<p>Planetoids can have moons and the minimum size is &quot;pretty small&quot;. 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 &quot;attracting the moon&quot; but the moon probably formed as a result of impact splitting a &quot;rubble pile&quot; 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.</p>
<p>However one aspect of your question is different:  you ask for a &quot;round&quot; 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)</p>
<p>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</p>

CC BY-SA 4.0
2020-06-29T06:32:21.150Z
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.
Polygnome
CC BY-SA 4.0
2020-06-29T15:02:25.233Z
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 :-)
Carl Witthoft
CC BY-SA 4.0
2020-06-28T20:53:40.293Z
2020-06-28T20:53:40.293Z
James K
<p>In <a href="https://arxiv.org/pdf/1810.03304.pdf" rel="nofollow noreferrer">Kollmeier &amp; Raymond (2018)</a>, 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 <span class="math-container">$10^{−5}$</span> times the primary body's mass, using the rule of thumb from <a href="https://www.sciencedirect.com/science/article/abs/pii/0019103573900535" rel="nofollow noreferrer">Reid (1973)</a>. So, for example for a submoon of the Moon, which has mass of <span class="math-container">$7.342×10^{22}$</span> kg, could have a maximum mass of <span class="math-container">$7.3420×10^{17}$</span> kg, and  that submoon could have a subsubmoon of <span class="math-container">$7.3420×10^{12}$</span> kg. There could be, also, a <em>subsubsubmoon</em>, with a mass of <span class="math-container">$7.3420×10^{7}$</span> kg, or <span class="math-container">$10000$</span> 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.</p>
<p>Edit: the smallest moon in the Solar System is Deimos, with a mass of <span class="math-container">$1.4762×10^{15}$</span> kg and a diameter of 12.4 km. So, theoretically, it could have a submoon of <span class="math-container">$1.4762×10^{7}$</span> tons.</p>

CC BY-SA 4.0
2020-06-29T14:47:26.113Z
2020-06-29T14:55:21.543Z
2020-06-29T14:55:21.543Z
Carlos Vázquez Monzón
<p>I was wondering how massive something have to be so that it it can attract moons by pop culture standards (ellipsoid/round shape).</p>
<p>Could a planetoid have a moon? What is the relation between the mass of a moon and planet?</p>

CC BY-SA 4.0
2020-06-28T23:28:25.493Z
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?](https://astronomy.stackexchange.com/q/22480/7982) and there's also [Is 486958 Arrokoth (2014 MU69 aka Ultima Thule) the only object determined to be binary by occultation?](https://astronomy.stackexchange.com/q/35466/7982) but I don't think these objects are round to "pop culture standards". Also see https://en.wikipedia.org/wiki/Binary_asteroid#
uhoh
CC BY-SA 4.0
2020-06-29T02:29:52.173Z
@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. :-)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2020-06-29T02:34:32.213Z
@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
CC BY-SA 4.0
2020-06-29T06:36:36.057Z
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.
Polygnome
CC BY-SA 4.0
2020-06-29T15:03:10.320Z
Natural vs.  manmade objects ?
Carl Witthoft
<planet><natural-satellites><newtonian-gravity>
What is the minimum mass of a celestial object so that it can have a moon?
inquisitor
# What is the minimum mass of a celestial object so that it can have a moon?

(Asked by: username_0 on 2020-06-28)

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?

**Tags:** planet, natural-satellites, newtonian-gravity
> **Comments:**
> **username_1**: 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?](https://astronomy.stackexchange.com/q/22480/7982) and there's also [Is 486958 Arrokoth (2014 MU69 aka Ultima Thule) the only object determined to be binary by occultation?](https://astronomy.stackexchange.com/q/35466/7982) but I don't think these objects are round to "pop culture standards". Also see https://en.wikipedia.org/wiki/Binary_asteroid#  
> **username_2**: @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. :-)  
> **username_1**: @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.  
> **username_3**: 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.  
> **username_4**: Natural vs.  manmade objects ?  

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## 2 answers

### Answer by username_5 on 2020-06-28. Score: 7

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

> **Comments:**
> **username_3**: 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.  
> **username_4**: 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 :-)  

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### Answer by username_6 on 2020-06-29. Score: 2

In [Kollmeier & Raymond (2018)](https://arxiv.org/pdf/1810.03304.pdf), 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)](https://www.sciencedirect.com/science/article/abs/pii/0019103573900535). 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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