The ruckus about Pluto
01/12/2008 . Source: Mark R. Leeper 
Mark recently got into a discussion with an older science fiction fan about Pluto. He had brought it up jokingly saying the ninth planet was now supposedly no longer a planet. I guess that the correct term now is planetoid. I could tell from the way he was talking that he was not happy that it no longer was a planet. A lot of people are unhappy about the new classification.
Now I knew this was an emotional issue for some people. There were protests when it was announced that Pluto would no longer be considered a planet. So what is a planet, I asked my friend. Well, he said there was a technical definition. What is the definition of planet? He did not know exactly. I did not say this to his face, but if he is not sure that Pluto fits the definition of a planet and is not even sure what the definition is, why does it bother him that Pluto does not fit the technical conditions of being considered a planet?
The answer is obvious though. He almost certainly was taught very young that there were nine planets. And from a young age he might have been able to list them. For science fans that is sort of like the A-B-C Song. Young kids with a science orientation can rattle off the names of the planets in order of smallest to largest orbits. It is not the planets going outward from the sun since Pluto has a strange orbit and is now or was recently nearer the sun than was Uranus.
But being able to rattle off the names of the planets is more a number trick than actually knowing the planets. You can probably list the people in your family youngest to oldest or oldest to youngest. No problem. But try rattling off the names of the eight planets in the opposite order and it will go noticeably slower. (And the alphabet song is almost no help at all in listing the letters of the alphabet in reverse order.) But any kid with a science orientation does know the names of the nine planets as we were taught them, and some of us feel a little betrayed that Pluto was demoted to a planetoid.
The question really centres on what actually is a planet. To those of us brought up on Donald Duck comic books or on Flash Gordon serials a planet is like another country where things are different. Another dimension is the same sort of thing as another planet. It is a place to set stories. I don't mean to imply everybody who thinks Pluto should be a planet is so unsophisticated, but that is how a lot of us start. I think few people could tell you what a planet really is and why astronomers no longer count Pluto.
The thing is I have had this really hard life, you know. I have had much bigger disappointments than finding out that I was wrong about Pluto being a planet. My response is just to wonder what is a planet and why doesn't Pluto qualify.
Well, a planet has to have enough gravity to be roughly spherical and it has to be in orbit around the sun. So far Pluto is a planet in good standing. But then there is the question of mass. If a body does not clean up its orbit it is not allowed to be a planet. And Pluto has been a bad boy and has not been cleaning up its orbit. A planet goes around in its orbit and its gravity picks up debris. Well, I hate to be a tattletale, but Pluto has not been cleaning up its orbit. It doesn't have enough mass to do that.
You see the existence of a ninth planet was predicted by Percival Lowell based on perturbations to Uranus's orbit. Lowell said that there was a big mass out there that was influencing Uranus. Lowell looked for it, but it takes a lot of time to find such things, particularly with the crude tools that were available in the early 1900s. Lowell died in 1916, convinced that there was a ninth planet out there somewhere.
In 1929 Clyde Tombaugh was hired by the Lowell Observatory to look for the predicted object. He had a new device called a "blink comparator" that helped him better to see moving objects. Tombaugh looked and there really was an object near where Lowell thought it should be. The object was supposedly the missing planet--then called Planet X. A schoolchild suggested the name Pluto, for the god of the underworld. The astronomers at the Lowell Observatory noticed that Pluto starts out with the initials of Perceval Lowell and so Pluto was officially our ninth planet.
As time went by it was discovered that there were errors in Lowell's calculation. Uranus was not the mass Lowell thought it was. There was no large massive object. And Pluto just did not have the mass that Lowell predicted. In fact Lowell had wrong information on the mass of Uranus and its perturbations were not from another planet at all.
Pluto was an ice ball from what we now know to be the Kuiper belt. That is this big ring-cloud of ice and rock that is generally outside the orbit of Neptune, 4.5 to 7.5 billion kilometres from the sun. That is about 30 to 50 times as far from the sun as we are. This one ice ball just happened to be in the right place at the right time. It was mistaken for Lowell's predicted planet, which really did not exist. The situation is sort of like what happened to Cary Grant in North By Northwest. Pluto is the George Kaplan of the Solar System.
But perhaps unused to the sudden attention it kept it real identity secret and let us play with the idea of Rocky Jones landing on it and finding space pirates. But it is not big enough to clean out its orbit. We now know that while it is unusually big for a Kuiper Belt object and in the game of cosmic billiards it ending up considerably sunward of most of its mates, it is really an unwitting fugitive from the Kuiper Belt.
Most of us grew up with this nice orderly view that the solar system was a star with nine planets orbiting around it. That is neat and simple. Well, even our solar system is not so neat and simple. It is a large collection of objects from being as big as Jupiter to as fine as dust. There probably are things big enough to be planets further out. We have found a chunk three-quarters the mass of Pluto out at 130 billion kilometers out. We could probably easily be missing a chunk the size of Mercury that far out.
The concept we previously had was that the solar system was a nice orderly comprehensible collection of a few objects and that school children could memorise everything large enough to be called a planet. That simplicity fed our egos probably in the same way it fed our egos that the Earth was the center of it all.
I think we now know that the solar system is just a big collection of junk loosely held together by the gravitational force of the sun. It has no more order than the dust I sweep up from my garage floor. Yes, there are some bigger objects, but they are just part of the detritus of what happened to be there. It is not simple, and school kids cannot memorise the entire set of large and important objects that are there.
I have to say that given my druthers I would still have it be a planet, but I must be grown-up about this. If it is not a planet, it is not a planet. And I am mature enough to say that even if it is an ice ball from the Kuiper Belt there is no reason that Rocky Jones can't find space pirates on it.
Mark R. Leeper
© Mark R. Leeper 2008 |
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