# What is the minimum mass of a celestial object so that it can have a moon?

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?

• 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#
– uhoh
Jun 28 '20 at 23:28
• @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. :-) Jun 29 '20 at 2:29
• @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.
– uhoh
Jun 29 '20 at 2:34
• 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. Jun 29 '20 at 6:36
• Natural vs. manmade objects ? Jun 29 '20 at 15:03

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.