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note: This question is asking for a representation of the astronomical classification scheme now used for solar system bodies as established (presumably) by the Working Group for Planetary System Nomenclature (WGPSN) of the International Astronomical Union or IAU. While graphical layout is mentioned as part of the question's motivation, the question is not about layout or graphics.

The Euler diagram shown in this helpful answer is hard for me to understand, I think mostly due to graphical layout. The diagram shows the mathematical relationship between various sets (in this case solar system body classifications) so it could be drawn with widely differing choices of layout and still retain exactly the same mathematical relationships.

I am wondering if there is some attempt here to indicate relative numbers or ratios by the degree of overlap? Look closely at the small overlap of Minor Planets with Comets. Is that meant to reflect in some way a relatively small overlap quantitatively? Did you notice it took a while to even make sure you were seeing the overlap correctly? This style of layout requires the use of color perception to help identify which label is associated with which contour while simultaneously trying to follow one line in a groups of perfectly parallel lines.

Have other versions of this Euler diagram representation of solar system body classification been published? A link to one or even inclusion of the diagram in the answer itself would be greatly appreciated!

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above: Example of an Euler diagram in this Wikipedia article, from here.

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above: Example of a somewhat easier to use and understand Euler diagram, from here, used in the same Wikipedia article.

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  • $\begingroup$ You could very easily make one yourself that fits your aesthetic desires. $\endgroup$ – zephyr Feb 8 '17 at 16:13
  • $\begingroup$ @zephyr That's an interesting idea! I'm looking for something "official" or even published and cite-able. I was hoping there might even be something available from the IAU. Actually I don't even know where exactly the expressions of what-can-or-can't-be-what-else (for lack of better words) are officially expressed. Is the solar system body Euler diagram based on absolute rules expressed clearly and explicitly somewhere, or does it include some interpretation? Honestly after a half-century of "knowing" the difference between asteroids and comments, suddenly I'm a little disoriented here. $\endgroup$ – uhoh Feb 8 '17 at 17:00
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    $\begingroup$ As I noted in a comment to the linked answer, that Euler diagram is incorrect. It implies that all minor planets and some centaurs are dwarf planets, and that small solar system bodies exclude minor planets. I guess I should make this part of an answer. $\endgroup$ – David Hammen Feb 9 '17 at 15:36
  • $\begingroup$ @zephyr I've been thinking about this more and more. I wonder if a better diagram could also be a candidate for a community wiki. A good diagram has to not only show the relationships of the sets, but might or might not also include the relative sizes of the sets (or log thereof) and one version might even include small images of examples if they exist. I don't know if the community wiki could produce the actual diagram, but it could collect the relevant data, relationships, and examples. The discussion at the other question has left my head spinning a bit, but that's a good thing. $\endgroup$ – uhoh Feb 10 '17 at 1:54
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Per the IAU (Resolution B5, adopted by the 2006 IAU General Assembly), there are five supposedly mutually exclusive categories of things in the solar system:

  • The Sun (only one), which is distinguished by the ability to fuse hydrogen into helium in its core.
  • The planets (eight, might possibly become nine if Michael Brown is correct), which are distinguished by having the ability to "clear their orbit" (a well-defined concept with numerical meaning) before the Sun turns into a red giant.
  • The dwarf planets (unknown number, most likely well over 100), which are distinguished as being bodies that don't qualify as the sun or as planets, but are sufficiently massive to more or less have pulled themselves into a roundish shape.
  • Small solar system bodies (unknown number, well into the millions) which are distinguished by not being distinguishable as any of the other categories.
  • The satellites of planets, dwarf planets, and small solar system bodies (unknown number, the number of which change over time as satellites are captured or rejected) which are distinguished by being in orbit about a planet / dwarf planet / SSSB rather than the Sun.

Contrary to that resolution, these boundaries are fuzzy rather than sharp, particularly when one looks outside of our solar system. (Note well: The IAU definitions pertain to our solar system only. There are currently eight planets in the entire universe, with the possibility of growing to nine if the conjectures regarding a largish planet beyond Pluto turn out to be correct.)

Consider the first boundary, that which separates planets from stars. This becomes fuzzy with the smallest red dwarf and the largest super-Jupiter. There's no clear-cut boundary here. In fact, there's a good amount of debate over this very issue. This boundary is quite sharp in our solar system, but it almost certainly is anything but sharp in star systems beyond our own.

The second boundary, the one between planets and dwarf planets (which, despite the name, are not planets) also has the potential for being fuzzy. In our solar system, that boundary is once again rather sharp. Any time you can come up with a boundary that is separated by five orders of magnitude you have a nice sharp classification boundary. However, given the hugeness of the universe, there is almost certainly some star system out there, somewhere, where this boundary is as fuzzy as can be.

The third boundary, the one between dwarf planets and small solar system bodies, is fuzzy in our own solar system. Objects less than 100 meters in diameter are, as a rule, anything but "round". Objects over a kilometer in diameter are, as a rule, quite "round" (when the shape is resolvable). Astronomers have found lots and lots of objects between 200 and 600 meters in diameter. Squint hard enough at the smaller ones and you might be able to convince yourself that they are "roundish". Focus intensely on the larger ones and you might be able to convince yourself that they are not quite round. There's no sharp boundary here, even in our own solar system.

The fifth category, satellites vs non-satellites, opens yet another can of worms. Planets (particularly the giant planets) can temporarily capture objects as satellites, only to lose them as satellites after a short time (tens or even hundreds of thousands of years is a "short time" in this context).


Note well: I'm not saying Pluto should once again be promoted to the status of "planethood". That very sharp boundary (at least in our solar system) says that it most definitely is not a "planet".

But what about asteroids versus comets, which is what started this series of questions? That boundary is ridiculously fuzzy. Fuzzy logic (a real and serious concept) doesn't come close to addressing how "fuzzy" this is. Centaurs and formerly active comets are the boundary cases. There is no boundary. There instead is a soft melding from one to another.

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  • $\begingroup$ There is no spoon ... backspace... strikethrough... boundary. Thanks - this is an answer I will enjoy reading after tomorrow morning's coffee, but I'm beginning to see already that comets and asteroids are not like cats and dogs. Instead small solar system bodies are dogs, and they come in an amazingly diverse variety. If an alien landed on Earth and started classifying animals, wow would they be surprised to find out all dogs (the pet kind) are one species. $\endgroup$ – uhoh Feb 9 '17 at 16:38

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