A few years ago, an astronomer’s son asked the type of question only kids and genius astrophysicists come up with: Can a moon have a moon? Juna Kollmeier of the Carnegie Institution Observatories couldn't answer her child's query, but she realized that investigating the idea could help answer questions about how moons form and even reveal some of the hidden history of the Solar System, reports Ryan F. Mandelbaum at Gizmodo.
The results, which she co-authored with astronomer Sean Raymond of the University of Bordeaux, were recently published in a short paper titled “Can Moons Have Moons?” on the preprint server arXiv.org, which hosts yet-to-be peer reviewed research. The study, however, has raised an even bigger question that now has the scientific Twitterverse riled up. Just what do you call the moon of a moon?
In their study, Kollmeier and Raymond looked at what would happen to a small submoon orbiting another moon. According to the paper, what they found is that in most cases there’s just not enough space for a submoon to orbit another moon. Tidal forces would pull the little moon toward the host planet, ripping the mini moon to pieces.
For a submoon to survive, it needs to be small—about six miles in diameter or less. It also needs to orbit a large moon with enough gravity to hold it in place and must be far enough away from the host planet to complete its own orbit. It turns out that several moons in our own solar system fit the bill and could host submoons, including Titan and Iapetus, which orbit Saturn, and Callisto, which orbits Jupiter. Even our own moon is the right size and distance from Earth to potentially host its very own moon.
Figuring out why none of our local moons have their own pet moons could tell us about how moons and planets form, the researchers write. And, they suggest, we should see if the recently discovered candidate exomoon circling Kepler 1625b has its own moon as well.
“We’re really just scratching the surface here with how we can use the absence of submoons to figure out our early history,” Kollmeier tells Mandelbaum.
In the paper, the astronomers simply dub the moons of moons "submoons." But Kollmeier tells Natasha Frost at Quartz that usage was just a personal choice, and that there is no official word, yet. Other terms for the moon of moon have been suggested, including moonmoons, moonitos, moonettes, and moooons.
“IAU [International Astronomical Union] will have to decide!” Kollmeier says.
It's already popped up in the scientific realm, too: Astrophysicist Duncan Forgan of the University of St. Andrews uses the term moon-moon in his recent paper also on arXiv.org, which was actually published the day before Kollmeier's, discussing the possibility of a habitable moon orbiting Kepler 1625’s candidate exomoon.
Twitter, for one, has glommed onto the satisfying un-hyphenated term “moonmoon,” which was suggested by New Scientist magazine, and the memes have already begun. One reason is that “Moon Moon” was already a popular meme from several years ago starring a derpy wolf, which has led to some strange mash-ups.
Sarah Laskow at Atlas Obscura explains that moonmoon is catching on because there’s something pleasing about “recursive places,” like islands within islands, volcanoes within volcanoes and rivers running through a sea. Laskow, for one, isn’t voting for moonmoon. Instead, she says she prefers either sub- or meta- as the prefix for recursive places, like submoon or metamoon. But she acknowledges that moonmoons may be in linguistic orbit for a while.
“Whatever the most people decide to call these fascinating places, that’s what will stick,” she writes.
Whatever comes out on top—moonmoons, grandmoons, moon-squareds, nested moons or who knows what—astronomers need to prove they exist before we call them anything.