Would an analogue of the definition for planets also work for moons?

  • 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?

  • Glorfindel

    Glorfindel Correct answer

    4 years ago

    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.

    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.

    @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.

    @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.

    @called2voyage - Almost certainly the twelve most recently discovered moons of Jupiter do not satisfy this definition.

    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.

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