Pluto might be a planet again. Or not.

Remember Pluto the planet? And then Pluto the not a planet? Well, it’s back. Possibly. Apparently an informal forum held the other week came down in favour of reinstating the ‘planet’ classification. Of course these things carry little weight with the International Astronomical Union.

Pluto in true colour, as seen by New Horizons. NASA, public domain, via Wikipedia.

What interests me is the way that the debate over whether Pluto is, or is not a planet also sums up the biggest flaw in modern human analytical philosophy; our need to categorise everything and fit it into patterns and slots as a part of being ‘scientific’.

In a way this is not surprising. We appear to be hard-wired to see patterns everywhere. Sometimes they even exist. The ‘evolutionary psychological’ explanation is that it conferred an advantage during our very, very long hunter-gatherer period. Humans who were better at identifying patterns were more likely to see either the next meal – or a sabre-toothed tiger – in the long grass. Today the same style of thought seems to drive everything from our need to classify the scientific world, to conspiracy theories – all of which are, of course, patterns assigned where none actually exist.

This thinking has basically been codified in the western tradition of analysis, founded in the approaches of the ancient world but given spice by the ‘age of reason’ as it evolved through the early modern period. This essentially systematises the need to assign patterns to nature and, with it, classifications. It is so much a part of western thinking, even at popular level, that we don’t usually realise it is an artificial construct. Everything, popularly, ‘must’ fit somewhere. Except, of course, when it doesn’t. The problem is that nature doesn’t co-operate with our neatly assigned thinking. Nature sometimes offers shades of grey, or a continuum that does not easily yield to the imposition of a human-made conceptual structure. That’s particularly true of matters astronomical, as we’re finding out.

Back when the current system of astronomical classifications evolved it all seemed simple; there were stars, planets of various sizes, and arrays of moons. Comets, asteroids and meteorites whirled around as part of the mix. And own solar system was pretty simple; a lot of empty space in which orbited eight planets, their moons, some asteroids and comets.

That picture wasn’t disturbed when Pluto was discovered in 1930. But as telescopes improved and space probes began looking more closely into the planets, the old system started to look a bit ragged. It broke altogether with the discovery of multiple bodies, all roughly Pluto-sized, in the outer solar system. Did we have nine planets? Ten? Dozens?

Comparison between stars and brown dwarfs. Not strictly to scale. Public domain, NASA/JPL/Caltech.

Hence the early 2000s push to re-classify Pluto and come up with a new category to capture the large Kuiper Belt objects. So emerged the somewhat annoying name ‘dwarf planets’. The issue was that this might have been fine for Eris, or Makemake, or some of the others; but it was an ill-fit against tradition that had always labelled Pluto a planet.

Curiously, during the same period, there was a similar stir-up in another side of astronomical classification; the nature of ‘brown dwarfs’. These are super-Jupiters which are almost, but not quite, stars. They were found in mass-ranges from a few times the mass of Jupiter – and still, we find, with cloud systems – through to nearly 80 times Jupiter’s mass. And it turned out that their mass also defined many of their characteristics. Below 13 Jupiter masses, an object can only be heated by gravitational contraction and, by IAU definition, is a planet. Above it, deuterium fusion is possible, creating a further source of heat, making it a ‘brown dwarf’. Brown dwarfs are further classified by mass and luminosity as M, L, T, and Y on the Hertzsprung-Russell diagram, which classes stellar luminosity largely according to the English alphabet. It’s possible for a high-mass brown dwarf to approach the characteristics of a low-mass red-dwarf star; the physical distinction is that a brown dwarf can fuse lithium 7, but not hydrogen. Above 80 Jupiter masses, hydrogen fusion is possible (via the proton-proton mechanism), and such objects are classified as true stars, low-mass red dwarfs.

The physics of these distinctions is undeniable; however the question is whether this makes a practical difference; Jupiter – like many low-mass brown dwarfs – also generates internal heat via gravitational contraction. Because of the physics involved (electron degeneracy pressure), even medium-to-high mass brown dwarfs are not much larger, physically, than Jupiter – though obviously denser. Despite a different heat generation mechanism, high-mass brown dwarfs are much the same luminosity as low-mass red dwarf stars; they even have the same classification, M. The problem, of course, is the old continuum-and-shades-of-grey vs hard ‘scientific’ divisions flowing from the western ‘age of reason’ tradition. Astronomical bodies seem to come in all sizes, and have properties dependent on their mass, which thanks to those same laws do provoke different physical outcomes at specific threshold levels – but which, otherwise, seem not to be too different in other ways. All, of course, is to do with the way we see things.

You see, I hope, what I am getting at. We imagine we can classify everything into patterns we understand; and it’s all too easy to suppose that the universe is what is at fault when our ideas don’t fit what we observe. The history of science is littered with such wreckage; the most obvious is the interplanetary ‘ether’. The 1887 discovery by Albert Michelson and Edward Morley that it was an illusion – helping fuel a growing revolution in physics that shortly inspired Albert Einstein to action – underscores the point that we often hit the limit of the conceptual model, and it breaks down.

I mention all this as examples. There are many others. The problem isn’t the universe – it’s the way humanity sees it, and our insistence on classifying everything in those terms.


Copyright (c) Matthew Wright 2019

9 thoughts on “Pluto might be a planet again. Or not.

  1. Pluto will always be a planet in my mind. I grew up with Pluto as a planet, and when it was demoted to “dwarf planet” I’ll admit I felt indignant, like someone was messing with my childhood. But the universe is what it is, and Pluto is still zipping around and breaking the rules way out at the edges of the Solar System, no matter what how we humans decide to categorize it. 🙂

    Liked by 1 person

    1. I was brought up with Pluto as a planet too. What worried me about the IAU was that they attempted to create a ‘scientific’ definition of ‘planet’ in circumstance where every variation of ‘things orbiting a star’ wasn’t known, and where in any event, they were unlikely to neatly fall into categories to suit human thinking. As you say, Pluto’s still going to be Pluto no matter what it’s called scientifically.

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  2. “…the biggest flaw in modern human analytical philosophy; our need to categorise everything and fit it into patterns and slots as a part of being ‘scientific’.” Labels, labels everywhere and as you point out, where there’s a label, can a pattern be far behind? Isn’t a name enough? As always, wonderful post, Matthew.

    Liked by 1 person

    1. Thanks, Karen! It remains one of my irritations that in the quest to label everything, humanity so often misses the wood for the trees. It is easy to blame the way the Age of Reason unfolded, 2 or 300 years ago, for conditioning everybody into such a pattern; but I think that was more an expression of the issue, not the creator of it.

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  3. Dig this much, Matthew. Thank you for your thoughtful work. You touch on many deep themes. Very instructive and a pleasure to read. I appreciate your ability to write about technical matters in a v. reader friendly way. A rare skill!

    Liked by 1 person

  4. Pluto disappeared from our local Jodrell Bank Radio Telescope visitor centre when it was decategorized from being a planet, which seemed a bit unnecessary. Also, as one interested in astrology, Pluto is a planet of great significance here. So I was very interested to see this great post. Thanks, Matthew.

    You make the point that we always try to categorise things and define patterns, but things never quite match our categorisations and patterns. This is very consistent with Kurt Godel’s theorem that any mathematical model of things is necessarily incomplete – there is no magic model of everything, or pattern that everything fits. Maybe that’s what makes life so interesting!

    Incidentally, I had the impulse to reblog this. Are you happy with your stuff being reblogged with comment?


    1. Hi Barry – yes, happy for a reblog, and I very much appreciate your asking first! On matters Godel, I always have to wonder whether his concept of innate incompleteness of knowledge was part of a twentieth century reaction to the earlier notion that, even if all had not yet been discovered, the universe would somehow soon yield to human intellect. (I often think about things in this direction – I began my studies with the sciences and music, turned to history, then studied philosophy post-grad under a student of both Popper and Wittgenstein, not that I necessarily now follow either, or my teacher, Peter Munz. Conceptual vision and understanding is something to be extended and explored, once the tools are offered, and of course the axiomatic suppositions that buoyed the frameworks of past analysis demand questions on the back of it.)


  5. Reblogged this on I can't believe it! and commented:
    Pluto disappeared from our local Jodrell Bank Radio Telescope visitor centre some years ago, when it was decategorized from being a planet. It no longer seemed to exist, which seemed a bit unnecessary. Also, as one interested in astrology, Pluto is a planet of great significance here, and has certainly not disappeared from use.

    So I was very interested to see this great post by Matthew Wright. He’s also questioning our tendency to classify everything into categories that may then obscure things of real significance, such as the astrological significance of Pluto!

    Liked by 1 person

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