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edit: I just saw this tweet and find it incredibly relevant :)

enter image description here

enter image description here

begin question: See this answer and then consider if there are known or likely solar system bodies that might be identified as both asteroids and comets.

If so (there seems to be an example there) would this be because there is insufficient information known about the body, and the ambiguity could be cleared up with enough data (say a visit by a spacecraft)? Or are the categories of asteroids and comets actually overlapping?

If the latter, is there work underway to improve or update these definitions?

Camille Goudeseune
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uhoh
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  • Possible duplicate of What is the difference between asteroids, comets and meteors? – StephenG - Help Ukraine Feb 07 '17 at 12:03
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    @StephenG no, I have absolutely not asked "What is the difference between asteroids, comets and meteors?" I am asking if that difference is recently being called into question, and if the definitions may need to be changed in the future. Not possibly a duplicate at all, no. – uhoh Feb 07 '17 at 12:47
  • @uhoh: While I agree in this case that this question is not a duplicate of the one mentioned above, note that it depends on the answers whether two questions are duplicates, not on the questions themselves. So your argument (“I have asked something else”) is beside the point (which should be “the answers to the other question don’t contain the answer to my question”). – chirlu Feb 08 '17 at 03:55
  • @chirlu those answer could not possibly serve as an answer to the question "Do astronomers generally agree that the distinction between comets and astroids is not so clear?" And of course, in light of the accepted answer here, those answers are also no longer even correct as written - technically speaking. It seems the term asteroid itself has been demoted. – uhoh Feb 08 '17 at 03:59
  • @CamilleGoudeseune It's always the same; at first my reaction is "Oh no you don't! You've completely removed my sentiment and eliminated my personal literary style" and in about two seconds it changes to "Oh! That's MUCH better!" :-D – uhoh Dec 09 '20 at 04:50
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    @CamilleGoudeseune I finally checked out your user profile, it's your score in The Optiverse! Everting a sphere is on my bucket list. Is the intermediate point of a 3-sphere eversion a “double” Boy's surface? – uhoh Dec 09 '20 at 05:09
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    This is why I chose my avatar. https://pics.me.me/never-date-a-scientist-kiss-me-let-me-taste-your-38169918.png Not the question asked, but at least meteors are well defined and distinct. – userLTK Dec 09 '20 at 10:32

2 Answers2

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If we're going to get technical, Asteroids are not really an official name anymore. In 2006, when the IAU redefined what a planet was (and thus demoted Pluto), they also decided to more formally define other terms to identify objects in our solar system. You can see a diagram of all the official terms and how they relate below. Notice the important factor that asteroids no longer made the cut. There is the collection of "Small Solar System Bodies" (SSSB) and within that falls comets and centaurs, but asteroids are not exclusively called out.

enter image description here

So if you want to get technical, asteroids are really SSSBs now and comets are SSSBs as well, but also happen to fit into the sub-category of comets. What defines an SSSB as a comet is its ability to form a coma based on the sublimation of icy surface volatiles. So you might distinguish an "asteroid" as an object which cannot form a coma (and orbits inside Neptune), but that still leads to an ambiguity because what happens when a comet runs out of surface volatiles? Does it now become an asteroid? Is it still considered a comet? There are no clear answers.

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zephyr
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  • OK this really clears up my confusion, thanks! I'm being humorous here, but I almost wonder if the "demotion of Pluto" was used to distract the public from noticing that Astronomers had just eaten the entire asteroid belt. I think this Euler Diagram has some room for improvement in readability - it relies heavily on color to indicate which labels go with which boxes, and to sort out those groups of parallel lines and slim overlaps, so could be ambiguous to a few percent of the population with various color-blindness. But your answer is very clear! – uhoh Feb 08 '17 at 03:05
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    Just as an aside, I agree with @uhoh: I tend to prefer diagrams where there aren't multiple boundaries that track each other for stretches like this one does; I can't easily tell at a glance which boundary goes where. This isn't meant to cast any aspersions on the answer, which is fine! – Brian Tung Feb 08 '17 at 03:09
  • @BrianTung I've asked this question. – uhoh Feb 08 '17 at 04:27
  • That Euler diagram from wikipedia is incorrect. It implies, for example, that all minor planets and some centaurs are dwarf planets. – David Hammen Feb 09 '17 at 15:31
  • @DavidHammen It does not imply all minor planets are dwarf planets, only that some are (given that the minor planet area extends beyond the confines of the dwarf planets area). It also puts a dashed box of centaurs in the dwarf planet box because there are a few centaurs (e.g., 10199 Chariklo, 2060 Chiron, and 54598 Bienor) which may possibly be dwarf planets (hence the dash, it is still unconfirmed). – zephyr Feb 09 '17 at 15:37
  • @zephyr -- It most certainly does. The diagram has minor planets contained within dwarf planets. This is so very, very wrong. There are over 700,000 minor planets (and almost certainly over a million of them). There are only a few tens to hundreds of dwarf planets, each of which is a "minor planet". But a tiny 10 meter diameter minor planet most definitely is not a dwarf planet, making that diagram incorrect. – David Hammen Feb 09 '17 at 16:37
  • @DavidHammen But then that tiny 10 meter diameter minor planet would exist in the portion of the "minor planet" box which is not encompassed by the "dwarf planet" box. That 10 meter diameter minor planet would exist in the intersection of the "minor planet" box and the "Small Solar System bodies" box. I think you may be misinterpreting how this plot works (which I admit, this particular diagram is not well designed). I maintain that this diagram is not wrong. – zephyr Feb 09 '17 at 16:45
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    @zephyr -- This is supposed to be an Euler diagram. That's not how they work. Minor planets are the superset rather than dwarf planets -- and they also encompass the non-cometary "small solar system bodies".The diagram does not depict that, so it is very wrong (but that's wikipedia for you). – David Hammen Feb 09 '17 at 16:47
  • @DavidHammen I'm sorry but I just don't see your point. The minor planets box is the super-set which contains both the dwarf planet box and the small solar system bodies box. The 10 meter diameter minor planet would be in the small solar system bodies box which resides in the minor planet box. This diagram does not imply that tiny minor planets are always dwarf planets. I just don't see what you actually think is wrong. – zephyr Feb 09 '17 at 16:51
  • @zephyr -- What? The minor planet box is inside the dwarf planet box. That means that minor planets are a subset of dwarf planets, and that is 100% wrong. Dwarf planets are a tiny subset of minor planets. The diagram would be far better if it didn't show minor planets, TNOs, plutoids, comets, and centaurs. – David Hammen Feb 09 '17 at 16:59
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    @zephyr the difficulty here is that it is claimed somewhere within WIkipedia that this is an Euler diagram. "An Euler diagram (/ˈɔɪlər/, oy-lər) is a diagrammatic means of representing sets and their relationships. Sets like in math class with union and intersection and proofs. So there are strict rules about what does or doesn't means what. David is just saying in words what this diagram says mathematically. – uhoh Feb 09 '17 at 17:00
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    @uhoh -- Thank you. That is exactly what I mean. – David Hammen Feb 09 '17 at 17:01
  • @uhoh -- In fact, the current version of the wikipedia article on Euler diagrams portrays this awful image as the prototypical example of an Euler diagram. The diagram on the British Isles is much, much better (but still not perfect). – David Hammen Feb 09 '17 at 17:05
  • @DavidHammen I had a strong feeling that was the case. Wish there was a way that stackexchange could automatically generate flags in Wikipedia. – uhoh Feb 09 '17 at 17:09
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    @DavidHammen I think I see the confusion. I'm interpretting the fact that the minor planet box spans both dwarf planets and SSSBs to mean it is the super-set (and that they diagram is just poorly created). I think we can all just agree that this diagram is bad and confusing. – zephyr Feb 09 '17 at 17:17
  • @zephyr wait - there is a problem in our conversation here somewhere. Either the diagram is morphing in front of me, or something else, but Minor Planets (Deep Purple) spans (roughly) the low two-thirds of the image. Small Solar System Bodies (Gray) spans (roughly) the bottom one third and Dwarf Planets (Green) spans (roughly) the center one third. Plutoids and Comets are the only two things that are subsets of anything else. In addition, Dwarf Planets and Minor Planets and Small Solar System Objects are all supersets of something. It's late I'm signing off now, but... – uhoh Feb 09 '17 at 17:32
  • @zephyr my supplemental answer. Looks like what you have said is correct, took me a while to deconstruct the diagram to see it though! – uhoh Feb 10 '17 at 02:30
  • @DavidHammen See my supplemental answer. I think we have a semantic bug somewhere in our conversation. It would not have taken place if the diagram were more readable - as the British Isles diagram for example. – uhoh Feb 10 '17 at 02:31
  • So, it's no longer the asteroid belt, it's the SSSB belt, or Sbelt for short. I'm going to start calling it the Sbelt, though it contains a dwarf planet too. – userLTK Mar 23 '18 at 04:17
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This is supplemental information to further clarify the answer. The drawing is confusing to me an likely to others. I've written a color filter in Python to help isolate only four of the boundaries.

enter image description here

import numpy as np
import matplotlib.pyplot as plt

colorz = dict()

colorz['red']    = 1.0,   0.0,   0.0
colorz['green']  = 0.0,   0.502, 0.0
colorz['blue']   = 0.0,   0.0,   1.0
colorz['purple'] = 0.435, 0.192, 0.596
colorz['brown']  = 0.612, 0.353, 0.235
colorz['pink']   = 1.0,   0.0,   1.0
colorz['gray']   = 0.471, 0.471, 0.471
colorz['orange'] = 1.0,   0.494, 0.0
colorz['yellow'] = 1.0,   0.8,   0.0

namez = dict()

namez['red']    = "Satellites (natural)"
namez['green']  = "Dwarf planets"
namez['blue']   = "Planets"
namez['purple'] = "Minor planets"
namez['brown']  = "Trans-Neptunian Objects"
namez['pink']   = "Planetoiods"
namez['gray']   = "Small Solar System bodies"
namez['orange'] = "Comets"
namez['yellow'] = "Centaurs"

fname = "Euler_diagram_of_solar_system_bodies.svg.png"

img = plt.imread(fname)

def fakeimg(img, colors, hw):

    img3 = img[..., :3]

    imgnew = np.ones_like(img3)

    for color in colors:

        col = np.array(color)[:3]

        mask = (np.abs(img3-col) < hw).all(axis=-1)

        imgnew[mask] = col

    return imgnew

cs = [colorz[c] for c in ['orange', 'green', 'purple', 'gray']]

imnew = fakeimg(img, cs, 0.01)

if 1 == 1:
    plt.figure()
    plt.imshow(imnew)
    plt.show()
uhoh
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