However, the New Scientist chose to call them "moonmoons". Some scientists have decided such moons should be called "moonmoons", and they believe there could even be some of them in our own solar system.
Astronomers Juna Kollmeier of Carnegie Institution for Science and Sean Raymond of the Laboratoire d'Astrophysique de Bordeaux ran a computer simulation to predict whether natural satellites such as the Earth's Moon can have a natural satellite of its own. Apparently, submoon was outshined by the more delightful term "moonmoon" based on a popular meme.
We know that moons orbit planets, planets orbit stars, stars orbit the center of the galaxy, and even galaxies sometimes orbit each other. There should also be enough distance between the main moon and the planet it orbits.
"Each of the giant planets within the Solar System has large moons but none of these moons have their own moons (which we call submoons)."
There are no known moonmoons in our Solar System, perhaps because the line they need to tread in order to exist is a very fine one.
Kollmeier and Raymond dubbed a moon orbiting a moon a "submoon" in their study but as expected, the internet has better ideas.
If the sizes don't match up just right, the smaller moon would be ripped away from the larger moon and shot into space or toward the planet the moon was orbiting.
A four-year old child asked the question in 2015, and that sparked her mother's curiosity. In a paper presented at the online library of preprints arXiv.org they lead the preliminary calculations of the conditions under which this is possible. Theoretically, any of Saturn's moons (Titan and Iapetus), Jupiter's moon Callisto or even our own moon could have a moonmoon, the astronomers wrote. The moonmoon needs to be close enough to the moon to be bound by its gravity, rather than the planet's; but not so close that it gets torn apart by tidal forces. Except they don't. So we might have to wait a bit before we can validate the theory. So as far as I'm concerned, moonmoons are now a thing.
"Further studies of the potential formation mechanisms, long-term dynamical survival, and detectability of submoons is encouraged".