spaceSpace and Physics

Moons Could Have Their Own Moons And We Might Call Them Moonmoons


Objects definitely not to scale. Jurik Peter/Shutterstock

Many planets, including our own, are orbited by one or more moons. But what if these rock and ice celestial bodies were themselves circled by smaller objects? Do such things exist, and, if so, what are they called?

According to astronomers Juna Kollmeier from the Carnegie Institution of Washington and Sean Raymond from the University of Bordeaux, the answer to the first question is: yes, maybe. As to the second question, they have a few ideas, from the scientific-sounding "submoon" to the much more entertaining "moonmoon", but more on that later.


In an analysis published in the pre-print database arXiv, Kollmeier and Raymond calculate the Goldilocks conditions that would allow a submoon to stably orbit its moon without being thrown off course or sheared into pieces by the gravitational pull – also known as tidal force – from the moon’s planet.

After making a few assumptions about the densities of moons and submoons based on what we know about the players in our own Solar System, the authors conclude that only large moons – those with a radius of 1,000 kilometers (621 miles) or larger – with wide orbits could accommodate long-lived submoons, whose size would be proportionate to 10 kilometers (6.2 miles) and larger.  

“Tidal [energy flux] destabilizes the orbits of submoons around moons that are small or too close to their host planet; this is the case for most of the Solar System's moons,” they wrote.

“A handful of known moons are, however, capable of hosting long-lived submoons: Saturn's moons Titan and Iapetus, Jupiter's moon Callisto, and Earth's Moon.”


They add that the newly discovered moon orbiting the exoplanet Kepler-1625b (a gas giant about six to 12 times the size of Earth, orbiting a Sun-like star located about 8,000 light-years away) may be able to host a submoon as well, though they don’t know enough about the object to be sure.

“This system where you’ve got a giant planet and a Neptune-sized moon that’s kind of far away from the planet is sort of the best-case scenario for a moonmoon,” Raymond told New Scientist.

However, even if it’s theoretically possible for a submoon to survive the competing forces from a moon and a planet, the likelihood of one forming in a moon-planet environment with the correct configuration is quite low.

“Something has to kick a rock into orbit at the right speed that it would go into orbit around a moon, and not the planet or the star,” Raymond said, adding that the submoon would also get lost or destroyed if the moon in question migrated during its evolution, as the Earth’s moon did.


At the end of their paper, Raymond and Kollmeier call for further studies investigating the conditions that could lead to submoon creation and persistence, as well as those trying to locate some real-world (real-galaxy?) examples.

In the meantime, in the few days since the duo’s article draft went live, the Internet has erupted into a frenzy over the possibility of submoons. And, thanks to a declaration by New Scientist, everyone is gleefully calling them "moonmoons", a name that has been floated in the astronomy community before.


Speaking with Quartz, Kollmeier said that she and her co-author would be happy to stick with submoons, though they also approve of moonmoons, moonitos, moonettes, and moooons.

“IAU [the International Astronomical Union] will have to decide!” she added. Clearly, she has a sense of humor, but us poor laypeople urgently need to know whether "moonettes" is a serious contender or a joke. Because, wow, that would be great on a T-shirt.


[H/T: New Scientist]


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