Thoughts on the Status of Pluto...
After reading Alison Tuffnell’s interesting article about the on-going debate surrounding Pluto’s status as a planet, which has been reignited of late with the recent discovery of a fifth moon orbiting the distant object, I was inspired to begin thinking about the issue myself.
It has been suggested that moons ought to play a part in the classification of planets, which currently must orbit the sun, have enough mass to maintain a spherical shape, and have cleared their orbit of any other objects in order to be considered fully-fledged planets. Pluto fits the first two criteria, but not the third. After doing a little research of my own, I have come to the conclusion that regardless of its impressive number of moons, Pluto should not be reinstated as a planet, but should remain within its current classification. Furthermore, I am of the opinion that adding moons as a criterion would only serve to complicate what is already a highly complex, subjective and hotly debated aspect of our understanding of our solar system.
Interestingly enough, it turns out that Pluto is not the first body in the solar system to have been labelled a planet when it was discovered, and then to subsequently have lost its planetary status. In 1801, based on the suspicion that there must be planets in the large orbital gap between Mars and Jupiter, Giuseppe Piazzi discovered a planet that he named Ceres. Following on from this, bodies later named Pallas, Juno and Vesta were discovered in 1802, 1804 and 1807, all of which were considered at the time to be planets, along with others. These have since been reclassified. As more were discovered, and as advancing optical technology enabled us to better estimate their sizes, it became apparent that referring to each relatively small body discovered in the region of space between Mars and Jupiter as planets was impractical. We now refer to them as asteroids, though Ceres has since been reclassified a second time as a dwarf planet, as it fulfils two of the three planetary criteria.
Ceres, Pallas, Juno and Vesta were considered to be planets until it became clear that they were in fact part of a much larger group of objects all orbiting the sun in approximately the same region of space. The same can be said of Pluto. When first discovered in 1930, Pluto was the furthest known body to orbit the sun, and, like Ceres, the only known body to orbit at roughly that distance. It took somewhat longer for other objects to be discovered, but nevertheless, it was hypothesised by Gerard Kuiper, as early as the 1950s, that there there should be a belt of icy objects beyond the orbit of Neptune. Pluto, we now know, was the first object to be discovered in what has since become known as the Kuiper Belt.
Since 1992, many objects like Pluto have been discovered within the Kuiper Belt, indeed, four of the recognised dwarf planets, Pluto, Eris, Haumea and Makemake all orbit within it or the so-called “Scattered Disk”, a sparsely populated ring of material with much more eccentric orbits than the bulk of Kuiper Belt objects. It seems then, that history is repeating itself: to refer to even only the largest bodies in the Kuiper Belt (many more of which are waiting to be discovered) as planets would be impractical. Potentially we could end up with dozens, if not hundreds, of recognised planets, most of them incredibly tiny. Those referring to every object in the asteroid belt as a planet, with a proper name, astronomical symbol and overlapping places on orbital diagrams faced the same mounting confusion.
So, this takes care of the historical precedent for Pluto’s demotion, as well as a very obvious practical reason. Moreover, it would be hypocritical to reinstate Pluto as a planet and yet leave other, very similar-sized objects in very similar orbits to remain dwarf planets or Kuiper Belt Objects. If Pluto is to be re-classified, something needs to be found to set it apart from the other officially recognised dwarf planets discovered, and other objects which are estimated large enough to be worthy of the label but are yet to be ratified. The presence of moons might be a way to do this; unfortunately, however, many other dwarf planets also have moons of their own.
Of the five officially recognised dwarf planets, three have discovered moons. Pluto has five, Haumea has two and Eris has one. Of the “unofficial” dwarf planets, Orcus and Quaoar each have one known moon and it is hypothesised that Sedna has at least one moon, but it has so far not been observed. This is an impressive total of at least ten, possibly eleven moons, and is consequently very strong evidence that other, as yet undiscovered, dwarf planets will have moons as well. This problematizes the process of including moons as a criterion to judge planetary status because if Pluto is made a planet again, it follows that Eris, Haumea, Orcus and Quaoar will have to be re-classified as planets as well, along with any other newly discovered dwarf planets with satellites. Essentially we’d be back where we started in 2006.
Dwarf planets are also not the only sub-planet-sized objects in the solar system that have been observed to have moons. In a 1993 fly-past of the asteroid Ida, the Galileo space probe discovered that Ida in fact also had a tiny moon, named Dactyl, which was less than two kilometres across. Since then, the NASA website claims that we now know of “more than one hundred and fifty” asteroids with at least one moon, and some have two. Furthermore there are binary asteroid (bearing a resemblance to Pluto and its largest moon, Charon) and even triple asteroid systems, in which the objects orbit around a common point. What this goes to show, is that although statistically it is still rare (given that there are estimated to be trillions of objects orbiting the sun), moons are not as rare a thing as we might think. Certainly as a method to distinguish planets from everything else in the solar system, as well as a “tie-breaker” in cases such as Pluto’s, it is flawed, as a body demonstrably need not be a planet to have moons.
It is perhaps only for the sake of sentimentality that Pluto might be re-instated as a planet. After all, for much of my childhood I learned to refer to it as “the ninth planet in the solar system,” and of course, there are countless others who have grown up and lived with the conception of Pluto as a planet for much longer than I.
Ultimately, the views of science cannot be swayed by sentimentality, and astronomy is no exception. Instead, science changes its views based on the latest and most reliable available evidence, and the fact is that the evidence now points towards Pluto bearing a much closer resemblance to the objects of the Kuiper Belt than to the other eight planets of the solar system. It makes sense therefore, to classify it with the objects to which it is more similar. I am no expert, however. I would like to conclude by saying that many transitions take time and, as the term asteroid and planet were used interchangeably for much of the latter 19th century, I suspect that Pluto will be known informally as a planet for years to come. But it’s not.
References/suggested further reading:
- http://www.theyorker.co.uk/lifestyle/scienceand%20technology/12013 - Alison Tuffnell’s article, here on The Yorker.
- http://solarsystem.nasa.gov/planets/index.cfm - An incredibly useful site, with pages on all the main objects in the solar system, well-written without too much astronomical technobabble, and the main source for this article, particularly the pages on the asteroid belt, dwarf planets and the Kuiper Belt and Oort Cloud.
- http://www.space.com/12692-dwarf-planets-solar-system-tour.html - With interesting profiles of the five dwarf planets, on this specific page, the main website is also a fantastic resource on many topics related to space.
- http://www.sciencedaily.com/releases/2012/05/120510224957.htm - A more in-depth look at Vesta, one of the largest objects in the asteroid belt.
- Finally, although you should be wary of it, Wikipedia is a good starting point if you want to find out more about the bodies in the solar system, with a wealth of information, and being a comparatively obscure topic, vandalism is less common.