Question: I’m just starting to learn about astronomy and have a question. I’ve heard about new moons being discovered around other planets, so how do we know Earth only has one moon? — JV, Nantes, France
Answer: That’s a very interesting question. Obviously, we’ve known about the Moon (aka “Luna”) for ages, but does Earth have any other moons? The short answer is: We don’t know. The long answer is somewhat complicated, since it really comes down to what you mean by “moon.”
Here’s the problem. The International Astronomical Union (IAU), which decides on the definitions of astronomical terms, does not have an official definition of “moon.” Still, there’s a few things astronomers generally agree on:
Moons orbit planets, whereas planets orbit stars.
Moons must be solid bodies with a distinct surface.
Moons must be individual entities, as opposed to the myriad tiny particles that compose ring systems.
Beyond that, there’s a lot of room for personal opinion. For example:
How big does it need to be to qualify as a moon?
Does it need to be spherical, or do weird shaped bodies like Phobos and Deimos count?
What if, rather than orbiting a planet, it’s one of two bodies ...