Moons can be inert pieces of rock, hunks of ice or volcanically active. They can be caught in slow death spirals or potential homes to life. Atmospheres can be thick, thin or non-existent. Even size doesn’t matter all the much, as long as the moon is relatively small enough to be held by a planet’s gravitational pull. This is because the only real rules about being a moon are that the object is naturally occurring (ruling out the International Space Station, for instance) and that they orbit a planet specifically (ensuring that planets aren’t moons of their local stars). It all seems logical enough from our single-mooned planet, but since these satellites could potentially be large enough to trap objects in their own orbit, it raises questions about why we’ve defined moons like this, and if secondary moons of moons are even a possibility to consider.
Even with just a handful of planets, our solar system is still home to hundreds of moons. Making up the lack of moons around the inner planets, a gas giant like Jupiter has at least 78 moons in orbit. The more we look, the more we seem to find, indicating that a big planet doesn’t have a lot of trouble scooping up satellites. The lack of moons around those moons indicates that the latter relationship is much harder to maintain. This is essentially due to the limited range where a smaller object can fall into orbit around a moon without being pulled towards the larger planet nearby. Called the Hill sphere, this distance is highly dependent on the relative size and distance of the larger body in the equation, meaning the Earth’s smaller size and proximity to the Sun give our planet a smaller Hill sphere than Jupiter.
Even if an object does fall into a consistent orbit around a planet or moon, there’s also the issue of tidal forces. On Earth, the Moon ends up “pulling” more on the axis that is pointed at the Moon, which gives us high and low tides as the planet’s water essentially shifts around to always be aligned with the Moon. On a planet-wide scale, this results in internal friction and the release of heat, slowing the rotational speed of the objects in question. Over the course of millions of years, an orbiting object is then likely to be slowed enough to fall out of its orbit altogether. Combined with an already tight Hill sphere, this further reduces the odds of finding a moon of a moon.
Models for moonmoons
This isn’t to say that these moons are impossible. Researchers recently modeled a number of scenarios and believe that moons-of-moons are likely to eventually be found around planets in other solar systems (or even around some of the many moons of Jupiter and Saturn!) Naturally, if these objects are going to be found and studied, it would help to define them, starting with a name. Some people are advocating for terms like “sub-moons” or “mini-moons,” but a standout contender may actually be based on a weird meme about werewolf names: moonmoons. Moonmoons may not be the most descriptive term out there it certainly makes a yet-unseen form of space rock seem very endearing. Sometimes even science is subject to popularity contests (just ask Opisthoteuthis adorabilis).
Source: Can moons have moons? (Intermediate) by Sabrina Stierwalt, Ask an Astronomer