As of today, there are no confirmed moons in our solar system with a name that starts with the letter X. This makes X one of the rarest letters in planetary moon nomenclature — a distinction it shares with very few other letters. With over 290 known moons orbiting the planets of our solar system, the complete absence of an X name is a curious linguistic gap worth exploring.

The reason comes down to how moons are named. The International Astronomical Union (IAU) assigns names based on established mythological traditions tied to each planet. For example, moons of Jupiter are named after figures from Greek and Roman mythology connected to the god Jupiter, while Saturn’s moons draw from Greco-Roman, Norse, Inuit, and Gallic mythology. The letter X simply does not appear at the start of many names in these traditions. Most mythological figures whose names begin with X come from outside the naming pools the IAU uses, and the few that do exist have not yet been applied to any discovered moon.

There are, however, moons whose names contain the letter X without starting with it, such as Proxima’s theoretical companions or the Uranian moon Caliban — but none that lead with X. As astronomers continue to discover new moons, particularly around the outer planets, it remains possible that a future moon will carry an X name. Until then, the letter stands as a blank space on the list — not because astronomers overlooked it, but because the mythology simply did not produce many names that begin there.