In 1992, after some years of searching, Hawaii based astronomer Dave Jewitt and his collaborator Jane Luu discovered what would turn out to the first of many small, icy bodies in the outer solar system. Known initially by the temporary designation 1992 QB1, the new object was the first member of what has become known as the Kuiper Belt, a disk of icy planetessimals left over from when the outer planets formed 4.5 billion years ago. Soon, other astronomers joined the hunt and within a decade the number of Kuiper Belt objects which had been discovered had risen to over 400. Estimates of the population suggest that there might be as many as 10,000 such objects altogether. Some of them, known as "Plutinos" are in orbits that, like Pluto, cross (or came close to) the orbit of Neptune. Others are in more or less circular orbits a bit further out. A few objects are known which travel great distances from the Sun into what is called the "Scattered Disk"
Following the rules of the International Astronomical Union (the IAU), these newly discovered distant objects were initially given temporary designations based on the year they were found. However soon the orbits of a few dozen of these Kuiper Belt Objects, or KBOs, were known well enough that they could be given permanant minor planet numbers and names just like many other asteroids. While the naming of individual objects is a matter for the discoverers, one naming issue caught the attention of the press and public. It concerned the planetary status of no less an object than Pluto.
The issue arose when it became clear that Pluto was just one of a number of objects whose orbit crossed that of Neptune. In many respects, Pluto's orbit is quite indistinguishable from many other Plutinos and some astronomers soon began to refer to Pluto as simply the largest of the known objects in the Kuiper Belt. Brian Marsden, who was at the time head of the Minor Planet Center, remarked on this as early as 1992 when discussing the description of 1992 QB1 as the first Kuiper belt object. He told a reported from the Boston Globe that, "It was probably unfortunate that Pluto has been considered a planet," and asked, "Is 1992 QB1 the first Kuiper belt object, or was Pluto the first?"
Things began to heat up when the increasing rate of discoveries of main belt asteroids meant that the score of numbered minor planets was rapidly heading towards 10,000. Since there was a tradition of naming asteroids with "round" numbers after someone or something special, Brian Marsden suggested that Pluto might also be numbered as minor planet 10,000 to recognize its status as part of the Kuiper Belt. Marsden discussed the issue with members of the appropriate IAU naming committee who agreed that this idea was viable. The idea of numbering Pluto soon began to circulate within the wider solar system community.
It was not a new issue. The question of Pluto's status as a real planet had come up from time to time before, especially when it was realised that the planet was much smaller than had been thought around the time it was discovered. However the issue had never really come to a head, perhaps because until the minor planet 10,000 idea surfaced, no-one had any better ideas. Brian Marsden insists he never suggested demoting Pluto from the list of planets, suggesting merely that giving it a minor planet number would make things tidier when the other Kuiper Belt Objects began to be numbered and catalogued. The idea was seen by him as a compromise between the physical reality of Pluto as a large trans-Neptunian object and its traditional identity as a planet. Despite his good intentions, the proposal ignited a firestorm of criticism which, like most such debates, generated more heat then light.
On the one hand stood the group in favour of numbering Pluto. Some of their arguments ran as follows. Pluto is orbitally indistinguishable from many other objects in the Kuiper Belt. Indeed in orbital terms only an expert can pick Pluto out from dozens of similar objects. Orbitally speaking, there is just nothing special about it. Hal Levison, who studies the orbits of outer solar system objects for a living said, "I firmly believe that if Pluto were discovered today we wouldn't be calling it a planet." This sentiment was echoed by Brian Marsden, who rules that if discovered today Pluto would have got a temporary designation, then a minor planet number, just like any other similar object.
Next comes the issue of Pluto's size. When it was first discovered Pluto was thought to be much bigger than we now know it to be. Indeed the estimated size of Pluto has shrunk steadily for many decades and a light-hearted paper published in the 1980s showed that the decrease in the estimated size of Pluto could be fitted by a curve that would predict the planet's complete disappearance quite soon. On the question of size, Marsden says, "If you want to consider a planet as something that is spherical and has collapsed under gravity, then you can have many such objects, fifteen, twenty, maybe more. There are plenty of main belt asteroids which would meet these standards." He describes the original naming of Pluto as a planet as "irrational" and feels that it only came about because the Lowell observatory did a very good public relations job of saying that they had found the Planet X predicted by Percival Lowell around the turn of the 19th Century. Of course, we now know that Pluto was not Lowell's Planet X. Pluto was found by chance, or rather by reason of the very careful search that Clyde Tombaugh had made.
On the other side of the divide stood the traditionalists who argued, often passionately, that Pluto's status as a planet should not be imperiled. It is clear that Lowell observatory remains very proud of "its" planet and Robert Millis, director of the observatory, says that numbering Pluto makes, "No sense," remarking that, "Pluto deserves to be considered as more than a minor planet." Alan Stern, a staunch defender of Pluto's planetary status, agrees. He says that, "It's not as if the Lowell staff knew Pluto was only 1000km or so in size when they found it and they were pushing it on people. They really thought they had discovered a new planet." Whatever we know now, the planetary lobby say, Pluto has been classified as a planet for over 60 years and to reclassify it now would be foolish and a break with tradition. Jim Scotti of the Spacewatch telescope in Arizona agrees, "People categorise things to bring some kind of order to things and Pluto was classified as a planet so let's keep it that way." Marsden counters that this arguement is not strictly correct. What we now call Minor Planet 1 Ceres was once happily designated the eighth planet (Neptune had not yet been discovered) and the next few asteroids were regarded as planets too. It was only with the discovery of more and more asteroids that Ceres, Vesta, Pallas, Juno were reclassified as a minor planets. Indeed, astronomy textbooks published as late as 1847 referred to eleven primary planets.
Continuing the defence of Pluto's planetary status are those who point out that Pluto is different from the rest of the objects in the Kuiper Belt in several important ways. Firstly, it is much bigger, with a now well established diameter of 2300km and a mass 0.00237 times that of the Earth, Pluto is small in planetary terms, but still several times bigger than its nearest rival in the Kuiper Belt. The completeness of the searches by Clyde Tombaugh in the 1930s (which found Pluto) and Charles Kowal in the 1970s (which found the strange comet-asteroid hybrid Chiron) make it very unlikely that a brighter object will ever be found. Secondly, Pluto has a satellite, and at the time no other trans-Neptunian object was known to have one. This is no longer a clinching argument as some main belt asteroids such as 243 Ida have tiny satellites, and a binary Kuiper Belt object was discovered in 2000. However Pluto's satellite Charon is large. In fact, Charon is so large compared with Pluto that many astronomers refer to the system as the "Pluto-Charon binary" regarding it a sort of double planet rather than an ordinary planet plus a moon. Finally, Pluto has an atmosphere, albeit a thin one that will freeze out on the surface sometime in the early decades of the 21st century as Pluto recedes from the Sun. So there you have it say the traditionalists, Pluto is spherical, bigger than the other things around it, has a moon and an atmosphere. What more do you want to make it a planet?
The issue rumbled on for a while with Brian Marsden urging that Pluto get the coveted minor planet number 10,000, but overall sentiment was against him. As the debate spread, more and more astronomers pitched in. Asteroid astronomer David Hughes from England said that, "It was just astronomers admitting to what they have known for a long time," while others claimed demoting Pluto was "stupid." Soon the debate started to spread outside the tight-knit community of solar system astronomers via the press and internet, becoming simultaneously more heated and less informed as it did so. Eventually the General Secretary of the IAU, Johannes Anderson, felt obliged to make a formal statement of the IAU's position. Although thought to be personally in favour of numbering Pluto, on 3rd February 1999 Anderson stated that, "No proposal to change the status of Pluto as the ninth planet in the solar system has been made by any Division, Commission or Working Group of the IAU responsible for solar system science."
Despite this announcement, various attempts were made to mount a vote on the issue. Many people felt this was not a sensible way forward. Alan Stern remarked, "If we were to take a vote and rename Brian (Marsden) an amateur astronomer it wouldn't change the things he has done in his career, but it would be perjorative. It would taint things." Marsden says he never meant to be pejorative. He thinks that Pluto is a most interesting object, but that it's just not a planet. None the less, votes were taken. One conducted by the Minor Planet Center came out strongly in favour of numbering Pluto, although an informal poll taken at a meeting of asteroid astronomers in Germany during 1998 came out with the opposite result. (The result from the German poll was 20 to 14 with a lot of abstentions.) According to respected cometary scientist Mike A'Hearn the debates have been remarkably emotional and the most interesting conclusion of the whole affair is the demonstration that astronomers are less rational than he thought they were.
Part, indeed most of the problem is that there is no formal definition of a planet. Furthermore it is very difficult to invent one which would allow the solar system to contain nine planets. Alan Stern feels that moving away from the rhetoric and actually defining what makes something a planet would help to crystalise our thinking. He suggests that for an object to be classified as a planet requires it to have three characteristics. It must be in orbit around a star (thus removing the larger satellites from contention), it must be too small to generate heat by nuclear fusion (so dwarf stars are excluded) and it must be large enough to have collapsed to a more or less spherical shape (which excludes comets, and most of the asteroids). These criteria would admit a few of the larger asteroids and probably some of the Kuiper belt objects as well, but adding a requirement for a planet to have a minimum diameter of 1,000km would remove the larger asteroids from contention while retaining Pluto. However setting a diameter of 1000km is very arbitrary (why not use 1,000 miles?) and it has no physical meaning in terms of how the objects formed or evolved. After all, if Pluto's companion Charon was just a bit larger, would it be called a satellite, or fully confirmed as half of a binary planet?
Astronomers catalogue things to try and rationalise the Universe and to understand how it works. Objects which fall on some borderline, like the comet-cum-asteroid Chiron, can be studied by people with different perspectives and often provide crucial tests for cherished theories. From the point of view of the evolution of the solar system, it is sensible to consider Pluto along with the other Kuiper Belt Objects, but from the point of view of how large solid bodies behave internally, Pluto is best considered a small planet. What the whole debate may be telling us is that there are at least three types of planets; rocky terrestrial planets like the Earth and Mars, giant planets like Jupiter and Neptune and a recently recognised class of ice dwarfs which encompasses Pluto, Charon, some of the large icy satellites and the large trans-Neptunian objects.
The arguments eventually subsided due to a mixture of scientific realism, tradition and perhaps an eye towards astronomy's public relations. From a scientific point of view it doesn't matter what we call Pluto. Reclassifying it won't actually help us understand its composition, origin or future. The people whose discovery started the whole issue don't seem to care. Jane Luu says, "Sure, we can call it a planet, it's no skin off my back," as if to dismiss the issue as trivial compared with actually studying the object itself. Dave Jewitt is firmly in the camp of those who regard Pluto as a large Kuiper belt Object, but says he is happy if people want to call it a planet. "People are confused about lots of things," he says. Julio Fernandez, who predicted in the 1970s that a population of objects must exist beyond Neptune is also in favour of the status quo. "Astronomy is full of things with names that later proved to be incorrect or just plain silly," he points out. "The lunar Maria, or seas, were named when astronomers thought they were just that, large bodies of open water."
The witching hour came and went when minor planet number 10,000 was finally reached. It was assigned without fanfare to a small main belt asteroid which was named Myriostos, which means 10,000th in Greek. Of course astronomers may live to regret this decision if an object bigger than Pluto is ever found. Certainly there is no reason why a few large Kuiper belt objects might not exist in the dark outer reaches of the solar system. The gravitational forces which eject objects into the Kuiper Belt during the planet building process do not care how big they are, as long as they are small compared with one of the giant planets. So a Pluto sized, or even Earth sized object may well be lurking out in deep space waiting to be discovered. If one ever is, then Brian Marsden may have the last laugh.
Dr John Davies has been studying Kuiper Belt objects since soon after the first one was discovered. Asteroid 9064 is named Johndavies in recognition of his work in studying comets and asteroids. Beyond Pluto is his 5th book.
This material is modified from Chapter 12 of "Beyond Pluto" by John Davies, published by Cambridge University Press.Beyond Pluto is about the discovery of the Kuiper Belt. The extract appears in FTL with permission but should not be reproduced further without the permission of the author.