Two things happened in 1930 (well, ok, more than two things to be precise): Disney gave Mickey Mouse a big floppy, friendly puppy-friend; and a young astronomer, Clyde W. Tombaugh, 22 years old, and working at the Lowell Observatory in Flagstaff Arizona, assigned to a meticulous task, after much arduous work, identified a new planet. Neither had a name at first. A few months after Tombaugh discovered it, an 11 year old British school girl (Venetia Burney), with an interest in classical mythology, suggested the name Pluto for it because the Roman god Pluto could disappear at will, and was thus as elusive as his namesake planet which had proved so hard to find; I’m not sure when and how Mickey’s dog acquired its name.
It was all so simple then, any large body orbiting the Sun was a planet, and there were 8 of them until Pluto came along. New discoveries since however, seem to have rewritten the rules somewhat: Firstly, Pluto’s orbit around the Sun is very different from those of the other planets in 2 important ways: 1. it lies in a different plane from the rest of the gang, and 2. it’s far more eccentric (so much so, that Pluto’s orbit crosses that of Neptune).
Secondly, not only is Pluto’s orbit different from that of the other planets, its size is also radically different. Furthermore, comparatively speaking, its largest moon (Charon) is huge, and in fact, at 1207 km in diameter, it actually is a fairly large moon when compared to many of the 140-odd other moons in our solar system. Additionally, Pluto always faces the same side to its moon (unlike any other planet).
Furthermore, Pluto too, just like Uranus, spins on its ‘side’, which is again unlike most of the other planets. This of course ought not to matter in terms of determining Pluto’s planetary status, but perhaps it does have some bearing on the matter after all, as it may speak to the origins of Pluto (and perhaps Uranus too for that matter). It’s hard to say for the moment, but it’s certainly possible that such an inclined spin (as well as the unusual orbit) implies an origin different from that of the other planets. Of course whether that ought to disqualify Pluto from planetary status is another matter entirely, a matter we’ll get to in a while.
In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System, and the recognition of Pluto’s relatively low mass, its status as a major planet began to be questioned. In addition, astronomers had been gradually turning up other, larger and larger objects in the Kuiper belt. In October of 2002 astronomers at the California Institute of Technology announced that they had discovered a new body, which they have named ‘Quaoar,’ after a Native American god. In March of 2004, the same team discovered another new sphere which they named ‘Sedna’, after an Eskimo goddess. Both of these objects were smaller than Pluto, previously considered the smallest planet, but both of these objects were bigger than Ceres, which has long been considered the largest asteroid. Being stuck in some kind of limbo, astronomers couldn’t decide whether these new bodies should be called planets or asteroids.
In 2005, Mike Brown et al. discovered 2005 FY9, which was only a little smaller than Pluto, and it became apparent that it was only a matter of time before something larger than Pluto showed up. A bit later thatyear, the bombshell arrived in the guise of 2003 UB313 (which, as you might deduce, was first observed in 2003) it was determined to be bigger than Pluto, and was given the name Eris (rather clairvoyantly as it turns out, as Eris is the name of the Greek god of discord). Eris is about 2600 km in diameter (as compared to Pluto’s 2,300 km), and about 27% more massive than Pluto. For Pluto’s planetary status, this was a disaster. Made of the same ice/rock mixture, here was another object which orbited the Sun. The choices were simple: 1. include these new objects as planets and increase the total number tentatively to 12 (until more are discovered that is); 2. stick with tradition, and say there were only 9 traditional / familiar planets (a proposal which was quite rightly seen as utterly unscientific); or 3. define the term ‘planet’ in a more rigorous (but not necessarily more realistic or scientific) way. At the XXVIthGeneral Assembly of the International Astronomical Union, which was held from August 14 to August 25, 2006 in Prague, Czech Republic, on the very last day of a 2 week long meeting, the handful of astronomers who were left behind voted to declare Pluto a minor planet.
This decision provoked episodes of violent and bitter discord amongst vast (relatively speaking) numbers of aggrieved, barb-tongued, telescopewielding astronomers. There were 2 dimensions to it, one political, the other definitional: for certain things, there is a clear physical basis upon which to base a definition: A water molecule always was, and always will be 2 hydrogen atoms connected to an oxygen atom, no matter what we call it, and it will always have the properties we are familiar with under similar conditions. It doesn’t matter where in the universe one is, this is always true; for certain things, there is a degree of uncertainty, but the object is still fairly readily classifiable: a particular species of animal is fairly easy to identify usually, even though there are some variations present from individual to individual which might eventually lead to further speciation; but for certain things, there is no natural definition possible at all, planets being one of them. Some scientific terms, you see, are nothing more than labels we have invented to help us sort out the world out there, even though the world out there, at least in this regard, has no innate order to it, and this is the key to the entire problem.
What constitutes a planet? There is no natural definition, and as such we have to come up with a reasonable one instead. What’s reasonable though, is not always clear. Must a planet necessarily orbit a star? What about a chunk of rock the size of the Earth say, which is wandering around in interstellar space, not gravitationally bound to any star? Would that be a planet? By the way, there is some evidence that there may be enormous numbers of just such bodies out there, so this discussion is not necessarily merely rhetorical. How about size? Ought there to be a lower limit to how small a thing could be and still be considered a planet? If so, why? That is to say, on what scientific & objective basis? To quote Dan Wilmore: ‘the more I think about it, the more I find myself questioning the entire idea of scientific categories. Some scientific terms are closely rooted in nature, so that the element carbon will always have six protons, and the element oxygen will always have eight protons, and there will never be a grey area where carbon is ‘oxygenish’ or oxygen is ‘carbonish’ — instead, carbon in all of its isotopes will always be carbon, and oxygen will always be oxygen, no matter how you combine it with other elements. Other scientific terms, however, are nothing more than labels that we invent only to help us sort out natural bodies which do not themselves have any sort of innate order. The International Astronomical Union has chosen a set of distinctions which have classified the solar system in a manner which satisfies some astronomers but not all of them. Right now it seems likely that the controversy will continue as new objects are discovered. (http://www.willmore.net/the_ten_planets.htm)’
The International Astronomical Union at their meeting in 2006, came up with the following definition of a planet:
• It needs to be in orbit around the Sun;
• It needs to have enough gravity to pull itself into a spherical shape;
• It needs to have cleared the neighbourhood of its orbit. (What does ‘cleared its neighbourhood’ mean? As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity.)
Seems reasonable, right? Well, not really, it’s the 3rd rule which is problematic. Pluto you see, does not clear it’s own orbit. However, every 228 years, Pluto crosses inside the orbit of Neptune, so in fact, Neptune, by this rule,ought not to be considered a planet either; in fact, most of the other planets (including Jupiter, the giant of our planetary system) have not yet cleared their orbits, since there are asteroids and things out there which are also orbiting the Sun, with whose orbits these planets still interfere (and thus are still in the process of ‘clearing’).Indeed, what’s the scientific basis for requiring that a planet clear its orbit? As Laurel Kornfeld put it: ‘As for the requirement that an object “clear its orbit” to be considered a planet, this determination is highly subjective. Applied literally, it would exclude all the planets in our solar system. It most certainly excludes Neptune, which does not clear its orbit of Pluto. There is no reason that “clearing its orbit” should be a criteria for planethood. This is an extremely arbitrary determination. The further away an object is from its parent star, the more likely it is to have other objects in its orbital path.’(Laurel Kornfeld in comments section of: http://www.universetoday.com/13573/why-pluto-is-no-longer-a-planet/)
Another problem has to do with the decision that ‘dwarf planets’ are not planets, even though dwarf stars are still stars, and the even more arbitrary and unscientific decision that these rules were only to apply to our solar system.Furthermore:‘along with the definition being both linguistically and scientifically flawed, so was the voting process. Although there are over 10,000 Astronomers in the IAU, only 237 of them voted and approved this definition. Therefore, there was NOT a majority consensus of what a planet is. Hundreds of Astronomers around the world (and this planetarium) have signed petitions to ignore the new definition and still refer to Pluto as the ninth planet in our Solar System….‘The Suits-Bueche Planetarium recognizes the fact that Astronomy changes as our knowledge grows, but we do not go along with the IAU’s flawed voting process and flawed definition. Therefore, our official policy is that Pluto is STILL the ninth planet in the Solar System!’(Clay Dante Tuvalu McDermott in comments section of http://www.universetoday.com/13573/why-pluto-is-no-longer-a-planet/)
Indeed, according to many astronomers: ‘what was truly sad was the fact that the demotion of Pluto was done in a controversial political, not scientific, manner. The definition adopted was done so on the last day of the IAU’s two-week conference by four per cent of its members, most of whom are not planetary scientists. The terms of the resolution adopted were changed over and over right onto the last day. No electronic voting was allowed, so the 96 per cent of IAU members not present in the room had no say in the matter whatsoever. Immediately, over 300 professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto, signed a petition saying they will not use the new definition, which Stern described appropriately as “sloppy science that would never pass peer review.” ‘As somebody else on this board stated, the IAU definition, which states that a “dwarf planet” is not a planet at all makes no linguistic sense. That is like saying a grizzly bear is not a bear. And the only reason it is considered “truth” is that the resolution that would have established both “classical planets” and “dwarf planets” under the broader umbrella of planets was voted down by the IAU 333-91. That’s not exactly a ringing endorsement. ‘In astronomy, dwarf galaxies are still galaxies, and dwarf stars are still stars (our sun is a dwarf star!). ‘There is no reason why an object cannot be both a Kuiper Belt Object and a planet if that object has achieved hydrostatic equilibrium, meaning it has enough self-gravity to pull itself into a round shape. These objects have geological processes and compositions far similar to those of the planets than those of asteroids or comets. Pluto is not a comet; its orbit does not take it into the inner solar system, as the orbits of comets do.’ (Laurel Kornfeld in comments section of http://www.universetoday.com/13573/why-pluto-is-no-longer-a-planet/)
Laurel Kornfeld concludes that: ‘The smart way to deal with the discovery of these new round KBOs (Kuiper Belt Objects) is to keep the term “planet” as a broad category and then establish multiple subcategories, such as terrestrial planets, gas giants, ice giants, and ice dwarfs. Moons of planets that have achieved hydrostatic equilibrium could be considered secondary planets since they revolve around other planets rather than around stars. If this leads to over 100 objects falling into the broad category of planets, so be it. ‘This debate will not settle down because a flawed definition adopted through a very flawed process is a poor substitute for true science. ‘In conclusion, I will advance the definition put forth by my astronomy instructor Al Witzgall, which makes the most sense and should be the one ultimately accepted by science. In his words, a planet is “a non-self-luminous spheroidal object orbiting a star.” That’s it. And under that definition, Pluto is a planet. (Laurel Kornfeld in comments section of http://www.universetoday.com/13573/why-pluto-is-no-longer-a-planet/)
There is also a possible political dimension to all of this: Most American astronomers disagreed with the IAU decision. All of the new objects, from Pluto to Eris, has been discovered by Americans in America; and if all of the objects from Pluto to Eris were removed from the list of planets, all of the remaining planets would have been discovered and named by Europeans, and none of them would have been discovered and named by Americans. Is this really part of the reason for the discord? It’s hard to say at the moment, but it’s certainly very far from the first time that the scientific process has been influenced and muddied political considerations. So, is Pluto part of Kuiper Belt or part of the planets? Does it even really matter? I doubt if the universe really cares. There is no doubt in my mind though that sooner or later we will most definitely discover other objects out there, either in the Kuiper Belt, or in the Oort Cloud bigger than Pluto, and perhaps even bigger than the Earth. What will we do then?
Maybe, it’s all Gustav Holst’s fault. Pluto was discovered four years before Holst’s death, but he was not interested in writing another movement for his Planets Suite; maybe he ought to have done so though, instead of stopping at Neptune. With an original Holst-movement named after it (and not one written as an afterthought in 2000 by Collin Matthews), perhaps Pluto would not have been treated in so cavalier a fashion!