I was going to blog about the larger asteroids at this point, but in recent days it’s been borne in upon me that there’s a current issue in astronomy, perhaps over-emphasised but definitely there, over whether Pluto was unfairly demoted. The reason I mention this now is Steve’s comment about what the difference between Phobos and Deimos and asteroids might be. It’s a very good question and I’ll address this first.
Phobos and Deimos, the moons of Mars, are a little puzzling. There are two hypotheses about where they come from. One is that they’re main-belt asteroids which were captured by Mars. At first glance this sounds very sensible and logical. After all, Mars is next to the asteroid belt, it could be expected to gather up a few stones from it from time to time and the pair seem to be only the latest representatives of a whole series which have scarred Mars with chains of craters as they broke up and impacted. However, there are problems with it. Firstly, the common type of asteroid found near the edge of the belt closest to Mars is different from the type of asteroid Phobos and Deimos would be if they are asteroids. That type is found near Jupiter. This is due to the inner belt being warmer than the outer belt, so the composition differs because temperature makes a difference to them. Secondly, both moons have almost perfectly circular orbits over the Martian equator, and if they were captured, they would usually have come in at a high angle to the equator and have markèdly elliptical orbits. This can be seen with Nereid, Neptune’s third largest moon, and Saturn’s moon Phoebe orbits backwards compared to most other bodies in the system. Therefore, if Mars’s moons are asteroidal in origin, something needs to be evoked to explain that. A simpler explanation would be that they emerged from the cloud which was forming Mars. This would be spinning in the same plane as any moons which formed from it, and if they were formed in situ they would be more likely to have almost circular orbits. However, as Steve astutely pointed out, the actual nature of the bodies themselves is very close to being asteroidal, and in fact is asteroidal, so maybe it doesn’t matter in most ways. In the sense of the physical nature of the two moons, they basically are asteroids. The way in which they aren’t is to do with their history and orbits, which may not be a sensible thing to focus on. The only thing which goes against this is that both are directly affected by orbiting Mars. Phobos has streaks because of the tidal forces of its planet, and Deimos accumulates fragments and dust from itself as it moves through its rather short orbit. If they were orbiting in the asteroid belt itself, neither of these things would be happening. All that said, I can totally see the argument that they are in fact just asteroids in an unusual place which are also moons rather than minor planets. So I agree with you Steve.
This connects to a wider issue which affects Pluto, and it also affects a number of other worlds in the system which if addressed could solve the problem of knowing what to call the big round things in our Solar System. It could also address the peculiarity of our own “moon”. The 2006 CE definition of a planet by the International Astronomical Union is:
The IAU members gathered at the 2006 General Assembly agreed that a “planet” is defined as a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
This definition was motivated by the discovery of a number of relatively large trans-Neptunian objects. Eris, discovered at the start of the previous year, has now been established to have a diameter of 2326 kilometres and a mass of 1.6466 x 1022 kilogrammes. Sedna, discovered in 2003, has a diameter of around a thousand kilometres and an unknown mass because unlike Eris it seems to have no moons. Sedna is less of a threat to the status quo but Eris was initially thought to be larger than it has now turned out to be. For comparison, Pluto is 2376.6 kilometres and it has a mass of 1.303 x 1022 kilogrammes, so it’s actually slightly larger than Eris but also less massive, so the question arose of whether it would be acceptable to admit a potential host of newly discovered planets, thereby reducing the “specialness” of planets, or to invent a new category. This last idea, of “dwarf planets”, seems very odd to me because the category of “minor planet” had existed for a very long time up until that point and instead of inventing an entirely new class of object, it would’ve made more sense, if they were going to do this. Whether or not I agree with the decision, there seems to be no merit in creating a whole new category of “planet” when “minor planet” already existed. I honestly don’t know why they did this.
Many people have disagreed with the decision to demote Pluto. It did elevate Ceres, previously considered a mere asteroid, at the same time. Before that point, for most of its history since discovery Ceres was considered an asteroid, but it’s the only body in the asteroid belt which has managed to make itself round due to its own gravity (there might be other bodies which just happen to be round-ish through chance because asteroids are irregular and could hypothetically be many shapes, including spheroidal), so it probably does deserve special recognition.
In spite of this definition, which is quite unpopular, a paper has recently been published on the subject arguing that Pluto, among other worlds, does in fact merit planethood. The paper can be found here. It’s sixty-eight pages long and I haven’t read the whole thing but the general gist of it seems to be that there used to be a scientifically arrived-at understanding of what a planet was, but over a period in the early twentieth century when astronomers focussed more on what was happening outside the Solar System, the popular uneducated public understanding of what a planet was took over. I have to say this doesn’t reflect my perception of what happened based on my knowledge of astronomy. I’m aware of the controversy about the canals, the discovery of Pluto, the idea that Mercury always faced the Sun and so on, all ideas which resulted from astronomical research at around that time. I’m aware of the research that was being done at the time about stellar evolution and the realisation that there were other galaxies, but it really doesn’t seem like they were concentrating that much on that more than this Solar System, but anyway, that’s what this paper claims.
Further, it claims that because they adopted a kind of folk understanding of what a planet was, it had led to them adopting earlier, non-scientific ideas about it. So for example, the public was really into astrology and had only recently got used to the idea that the Sun was at the centre of the Solar System rather than Earth. The authors of the paper give examples of how scientific classifications differ from public ones. For instance, most people think of fruit and vegetables as two different things but when it comes to botany, vegetables include fruits, which are the reproductive organs of plants, so from a culinary viewpoint fruit and veg are separate but scientifically they aren’t. To this I would add a couple of things which are I hope relevant to astronomy. One is that I think of a lot of things as fruit, such as tomatoes, aubergines, courgettes, peppers and tomatoes, which other people seem to think of as vegetables because it makes sense to me to think of them nutritionally and in terms of flavour in that way. The other is that the culinary arts are also sciences, and it seems a bit hierarchical to see them as inferior to botany for some reason. After all, we all need to eat. Applying that to astronomy and planets, that would mean that although some things are planets and some things aren’t according to astronomers of a particular vintage, that doesn’t mean there isn’t another branch of science which would view them differently. For instance, everything is subject to the laws of physics, and geology would seem to apply pretty much equally to planets, moons and asteroids in their own way. They’re just bodies in space like everything else. Therefore, I’m not convinced about this. Also, the general public were specifically irritated at the idea of Pluto not being a planet any more, so I don’t see how exactly they were using the public view of what planets were if they managed to annoy so many non-astronomers with their assertion that Pluto wasn’t one.
What seems to have happened is that the problem crept up on astronomers and they kind of panicked and made a fairly slapdash and hasty decision. As various large bodies were discovered on the edge of the Solar System, they became uncomfortable with the idea that they were probably going to end up with a very long list of planets, which seemed unwieldy and not very “neat”, and they also perceived it as an imposition on education that people were going to have to learn about so many worlds. They seemed to feel like this would be regarded as off-putting. The paper compares the situation with how mammals are defined. The official definition of a mammal is now rather abstruse, because it actually hinges on how many bones are in the jaws and the ears, but this is partly because of the need to identify fossil mammals. The widely-used definition is “animals who suckle from their mothers as infants, maintain a different body temperature from their environment, are often covered in fur or hair and mostly give birth to live young”, and the first criterion is the most important. There are exceptions to most of these. For instance, some hibernating mammals don’t keep their body temperatures above their surroundings and humans, whales and elephants are largely hairless, but this is a fairly good definition. However, claim the authors, astronomers have taken a weird approach to planets, having concentrated on whether they dominate their local region, which is in any case vague because what’s local? They’ve also looked at how they move. If mammals had been defined in this way, as warm-blooded vertebrates who walk in herds on land for example, a lot of mammals would’ve been excluded. Bats and whales would then not be mammals and any mammal who has a largely solitary life, such as leopards or sloths, would not then count as mammals either.
Looking at the history of the idea of planets, for a long time any round object in the sky which didn’t appear to stay in the same place was a planet. This used to include Cynthia and the Sun, when people thought Earth was at the centre of the Universe, and it didn’t include Earth. Later on, the four largest moons of Jupiter were discovered and also referred to as planets, and even the thick parts of the rings on either side of Saturn due to the poor quality of telescopes at the time. Later still, Ceres was called a planet because it seemed to fit into Bode’s Law, and turned up where it was expected. By that time, however, the known satellites had been relegated to moons, and soon after Ceres was also demoted because it was realised that there were thousands of other bodies between Mars and Jupiter, some even quite large.
The 2006 definition also has a rather silly consequence which a few people have noticed: it means Earth isn’t a planet! As I’ve mentioned before, from the Sun’s perspective Cynthia doesn’t orbit Earth, but the two weave in and out of each other’s orbits. I’m not completely clear what the astrological influence is supposed to be, but I think it’s the emphasis on orbits, i.e. the kind of definition which would’ve excluded bats, whales and leopards from being mammals. Whatever the definition of a mammal is, it seems to make more sense to use their anatomy and physiology than other more dubious criteria. Both of the definitions I mentioned above do this. The first is rather abstract and strange to most people, although there are good reasons for it – mammal jaws and teeth survive better than the rest of their bodies so it’s like identifying a body by dental records – but both of them focus on what their bodies are like, which seems entirely sensible compared to that fictional other definition.
What, then, is proposed as a more sensible definition of a planet? Well, it’s closer in spirit to that way of defining a mammal. A planet is a geologically active body. I have to admit I’m not sure about this because of various things, such as “eggshell planets”, and I’d also want planets to be round and I can’t tell if they also stipulated that. What it means (I’ll get back to eggshell planets in a moment) is that Pluto’s Sputnik Planitia which is created by frozen nitrogen and is active even though the Sun isn’t strong enough at that distance to have that effect. In talking about asteroids, I’ve mentioned the fact that the larger ones tend to be layered like Earth is, but the smaller ones are either rubble piles or mixtures of different minerals and other substances which aren’t separated out in the same way. A geological process has done this sorting in the larger ones, and consequently Ceres, for example, could count as a planet: it has been geologically active.
This applies also to some moons. Io, the innermost large moon of Jupiter, is intensely active with continual volcanic eruptions, to the extent that it’s thought to “turn itself inside out” every few years – some much of its interior is spewed onto the surface that the former surface becomes the interior and proceeds to get thrown out itself a few years later. This is because of the tidal forces effectively “wringing” the moon all the time, with the other large moons in the Jovian system along with Jupiter itself wreaking havoc on the place. By this standard, Io is definitely a planet, albeit a planet which is also a moon.
I’ll now permit myself a digression into eggshell planets. An eggshell planet is a surprising kind of “planet” which kind of “does nothing”. It isn’t necessarily possible to tell from a distance which planets are like this. Earth’s crust is divided into plates, and other planets have a thick, solid layer all the way round, but there is another possibility or which at least three examples may have been found already. This is where the crust is thin and fragile, and so cannot have plates or thick layers, and also can’t even support mountains or hills, so the surface is solid and also smooth, and nothing happens there – no volcanic eruptions, continental drift or erosion, because there’s nothing to erode. The question arises of whether this even counts as a planet under this new definition, since it isn’t geologically active. However, there are no such planets in our Solar System as far as anyone knows, and they’re probably quite rare.
There are three categories of planets suggested in this new definition: terrestrial planets; giant planets; satellite and dwarf planets. The last category is the largest. It includes the large moons of Jupiter, Ceres, Titan, Pluto, Charon, Eris and Sedna, and in fact there are more than a gross of these. Far from the expected response, apparently people tend to be quite excited at the idea that there are so many planets around the Sun. The giant planets include Jupiter, Saturn, Uranus and Neptune, so no surprises there, although this clear-cut division may be an artifact of how our own Solar System is, with its complete absence of the very commonest type of planet, the mini-Neptune, intermediate between Earth and Neptune in size.
There are five planets in the terrestrial category rather than four, because once the criterion for dominating its orbit has been removed, Cynthia becomes eligible, which makes me very happy! Cynthia is not even in the same group as the satellite and dwarf planets, but a planet just like Mars and Mercury. This also means that the Apollo astronauts landed on another planet, not just our moon. As well as that, Earth now has no moon!
It seems that the process leading to the decision to redefine planets was not very scientifically grounded and was in fact rather acrimonious. The orbital dynamics people took umbrage at the geophysical definition and there were only a few days available for debate, forcing people to take sides quickly without due consideration. Planetary scientists were underrepresented because they’re apparently not officially astronomers, which is a bit astonishing. Another motivation was to keep the number of official planets low because the IAU didn’t expect the alternative to go down well with the public because previously, i.e. in Victorian times, they’d felt more comfortable with a small number of planets. They were used to seven at that point, including the Sun and Cynthia. This is probably no longer the case, so in 2006 they made a decision based on misjudging the mood of the general public.
To finish, I’m going to make a commitment. Henceforth I will be referring to every spheroidal body in the Solar System as a planet, although I will also acknowledge what kind of planet it is, such as a moon or dwarf planet. And Pluto is a planet!
I’d be delighted to hear your views on this.
A Box Of Chocolates 