There are more things in heaven than were dreamed of in the philosophy I was taught at school

There has been quite a bit of press coverage in the last couple of weeks about the discovery of an object in the outer solar system, which has been given the astronomical name 2003 UB313, but which has been popularly dubbed “Xena”. In some circles this has been described as a new planet, and in others its discovery has been given as comprehensive proof that Pluto is not a planet and that there are only eight planets in the solar system. Personally I have two opinions here. Firstly, I think it should be “Rupert” and not “Xena”. And of the two viewpoints given, I tend to agree with the second, which is that the new discovery reduces the number of planets to eight. Although thinking about it some more, I am not sure that either viewpoint is right. A better interpretation might be that it reduces the number of planets to four. Or perhaps to zero. It all depends on your point of view.

Why do I think this? In order to properly understand the question, an astronomical primer is in order. Many of our readers will already know this stuff, but this is all quite interesting and is nice to put it all down in one place.

Let me describe the solar system. For the moment, I am going to leave Pluto out, as it does not fit into what I am to initially describe. The solar system is generally considered to contain two types of planet. One is the inner planets (Mercury, Venus, the Earth, and Mars). These orbit the sun at distances between 50 million kilometres and 250 million kilometres, and have radii of between 2500km and 6500km. They have surfaces made of solid materials (ie rock) . The second type of planet is the “Gas Giants” (Jupiter, Saturn, Uranus, and Neptune). These orbit the sun at distances of between 0.75 billion and 4.5 billion kilometres, and have radii of between 25000 and 70000 kilometres. They basically consist of atmospheres that get denser and denser as the altitude gets lower and lower, and which gradually thicken until at some rather indeterminate point they go from being a gas to a liquid to a solid or to even more exotic things that defy simple classification. These planets are orbited by many small rocky moons, and planetary rings. Three of them (Jupiter, Saturn, and Neptune) are also orbited by larger moons that would count as planets in their own right if they orbited the sun and were part of the inner solar system.

The orbits of these planets (of both kinds, but with one exception that we will get to) have two notable facts about them. First, they all circle the sun in approximately the same plane, known as the “plane of the ecliptic” or just “the ecliptic”. As a consequence, if there are a number of planets visible in the sky at the same time, they tend to be in a fairly straight line. Secondly, the orbits of the planets are approximately circular.

But they are not exactly circular, and they are not exactly in the same plane. Mathematicians have ways of quantifying both these things. The first of these is relatively simple. Simply measure the angle between the ecliptic and the plane of the planet’s orbit, and quote this number as the orbital inclination. With the exception that I will get to in a moment, the planets discussed already have small orbital inclinations of up to about three and a half degrees. See here for detailed planetary statistics of various kinds, including inclination.

While Copernicus was the first modern scientist to recognise that the Earth and other planets went around the sun, his theory did not quite successfully explain the movements of the planets in the heavens. That took someone with better mathematics. Johannes Kepler managed to fix this problem in around 1620 by explaining that the orbits of the planets are not precisely circular, but are ellipses. Mathematically, the shape of these ellipses are described by a number called the eccentricity of each ellipse. This number has a a precise mathematical definition that falls out of certain solutions to Newton’s equations of gravity and motion fairly naturally, but it is enough to explain that it is zero for an exact circle and it gets greater as the ellipse gets longer and thinner. When one end of the ellipse is stretched all the way to infinity the eccentricity becomes equal to one and the orbit becomes a parabola rather than an ellipse. (At this point we are digressing towards something called the mathematics of conic sections, which I will merely provide a link to rather than describe in detail).

So what is the exception? That would be Mercury, which has an orbital inclination of 7 degrees and an eccentricity of about 0.2. This makes it highly unusual, but as it is broadly similar in composition as the other inner planets and there are no other objects like it to be classified into a third class of planets, it is generally considered one of the inner planets.

As it happens, there is a simplification in the above analysis. While the orbits of planets around the sun are almost ellipses, they are in fact not quite exact ellipses. If a planet was orbiting the sun and was not affected by the gravitational pull of any other object, the orbit would indeed be an ellipse. But as it happens the planets exert gravitational effects on one another, and these cause perturbations from exact elliptical orbits. (In a strict sense, the orbit of a given planet is influenced by the gravitational pull of every other object in the universe, but in practice it is the gravitational pull of the other planets that matters).

Prior to the eighteenth century, planets beyond Saturn were unknown to science. Calculations were made by various mathematicians as to the effects of the gravitational pulls of the planets of each other. (Without a computer this is very difficult to do accurately). However, it seemed that it was not possible to explain the orbit of Saturn. One potential explanation was that another unknown planet was influencing the orbit of Saturn. The approximate location of this planet was determined by further calculations. Astronomers started looking for it, and Uranus was discovered William Herschel in 1781.

Of course, mathematicians then did another series of calculations, and discovered that they couldn’t properly explain the orbit of the new planet Uranus. Once again the explanation was that there was another planet, and Neptune was discovered in 1846. By this point a pattern had been established. Calculations of the orbit of Neptune could not be properly explained and another planet further out was predicted. Astronomers looked, and in 1930 Clyde Tombaugh at the Lowell Observatory in Arizona discovered a ninth planet. The name it was given, Pluto, was coined by an eleven year old girl in Oxford named Venetia Burney.

Except that the calculations that the discovery of Pluto was based on turned out to be wrong. The mass of Neptune used in the relevant calculations were incorrect, and Neptune’s orbit could be adequately predicted without any need for an additional planet. It just happened that there was something where Tombaugh happened to be looking. That something was very peculiar by the standards of the other planets. Pluto has an eccentricity of 0.25 and an orbital inclination of 27 degrees. These are greater than any other planet, including Mercury. And Pluto has less than a tenth of the mass of any of the other planets, although it took a few decades for this to be determined.

Pluto was odd, yes. But this was not initially a huge issue. Mercury is odd, too. If there is an odd planet at each end of the solar system, so what. That just makes things interesting. But there was another possibility. Pluto was discovered because one small piece of the sky was being intensively examined. There were many other pieces of sky which were not being intensively examined. (Many pieces of sky a long way out of the ecliptic were and are seldom examined). What if there were other Pluto like objects, and rather than being lucky, Tombaugh had instead simply discovered one of many such objects. This does actually appear to be what happened, but we have only known this recently.

As well as the planets, the solar system contains many objects that orbit the sun that are too small to be classified as planets: these are known as asteroids and planetoids. The two terms are distinct, but overlap somewhat, and a third expression “minor planets” is sometimes used to describe them and other objects we will get to later. Most of the largest asteroids orbit between the inner and outer planets (ie between Mars and Jupiter).

But in recent years, a new class of minor planet has been predicted and discovered in the outer solar system

As knowledge of how the solar system formed has grown, then it has become steadily clearer that something called the Kuiper belt exists beyond Neptune. This consists of small rocky objects that have been ejected from the inner solar system by the gravity of Jupiter. These objects typically have high orbital inclination and eccentricity. Quite a lot of these objects have been discovered over the last fifteen years. It takes quite a lot of effort to find them because they are both quite small and distant and thus very feint, and also because their high orbital inclination means that they are further out of the ecliptic than are the planets. Interestingly enough, there appears to be an outer limit to the Kuiper belt, “the so called “Kuiper gap” or “Kuiper cliff”, beyond which there are no further such objects. It has been postulated that there is a larger object (perhaps the size of Mars or the Earth) orbiting at that distance which interferes with the orbits of Kuiper belt objects and does not allow them to remain in orbits beyond this point.

In any event, the mathematical analysis that predicts the Kuiper belt also allows for objects in it the size of Pluto, and larger, up to about the size of the earth. Thus it has been the opinion of many planetary scientists for a few years that Pluto was just another Kuiper belt object. While it would be okay to keep Pluto in the list of planets (despite being very dissimilar from all the rest) it would not be okay to do so if it was just one of a class of other objects. It would not be possible to include the rest of this class of objects in the list of planets, because many of them are smaller than other objects (eg some asteroids) that we do not count as planets. Other scientists and non-scientists had argued that as long as Pluto was larger than any other Kuiper belt object (and closer than the vast majority) it was okay to keep it on the planets list, largely for historical reasons.

But this position would go from eccentric to deeply misleading if Pluto was not the largest such object, and if there were other similar sized objects roughly the same distance from the earth. And over the last few years that position has got steadily shakier. More large Kuiper belt objects have been discovered, and Pluto’s status has become steadily less unique. In particular Quaoar was discovered in 2002, which was more than half the size of Pluto, although considerably further out. At the time there were a few calls to remove Pluto’s planetary status, which were listened to but generally ignored.

And on July 29 this year, the discovery of three new objects was announced. Two of these was a bit less than the size of Pluto, the other, 2003UB313, the aforementioned Xena, is clearly larger than Pluto. It is a little further out than Pluto, but in cosmic terms not dramatically so. Xena’s closest point to the sun is substantially further out than Pluto’s furthest point out, so in any meaningful way of looking at things they are neighbours. It is clearly an accident that Pluto and not Xena was discovered in the 1930s. (However, Xena has a very high orbital inclination – around 41 degrees). If Tombaugh had instead been looking in the direction of Xena, then he would have seen Xena. Xena can clearly be seen in many astronomical photographs going back decades now that people need to look at it. Incidentally, one of the other new objects announced last week, 2003 EL61 or “Santa”, has a moon, so attempting to use “Only planets have moons” as part of your definition of a planet doesn’t work. The truth is that we have reached the point where Pluto’s planetary status is not something that can be kept if we wish to keep any kind of consistency. It might be possible to give planetary status to Xena for now, but almost certainly another object of Xena’s size or larger will be found before long.

So, for now, we can keep some consistency by saying that Mercury is the smallest planet, and anything larger is a planet but that anything smaller is not. This is at least a consistent definition.

But even this presents us with problems, because there are clearly an awful lot of things that are orbiting the sun that we do not know about. What about that Mars or Earth sized object out at the far edge of the Kuiper belt? The chances are excellent that it is there, and I suspect that we shall actually see them before now. (Track the orbits of existing Kuiper belt objects, look at any large perturbations to their expected orbits, and narrow the positions down. Much more complicated forms of these calculations can be managed in these days of computers). Almost certainly such objects will be found that are larger than Mercury. At that point we either include these new objects as planets – something I think there would be a strong case for doing – or we change the definition of planets again, to something like, “objects that are orbiting the sun, that are the size of Mercury or larger, and that are no further from the sun than the orbit of Neptune. Or you could perhaps exclude Mercury, and work from “planets Mars size or larger, and….., and……

But it all perhaps gets down to a simple fact. Rather than just a few planets, there is in fact an immense amount of stuff orbiting the Sun. Any distinction between “planets” and “something else” is rather arbitrary. There are certainly distinct classes of object, including “Gas Giants”, “Inner planets”, “main belt asteroids”, “Kuiper Belt objects”. “comets” and more. Perhaps the very idea of a “planet”, which is a category that includes at least two of these distinct classes of object, is an idea that has outlived its usefulness. In any event, we have to add an awful lot of qualifications to the definition make it work. I suppose one could narrow down the definition further, and say that only the gas giants, “Jupiter, Saturn, Uranus, and Neptune”, are the only true planets, and that everything else merges into everything else with so little in the way of discontinuities that such a definition is the only one but makes sense.

But that leaves us with earth not being a planet, and as planets were originally considered to be “earth like objects”, then this is also a bit of a problem. It may be time to drop the idea of a planet entirely.