What is a Moon?

moons
A selected collection of solar system Moons. Though they may look different from each other, they are still moons of planets. NASA

What is a moon? It's a simple question that you might think has an easy answer. However, it's important to remember that the moon we know so well is not the only moon, and when it comes to defining these magnificent objects, the answer gets complicated. Let's explore the idea of a moon and what it means in our solar system.

That Bright Ball in the Night Sky

The first moon ever discovered was, unsurprisingly, our Moon.

Originally, people called it a planet. This is an artifact of the geocentric model of the solar system, essentially the belief that Earth is the center of everything. 

Our Moon actually shares much in common with planets in general, but there is one important distinction: it orbits a body that already orbits the Sun. It's a natural satellite, to be technical, and it's one of many in the solar system.

Definition of a Moon

There is actually no strict definition of what a moon is, but there are some common features shared among those objects we consider moons. The technical term is actually "natural satellite".

Moons are: 

  • Distinct, whole objects
  • Solid objects
  • In orbit around a more massive body (that presumably orbits a star)

Moons also come in all shapes and sizes. We tend to think of objects like our own Moon that are large and round, but moons like Phobos and Deimos (orbiting Mars) look more like small, irregularly shaped asteroids.

A moon's appearance can cause confusion, especially since there is no lower limit to the mass it can have. So, finding moons shaped like asteroids give us hints about their histories and the history of the solar system.

Some people ask if the bits of material that make up the rings of the outer planets could be considered moons.

It's a good question. Currently, chunks of ice and rock and dust that make up rings are considered solely part of the rings, and are not individual moons. 

Interestingly enough, moons don't just orbit planets. Nearly 300 asteroids (or minor planets) are known to have moons of their own. 

Are All Moons Really Moons?

But there are objects that are currently classified as moons, that actually may be better classified as some other type of object.

The classic example raised is the moons of Mars, as well as similar ones that orbit the outer planets and appear to be captured asteroids. While we call them moons, some planetary scientists argue that a new classification of object should be created, such as "binary companions", or even "double asteroid". 

Perhaps the most controversial example, however, is the Pluto/Charon system. Pluto was demoted from planet status in 2006 to dwarf planet status. Its smaller companion Charon was deemed its moon.

However, the step taken by the International Astronomers Union (IAU) to establish a strict planet definition has created a controversy with the system. By making a distinction between planets and dwarf planets—essentially small worlds that don't quite have the properties needed to be planets—the question also arises whether Charon should also be considered a dwarf planet.

One of the few distinguishing properties of a moon is that it must orbit another object. But in this case, Charon is nearly half the mass of Pluto. So rather than orbiting Pluto, both orbit a point outside of Pluto's radius.

At Earth, the center of mass of the Earth-Moon system is within Earth itself, but Earth still moves slightly in response to the mass of the Moon. This is not the case with Pluto and Charon, because they are so similar in size. Therefore some astronomers think that the Pluto/Charon system should be classified as a dwarf binary. That is not a commonly held position and there will continue to be confusion and disagreement until more strict definitions are agreed on by the planetary science community to guide the IAU. 

Edited and updated by Carolyn Collins Petersen.