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I was wondering how massive something have to be so that it it can attract moons by pop culture standards (ellipsoid/round shape).

Could a planetoid have a moon? What is the relation between the mass of a moon and planet?

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  • $\begingroup$ There is an example of an asteroid with a "moon" in What is the physical geometry of this apparent “eclipse” of a tiny moon of Asteroid Florence? and there's also Is 486958 Arrokoth (2014 MU69 aka Ultima Thule) the only object determined to be binary by occultation? but I don't think these objects are round to "pop culture standards". Also see en.wikipedia.org/wiki/Binary_asteroid# $\endgroup$
    – uhoh
    Jun 28, 2020 at 23:28
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    $\begingroup$ @uhoh that looks like the basis for a brief additional answer. Comments are neither permanent nor searchable, and yours has some useful info that should be made available to everyone in an answer. :-) $\endgroup$
    – Chappo Hasn't Forgotten Monica
    Jun 29, 2020 at 2:29
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    $\begingroup$ @ChappoHasn'tForgottenMonica Thanks for your suggestion, I know what you mean; but as the comment says I don't think these objects are round to "pop culture standards" so not a basis of an answer to this question. I think the comment will be there long enough for the OP to see it, and maybe they will ask a new question about non-pop-round natural satellites as a follow-up, and the two linked questions are now permanently displayed in the Linked sidebar, and will stay there even if the comment is deleted. I think that in this case the comment is the best compromise. $\endgroup$
    – uhoh
    Jun 29, 2020 at 2:34
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    $\begingroup$ Do you consider the secondary object of a binary system to be a satellite, or do you disqualify binary systems? I can see arguments for both PoVs. Also, when you talk about planetoids, which class of objects are you referring to? Dwarf planets, SSSBs, minor planets (the superclass of both), something else? "Planetoid" is a notoriously fuzzy term. $\endgroup$
    – Polygnome
    Jun 29, 2020 at 6:36
  • $\begingroup$ Natural vs. manmade objects ? $\endgroup$
    – Carl Witthoft
    Jun 29, 2020 at 15:03

2 Answers 2

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Planetoids can have moons and the minimum size is "pretty small". For example 2003 SS84is a small Near-Earth asteroid, with a diameter of 120m and a moon of about 60m in diameter, which orbits at a distance of 270m ever 24 hours. It probably didn't form by "attracting the moon" but the moon probably formed as a result of impact splitting a "rubble pile" asteroid. The moon, in this case, is similar in size to the main object. There is a wide distribution of size ratios from close to 1:1 to very small objects orbiting much larger ones.

However one aspect of your question is different: you ask for a "round" shape. That makes things harder, since a round shape requires a fairly large size to pull the object into a sphere. There is probably only one object except for the major planets, with a spherical moon, and that is Pluto with its moon Charon (and even Charon isn't perfectly rounded, but it is close)

So it is quite easy for a planetoid to have a moon. But it is hard for a planetoid's to be big enough to pull itself into ellipsoidal equilibrium

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  • $\begingroup$ This is a very intriguing object, thanks for pointing it out! According to their discovery paper, the authors think of this as a binary system. Same can be said about Pluto/Charon. $\endgroup$
    – Polygnome
    Jun 29, 2020 at 6:32
  • $\begingroup$ But I guess it's easy enough to create a system by tossing a microsphere and a beebee into space and giving them a little push :-) $\endgroup$
    – Carl Witthoft
    Jun 29, 2020 at 15:02
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In Kollmeier & Raymond (2018), it is stated that a moon can have its own submoon, and that submoon can have a subsubmoon, etc. It doesn't matter if the object is round or not, and there's no minimum mass per se, provided the secondary object has $10^{−5}$ times the primary body's mass, using the rule of thumb from Reid (1973). So, for example for a submoon of the Moon, which has mass of $7.342×10^{22}$ kg, could have a maximum mass of $7.3420×10^{17}$ kg, and that submoon could have a subsubmoon of $7.3420×10^{12}$ kg. There could be, also, a subsubsubmoon, with a mass of $7.3420×10^{7}$ kg, or $10000$ tons. As you can see, there is a moment that the smallest object has very little mass, so it can be gravitationally attached to almost anything.

Edit: the smallest moon in the Solar System is Deimos, with a mass of $1.4762×10^{15}$ kg and a diameter of 12.4 km. So, theoretically, it could have a submoon of $1.4762×10^{7}$ tons.

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