If a Moon Could Have Its Own Moon, What Would We Call It?

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A photoshopped image of a moon orbiting our moon. Researchers have proposed naming such a celestial arrangement a 'moonmoon' or 'submoon.'. (Photo: Original photo by Dave Grubb/Flickr)

Will we one day discover a moon that has its own, smaller moon? Researchers say it's not outside the realm of possibility and, just in case, they're already proposing names for such a bizarre orbital arrangement.

In a paper published in on the arXiv preprint server, astronomers Juna Kollmeier from the Observatories of the Carnegie Institution of Washington and Sean Raymond from the University of Bordeaux explain the complicated physics behind a moon orbiting a moon orbiting a planet. While they've chose the predictable title of "submoon" to classify this scenario, New Scientist is reporting that others have floated the much more enjoyable name of "moonmoon" instead.

The internet has also chimed in, with such wonderful suggestions as "moonito" or "mini-moon."

"Moonmoon" –– it's just fun to say. The only problem is that even should our moonmoon naming dreams come true, the chances of getting the opportunity to report on the term frequently are, well, currently non-existent.

As far as we know, our own solar system has no moonmoon candidates. Outside our solar system, we may have just discovered our first moon orbiting an alien world, what's known as an exomoon, but even that is an extremely rare occurrence. Until the James Webb space telescope arrives sometime in the early part of the next decade, the technology needed to spot a little moonmoon is still slightly beyond our reach.

And the math gets worse. When Kollmeier and Raymond ran the calculations on the possibilities of moonmoons putting down roots around an existing moon, they discovered a litany of specific factors that must first come into play. For one, the moonmoon must be close enough and small enough to its parent body to be captured in its gravity, but not so close that it would be torn to pieces by tidal forces.

A moonmoon orbiting our own Moon during moonrise. Researchers say that even if such a thing were possible, it's not a celestial relationship that would likely last very long.
A moonmoon orbiting our own moon during moonrise. Researchers say that even if such a thing were possible, it's not a celestial relationship that would likely last very long. (Photo: Original photo by Denali National Park/Flickr)

For a moon to even host a moonmoon in the first place would require an outside force to hit it in what essentially boils down to an orbital bullseye.

"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 told New Scientist.

As detailed in the paper, the researchers say Jupiter's moon Callisto, Saturn's moons Titan and Iapetus, and even Earth's moon all fit the size and orbital requirements to host a moonmoon. They may have even at some point had their own primordial moonmoons, but later lost them due to tidal or orbital shifts.

"To conclude, we note that while many planet-moon systems are not dynamically able to host long-lived submoons, the absence of submoons around known moons and exomoons where submoons can survive provides important clues to the formation mechanisms and histories of these systems," they write. "Further studies of the potential formation mechanisms, long-term dynamical survival, and detectability of submoons is encouraged."

As for the name, they're open to suggestions there as well.