As every scientist knows, children generally come up with the best questions. A few years ago, the child of Juna Kollmeier, an astrophysicist of the Carnegie Institution Observatories, asked a great question: “Can moons have moons?” Spoiler alert: It’s technically possible, and these “moonmoons” might even exist. But it’s not likely.
Can Moonmoons Exist?
First of all, let’s agree that defining a planetary body is an inherently problematic process. People are still arguing about Pluto’s planethood more than 12 years after an International Astronomical Union vote deemed it a “dwarf planet.” Even the idea of a moon is blurry. We’ve seen moons orbiting many types of objects — planets, dwarf planets, and even asteroids.
Loosely defined, then, a “moon” is a world that is orbiting another world. So could a moon orbit another moon? Kollmeier’s paper on preprint site Arxiv(which is, so far, not peer-reviewed) suggests a “moonmoon” can only exist if it’s less than six miles in diameter. Also, the moonmoon’s moon must have a strong enough gravity so that the moonmoon doesn’t go flying off into space. Most importantly, the moonmoon needs to have enough space not to crash into the moon, or to crash into the planet the moon is orbiting.
We know, it’s all hard to picture. And it’s even harder to simulate. But the paper shows that a few moons in our system do have enough room for a moonmoon. One possibility is Titan, a moon of Saturn that could host the building blocks of life. Even Earth’s moon has enough room for a moonmoon.
A Moonmoon by Any Other Name
Here’s another tricky problem: What’s the proper name of a moon circling a moon? In this article, we’ve settled on the term “moonmoon” because that seems to be the most popular option on the web. But Kollmeier’s paper calls these types of worlds “submoons.” In the publication Quartz, other names were suggested: moonitos, moonettes, and moooons. Smithsonian Magazinehad a few other clever options: grandmoons, moon-squareds, nested moons.
“IAU will have to decide!” Kollmeier joked with Quartz. But since we can’t even agree if Pluto is a planet, it looks like even an IAU vote may not be enough to convince the community.
Whether you want to call these little worlds moonmoons, submoons, or moonitos, one thing is for sure: We haven’t found any of them yet. Astronomers certainly aren’t giving up the cause, though. Another recent paper on Arxiv by the University of St. Andrews’ Duncan Forgan (submitted to the Monthly Notices of the Royal Astronomical Society) suggests we should look at Kepler-1625b, where the first “exomoon” — moon outside our solar system — may have been found. Happy hunting!