Sciency Words: Submoon

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:


After my recent post about exomoons and trickster moons, a reader commented asking about moons with moons.  Honestly, I couldn’t think of any reason why that wouldn’t be possible, but I felt like it must be an extremely rare thing. Otherwise we probably would’ve found something like that in our own Solar System by now.

And according to this paper entitled “Can Moons Have Moons?” the answer is yes.  Theoretically, under certain circumstances, a moon could have a very, very tiny moon of its own.

It’s important to note, however, that for an object to truly be considered a moon, its orbit must be stable.  For example, there are multiple objects that are in temporary orbit around Jupiter, but since those objects are not expected to stick around for more than a few years, or maybe a few decades at the most, they are not included in the official count of Jupiter’s moons.

In most cases, a small object caught in orbit around a moon will have a very difficult time maintaining that orbit.  The gravitational attraction of the nearby planet will just keep tugging and tugging, stretching the orbital path into a wider and wider ellipse.  It won’t take long before the moon’s gravity can no longer hold the small object it captured.

But according to that “Can Moons Have Moons?” paper, if a moon is relatively large (like our own Moon) and orbits relatively far away from its host planet (also like our own Moon), and if there aren’t a whole lot of other moons around to make gravitational interactions complicated, then yes: that moon could have a moon in a stable orbit.  A very, very tiny moon.  Something asteroid sized.

The research paper I’m citing proposes calling the moon of a moon a submoon, but that’s not an official scientific term.  Not yet.  It probably won’t be until an actual submoon is discovered somewhere out there.  Until then, other terms have been proposed, like meta-moon, nested moon, grandmoon, and moonmoon.  Moonmoon seems to be the most popular choice on the Internet, probably because of the Internet meme.  Which means when the time comes the I.A.U. will almost certainly not pick that one.  More likely, the I.A.U. will go with “dwarf moon” and insist that no further discussion of the matter shall be permitted.

For right now, I think submoon is the term with the most scientific legitimacy.  For the purposes of Sciency Words and other sciency writings, I think that’s the term to go with.  But what do you think?  What would you call the moon of a moon?

18 thoughts on “Sciency Words: Submoon

    1. I have to confess, I do kind of like moonmoon. I think it’s fun. But it’s probably not the term that the scientific community is going to end up using, and it’s not the term I’d want to use in my science fiction writing. I can’t picture battle-hardened space pilots doing risky combat maneuvers around a “moonmoon.”

      Liked by 1 person

    1. Since the I.A.U. has never written an official definition of “moon,” I can’t point to anything specific. I think moons and submoons have to be natural satellites, rather than artificial ones. I don’t think size is a consideration; a moon just has to be smaller than whatever it’s orbiting. But the law of gravity seems to place physical limits on how large a submoon can be.

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      1. Definitely if it’s not smaller than what it’s orbiting, it isn’t the moon, the other object is its moon. Although isn’t the center of gravity between Pluto and Charon somewhere between them? Nature seems to delight in frustrating our little categorization systems.

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      2. Yes, Pluto and Charon orbit a point in space between them. That point is much closer to Pluto than to Charon, but still…

        The I.A.U. considered classifying Pluto and Charon as a binary planet, which would have been a neat idea, I think. But if they’d done that, the distinction between moons and binary planets would have been very complicated.

        Liked by 2 people

    1. According to that paper I cited, Titan is on the list of moons in the Solar System that could hang on to its own submoon. Ganymede did not make the list, though, because the other Galilean moons would keep trying to tug a submoon away.

      Liked by 1 person

    1. No problem! Another possibility: maybe some of the other Galilean moons could get thrown out of orbit. That would make it easier for Ganymede to capture a tiny moon and hold onto it. Something tells me you wouldn’t be too upset if Europa got ejected from the Jovian system.

      Liked by 1 person

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