This is a follow up question to What exactly is a "moon"? The conclusions I draw from James K's answer is that the IAU should define what a moon is. They haven't done so yet, but they should.

The obvious way to go about defining "moon" would be to build on the definition of a planet:

  1. Planets must be in orbit around the Sun
  2. Planets must be round due to their own gravity
  3. Planets must have "cleared the neighbourhood"

This immediately suggests the equivalent definition of a moon:

  1. Moons must be in orbit around the host planet
  2. Moons must have [minimum size]. Unfortunately being round doesn't work since not all moons are round (c.f. Mars' moons, unless one reclassifies those as not moons)
  3. Moons must have "cleared the neighbourhood" of their own orbit, i.e. in their particular orbit around the host they are by far the most massive body

Would such a definition work? If so, why hasn't the IAU also defined moons? It seems so natural after all. If not, what's the catch?


3 Answers 3


In 2006 the IAU had a trilemma.

  • They could decide that Eris was a planet, and potentially allow for future discoveries of tens of new planets.
  • They could be inconsistent, declare that Pluto was a planet, but Eris (and Ceres) wasn't
  • They could come up with a definition of "planet" that would exclude Eris, and consequently also exclude Pluto.

Each option is problematic: We teach eight-year-olds to chant the names of the planets, would we want to make them chant the names of 20 or thirty Kuiper belt objects? Being willfully inconsistent would lead to further argument. Re-describing Pluto would go in the face of 75 years of tradition. There was no option that would keep everyone happy, but they chose the third.

We don't have a problem with a more relaxed defintion of "moon" we are used to new "moons" being discovered. Eight-year-olds don't need to chant their names. The casual definition of "if you find it, you can name it" works.

The issues with planets are particular to our expectations that "planets" are important. We have no such expectation with moons.

  • 1
    $\begingroup$ There was a precedent as well. Ceres, Pallas, Juno, and Vesta were discovered within a few years of one another, but then no others for about 40 years. Students were initially taught that these asteroids were planets. Then came an onslaught of asteroid discoveries, first just a few, but within a few years, dozens upon dozens were being discovered every year. Planets were supposed to be notable and interesting things, things that made kids to want to become astronomers. So Ceres, Pallas, Juno, and Vesta were downgraded from planet hood status. $\endgroup$ Jul 23, 2018 at 14:55
  • $\begingroup$ "We have no such expectations with moons" - doesn't the existence of the moon have important implications on life on Earth? $\endgroup$
    – Allure
    Jul 23, 2018 at 23:48
  • $\begingroup$ @Allure That seems like a bit of a non-sequitur. We find our moon to be important, that's why it gets to be called The moon with a capital T. But I don't see what that has to do with the more generally relaxed attitude towards the discovery of new extraterrestrial moons. $\endgroup$
    – Kyle
    Jul 24, 2018 at 8:16
  • $\begingroup$ I suggest you rewrite the answer and first mention the important argument, then the eight year olds. It currently reads as if the eight year olds having to remember names is more 'important'. $\endgroup$
    – user1569
    Jul 24, 2018 at 14:22

Singling out one point:

  1. Moons must have "cleared the neighbourhood" of their own orbit, i.e. in their particular orbit around the host they are by far the most massive body

This would mean that co-orbital moons won't exist anymore; e.g. Telesto and Calypso share their orbit around Saturn with the much more massive Tethys. With the proposed definition, only Tethys is a moon, but if Telesto and/or Calypso would be bigger (so that Tethys is no longer by far the most massive body) none of them would qualify as a moon, not even Tethys, no matter how big they are compared to other (potential) moons orbiting Saturn.

  • 2
    $\begingroup$ Not really. There's nothing to say everything we currently call a moon must remain a moon, just like how Pluto was downgraded to dwarf planet. $\endgroup$
    – CJ Dennis
    Jul 23, 2018 at 10:12
  • 1
    $\begingroup$ @CJDennis: Janus and Epimetheus are a more problematic case. With a mass ratio of only about 4 to 1 between the two moons, it's kind of hard to describe either of them as having "cleared its neighborhood". Yet either of them would clearly pass the proposed criteria on its own (well, at least mostly, depending on how strict one wants to be about roundness), if they didn't happen to share the same orbit. $\endgroup$ Jul 23, 2018 at 11:11
  • $\begingroup$ @CJDennis The current description of planet works for most of what we called planets. For the proposed definition of moon to be reasonable, you would need to show it works for most of what we call moons. I'm not entirely sure that is the case. $\endgroup$
    – called2voyage
    Jul 23, 2018 at 13:54
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    $\begingroup$ @called2voyage - Almost certainly the twelve most recently discovered moons of Jupiter do not satisfy this definition. $\endgroup$ Jul 23, 2018 at 15:49
  • $\begingroup$ This was the part of the definition that necessitated creation of the category "dwarf planets" (all other requirements met but this one). So it seems pretty simple: Either you'll have a category of "dwarf moons", or you'll not have this rule. Since moons don't really have the problem that was solved by "dwarf planets", seems reasonable to dispense with the rule and the extra category. $\endgroup$
    – T.E.D.
    Jul 23, 2018 at 18:58

I'd like to expand on @James K's good answer.

The question you need to answer before your question can be answered is: why do you want to have a definition of "moon"? What purpose does it serve other than pedantry? (Remember that there is a continuum of objects from the largest stars down to the smallest bits of space dust -- whatever you do, you're drawing lines dividing this patch of sand from that one. Giving groups of similar objects a collective name is useful, but why is it important to precisely define the line's position down to individual grains?)

Is it because there are some laws which apply to moons and not to smaller bodies? Because it's a step to making your organization the definitive source of naming policy? Because full-fledged moons are more attractive tourist destinations? To get publicity? Because there's a turf fight between the Department of Small-Body Astronomy and the Department of Lunar Astronomy that needs to be settled? Because several of your colleagues like making lists? Because the name will make a difference in the scientific study of the bodies? Because you're at this meeting and bored?

Some of the reasons may seem flippant, but many of them (most of them, I suspect) played a role in the IAU's definition of planet. So if you can answer why, you'll probably also be able to answer your question.

(My own guess is that having a formal definition is important to people who like to make lists, but not otherwise scientifically important.)

For myself, I'd say that a satellite is an object that goes around a larger non-stellar object.

  • 1
    $\begingroup$ Because if we consider any body orbiting a planet a 'moon' (even the smallest ring component) we consequently ought to also consider any celestial body orbiting a star a 'planet' (including the smallest meteoroid). The 2006 definition differs planets from 'dwarf planets' thereby further opening Pandora's chest. Better to judge a celestial body by itself, not its location or orbital parameters. Moons don't really orbit a planet anyway, they orbit a barycenter that may or may not be within the primary body. $\endgroup$
    – Greenhorn
    Dec 28, 2020 at 7:00

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