Defining Planets (Part I)
Published on Feb 2, 2009 at 4:48 pm.
6 Comments.
Filed under planets.
In 2006, the International Astronomical Union voted on a new definition of the word planet. I wrote about the matter at the time, but I now feel that I should revisit that discussion. Last week I wrote about an asteroid sharing Earth’s orbit and that this asteroid raised issues with the IAU definition of planet. I got a number of responses to that posting. One of the most talked about consequences of the IAU’s new planet definition is that Pluto, long on the list of planets, was removed from the list. One of my commenters who wants Pluto to remain a planet, Laurel, even has a blog about the status of Pluto. Recently, even Flying Singer, who has not weighed in until now, has decided that the definition of planet should be amended. His favored definition also includes Pluto. There are a lot of opinions and emotions involved in whether Pluto should be on the list. It is not as simply, though, as whether there are 8 or 9 planets, nor whether Pluto is on or off the list. Whatever decision applies to Pluto applies to all of the other bodies just like Pluto. And the matter did not start with the 2006 IAU meeting. I had already written about this over half a year before the decision, and I even gave a public presentation in January of 2006 about the controversy as to whether Pluto should be a planet or not.
Interestingly, most of the discussion about the new definition has centered on whether or not Pluto should be a planet, and what definition of planet should be used that would allow Pluto to remain on the list. Really, that is not how science should be done. we should first decide what makes a planet and then decide if Pluto is on the list. If it is, then fine. If not, then that should be fine, too. Personally, I think that the IAU definition is seriously flawed and needs to be revisited. I’ll be writing more about this for the next several posts. First of all, though, I want to talk about the history of how all this came about. All too often, the pro-Pluto crowd and the anti-Pluto crowd have squared off and hurled insults at one another without stopping to look at each other’s positions. That helps no one. As it turns out, there are very valid reasons for having a definition of planet that excludes Pluto. But, there are also very valid reasons for having a definition of planet that includes Pluto. But, as I said, creating a definition to either include or exclude Pluto is wrong. We need to put Pluto aside. Next, without regard for Pluto, create a definition of planet. Then, see if Pluto fits the definition.
This is a very complex issue, and anyone who argue that it is NOT complex is not being realistic, in my opinion. So, I cannot simply cover the topic in one post (no matter how long). I am going to string this out for the better part of this week. First, we need to talk about how we got into a situation where the definition of a planet was even an issue. Next, we want to talk about what sort of definitions could exist. As it turns out, there are several possible ways to define planet. All have something to say for them, and all of them lack in some area.
Oh, and astronomers are not the only ones who reclassify things now and then. Recently I heard from one of our biologists that there is a proposal to reclassify the species and genus of drosophila. For those that don’t know, that is the iconic fruit fly on which so much research is done. So, where are all the people who are screaming about Pluto? Why are they not concerned that drosophila is possibly being changed, too?
If you are not aware of the discussions that had been going on for the prior two decades (or more), then you might wonder why the IAU even decided to bother with redefining the term planet. Well, actually, as it turns out, they weren’t redefining planet; rather, they were defining planet. There never had been an official definition of planet. The term just happened, and astronomers generally agreed informally on what they meant when talking about planets.
Growing up, I learned the planets of the Solar System, like all the other kids in school. We had to learn nine of them. I’ve always been an avid reader of science fiction, and of course space adventurers were always visiting other planets. In the mid 1990s, I was teaching astronomy when word came out of the discovery of planets around other stars. Some years prior, planets had been found around a pulsar. We all knew what a planet was, or at least we thought we did. So, why did they have to come up with a new definition?
The problem is that, as I said earlier, there never really was a definition of planet. The word and concept the concept of planet comes to us from ancient times. People looked up at the sky and saw stars. Night after night, they saw the same stars in the same orientation to one another. The human mind tries to find patterns in what we see, so we see patterns in the random assortment of stars. These became the genesis of the constellations. People also looked up saw the Sun, the Moon, and stars. Clearly, the Sun moved. It was sometimes high in the sky, and sometimes not. The Moon, too, moved. Every night, people could see that it was a little farther to the east in the sky. But, while most stars seemed to always stay put, there were five star-like objects that appeared to move around. These wandering stars were special. It is from the Greek word for “wanderer” that we get the word “planet.” So, a planet was a wandering celestial body. By that definition, even the Sun and Moon were planets.
In ancient times, most people believed that the Earth was fixed (after all we don’t feel it moving, to we?) and that the Sun, Moon, and stars went around the Earth. Eventually, however, the heliocentric model began to gain support with the work of Copernicus, Galileo, and Kepler. It became apparent that Earth went around the Sun like the planets. That made Earth a planet, and the Sun not. Galileo’s discovery of four large satellites of Jupiter led to their being called planets, too. The Solar System, for a while had 11 planets, only five of them orbited other planets rather than the Sun. Soon, satellites of Saturn were discovered, and more satellites of Jupiter were found. Eventually, astronomers decided that planets orbited the Sun and moons orbited planets.
But, it is not so simple. In 1781, William Herschel discovered Uranus. This body was the first discovery of a new body whose designation as being a planet has stood the test of time (so far!). But, then astronomers found other bodies orbiting the Sun between Mars and Jupiter: Ceres, Pallas, Vesta, and Juno. These were called planets since they clearly orbit the Sun and not another planet. Quickly the list of planets soared to over a dozen. I think that I read somewhere a few years ago that at one time textbooks listed about 17 planets. But, these new bodies between Mars and Jupiter clearly were much smaller than the rest of the bodies labeled planets. Furthermore, there were a lot of objects between Mars and Jupiter. At the time that these bodies were first being discovered, many astronomers believed that a mathematical formula might exist that showed a pattern for the spacing of planetary orbits. This formula predicted a planet between Mars and Jupiter, but just one planet. So, these bodies could not fit the bill. Some astronomers speculated that perhaps there had been a planet there that had somehow been destroyed. That idea turns out not to be correct. Eventually, the shear number of bodies between Mars and Jupiter, together with their extreme difference in size compared with the other planets, suggested that they should have their own designation rather than called them planets. Informally, they became known as asteroids. It should be noted, though, that for decades professional astronomers have called asteroids minor planets, and the body of astronomers who keep tabs on them are called the Minor Planet Center.
Along the way, Neptune was found. Uranus was discovered by accident. Ceres was discovered by an accident of sorts, too. Astronomers were looking for a non-existent body predicted by a faulty equation. They just got lucky that Ceres was in the area that they were looking. But, soon more asteroids were discovered. Today, we know that the asteroid belt is full of these bodies, and more are being found all the time. But, Neptune was different. While a faulty trick of numbers was responsible for astronomers looking for a planet between Mars and Jupiter, Neptune was predicted to exist from the understanding of orbits and gravity. A slight wobble in Uranus’ orbit could be best explained by the presence of another body. That body turned out to be Neptune. Clearly Neptune is a body very much like Uranus. So, if Uranus is a planet, then so is Neptune.
Astronomers were not so quick to give up on the numerology of the Titius-Bode Law (not really a physical law). So, they began to look for something beyond Neptune. To complicate matters, there were a number of errors in reporting Neptune’s position. Actually, the errors were more in how the position was reported, rather than the position itself, and how the standard changed over time. If the conversions were not made properly, then it appeared that perhaps Neptune, too, had an unexplained wobble in its orbit. Several big names in astronomy at the beginning of the Twentieth Century were among those championing the idea of another planet beyond Neptune. So, a search was on for the missing planet. During one of the searches, Clyde Tombaugh discovered a tiny dot. That became known as Pluto. At first, it was heralded as the missing planet. Later, though, Pluto was found to be far too small to have explained the disturbance in Neptune’s orbit. Further analysis of the orbit showed that the disturbance was not there, anyway. So, Tombaugh got lucky in finding Pluto, much as astronomers got lucky in finding Ceres and the other bodies in the asteroid belt.
When it was first found, Pluto was assumed to be at least as large as the smaller of the terrestrial planets. Over time, though, it was found to be much smaller than had been first thought. But, that is not the sole reason for it being booted off the list of planets. For years, I taught my astronomy students that there were nine planets: four terrestrial planets, four gas giants, and Pluto. But, all of that changed. And that is the topic of my next posting.
Oh, and a very good reference for those interested in knowing the history of all of this can read David Weintraub’s excellent book Is Pluto a Planet? It is very informative, and the author explains why Pluto could or could not be considered a planet. Weintraub makes a case that Pluto is a planet. But, there are perhaps some better arguments that could be made for Pluto’s planetary status than the author gives.
Stay tuned.
-Astroprof
Image courtesy NASA, Cassini
Laurel Kornfeld on February 2, 2009 at 6:18 pm: 1
Weintraub’s book is very informative, and I like his logic. Wouldn’t it make sense now to say there are four terrestrial planets, four gas giants, and five dwarf planets, with dwarf planets being a subcategory of the broader term planet designating the smaller ones that are not gravitationally dominant but are still in hydrostatic equilibrium?
Astroprof on February 2, 2009 at 6:58 pm: 2
Laurel, that definition has a few problems, too, which I’ll talk about in a later post. However, it would be FAR better than what we have now. It may be that the term planet is archaic and no longer really applies as it has been used. I’ll say more in later posts, though.
Boxorox on February 3, 2009 at 10:11 am: 3
Weintraub wrote at length on this subject in “Is Pluto a Planet?” and it reopened some thinking that stared even when I was an undergrad at Vanderbilt in the early 80’s. I think it is necessary to be clinically objective in our treatment of celestial objects, just as specialists within their disciplines are for defining, classifying and typing flora and fauna. As such, it is in the interests of astronomy to come up with as universal a definition of planet and other “lesser” objects orbiting a star that might be at least loosely applicable to the objects we find orbiting other stars. We won’t really be able to put such ideas to the test until we get much better understanding of the populations of planets and such things which attend the closer stars, but this exercise will provide forcus for our own neighborhood.
Where it comes to considering Pluto, I find it very helpful not to consider Pluto. That is to say, that I have always tried to outline definitions and classification criteria which apply to a comprehensive understanding of what planets should be and word the rules such that they do weed out what we might consider odd-balls. In my view it has always seemed that Pluto was an odd-ball, not because of its size or composition, but by virtue (or lack thereof) of its orbit in that it crosses the orbit path of Neptune. I think this is an important distinction, since we have to consider that it is Pluto which crosses Neptune’s orbit, not vice versa. With that idea in mind, it seems clear to me that wording about a planet’s ability to clear its orbit of other objects should be avoided. We can disqualify Pluto as a planet by this constraint, but in so doing, don’t we also have grounds to disqualify Neptune for the same reason?
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