the largest of these objects. Then, the object 2003 UB313 was found. It is a bit larger than Pluto (original estimates were quite a bit larger, but it has been scaled back to only a little larger after more careful study). Clearly, if this Pluto had not been already on the list of planets, then 2003 UB313 would not even be up for consideration for planet status. But Pluto is on the list, so there was controversy. Michael Brown, one of the discoverers of 2003 UB313, doesn’t consider it a planet. So, if the astronomer who found it doesn’t think that it is a planet, why should the rest of us?
The problem is that there has never been a definition of just what contitutes a planet. The term comes from the ancient Greeks, and simply means wanderer. Planets were “stars” that appeared to wander around the sky from constellation to constellation. Eventually, it was determined that these bodies along with Earth all orbit the Sun, and so Earth is a planet, and planets are worlds. OK, no problem. Then, in 1801, Ceres was discovered. It was found to orbit the Sun. So, Ceres was heralded as a planet located between Mars and Jupiter. After a few years, though, astronomers realized that Ceres was simply vastly smaller than the other planets, so it did not really fit with them. Ceres was downgraded to an asteroid. For decades, astronomers, though, have not officially used the term asteroid. Rather, the official term has been minor planet, signifying that these bodies do go around the Sun like planets.
But, this doesn’t clear up the question of Pluto and 2003 UB313. And, there are a lot of things nearly the size of Pluto. What about them? So, the IAU set about to decide on the definition of the term planet.
Now, to most astronomers, the actual defintion of planet, and the classification of these bodies isn’t really that important. We know what they are. But, if you are going to go to the effort to actually define planet, then you should come up with a good definition. I and a host of others don’t like the definition that the committee assigned to this task came up with.
So, what should they have come up with? Well, let’s think about it. It doesn’t make sense to have a definition that has an arbitrary number in it. So, you can’t just draw a line arbitrarily and say that “Bigger than this is a planet, and smaller isn’t.” Because an arbitrary line will have several objects right at the threshold of the line. Obviously, it doesn’t make sense to declare one object to be one thing, and another nearly identical object to be something totally different. So, you should look at the distribution of objects in the Solar System. Group them various ways: by composition, by size, by mass, by color, or by whatever. Then, look to see if there is any natural divisions in those groupings. I believe that there are.
Look at the distrubution of planet sizes. The gas giants are huge, the terrestrial planets are big, and everything else is tiny. And there are a lot of tiny things. They should all go together. The division between the eight largest bodies and the rest is quite pronounced. The division is even greater when mass is considered. And, if you plot the sizes logarythmically, then the division is even more pronounced. So, I would propose a definition as such: A planet is one of a class of large bodies orbiting a star. This class is defined as being composed of bodies all of which are significantly larger than the majority of bodies orbiting the star, with a clear separation in size between the smallest planet and the size of the rest of the bodies orbiting the star.
In our Solar System, that means that there are 8 planets. I don’t think that it would be bad to drop Pluto from the list. I think that makes more sense than adding some of these other things (see my last three posts for reasons why I believe this). The argument that it would traumatize school children learning the planets of the Solar System is silly. It would be worse to make them learn a host of others, and then change the list as we find out more about them. From what little bits that I’ve heard members of the committee say, they were less worried about scientific precision in a definition and more worried about how demoting Pluto would affect kids as well as public sentiment. Since when do the uneducated masses get to decide scientific definitions???? That’s our job as scientists. We decide what the definitions are, and then we educate the rest of the population. That is my entire job description for heaven’s sake!!!!!
So, we can teach 8 planets, and then include Pluto in discussions of planets for its historical significance. We can just accept that Pluto shouldn’t have been on the list, and just take it off. There is precident with Ceres. But, there is an alternate defnintion that I would accept.
Astronomers are pretty conservative about lots of things. On more than one occasion, something was discovered, classified, named, only to discover that it wasn’t what we thought. But, usually we leave the name alone. For example, BL Laceratae objects are active galaxies. But, the name BL Lacertae is a variable star name, because that is what they were thought to be before better telescopes saw the galaxy around the active core. We still call them BL Lacertae objects, though. So, perhaps, we could define planet as I had before, but with a footnote that Pluto is a planet because it got on the list before we had a definition.
In that case the definition of planet would be: A planet is one of a class of large bodies orbiting a star. This class is defined as being composed of bodies all of which are significantly larger than the majority of bodies orbiting the star with a clear separation in size between the size of the smallest planet and the size of the larger members of the rest of the bodies orbiting the star, or if the body is Pluto. That leaves the Solar System with nine planets. Eight that fit the primary definition and Pluto, with everyone understanding that Pluto is only on the list for historic reasons.
It isn’t a perfect definition, but it is sure better than what this committee came up with after a year’s worth of work.
-Astroprof
Judy Walker on August 19, 2006 at 1:19 pm: 1
I am Scott McDonald’s mother-in-law and he sent me your article. Scott and I have astromony in common and discuss the heavens frequently. I have a telescope that Scott takes to your star parties sometimes. I am so glad he sent me your article as I just told him the other day that there are only eight planets and that Pluto is a KBO. I just wanted to let you know that I thoroughly agree with you and I enjoyed your article.
Powerprof on August 20, 2006 at 8:02 am: 2
I’ve been behind in my blog reading - but avidly following the Pluto debate — it’s great to hear a prof’s explanation (never took an astronomy class, regretfully!)
Aviatrix on August 20, 2006 at 11:27 pm: 3
I think you’d like this cartoon:
http://ars.userfriendly.org/cartoons/?id=20060820