What Is the Definition of a Moon?

Earth's Moon
Earth's Moon is just one of hundreds of moons in the solar system. NASA

What is a moon? That seems like a question with such an obvious answer that you might be tempted to think to yourself that the answer is pretty obvious: it's the object we see in the sky at night (and sometimes during the day) from Earth. Which is true, of course. However, that's just one answer.

It's important to remember that the moon we know so well is not the only one. Moons make up an entire class of objects in the solar system, and they can be found nearly everywhere.

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, which is an artifact of the geocentric model of the solar system. That old and discredited belief that Earth is the center of everything fell by the wayside when astronomers figured out that everything in the solar system orbits the Sun, not Earth. So, what do you call something that orbits a planet? Or an asteroid? Or a dwarf planet? By convention, they're also called "moons". They orbit bodies that already orbit the Sun. To be technical, the term is actually "natural satellite", which differentiates them from the types of satellites we launch to space. There are dozens and dozens of these natural satellites throughout the solar system

Moons Come in All Shapes and Sizes.

We tend to think of objects like our own Moon that are large and round. Many satellites in the solar system are like that. However, others are weirder-looking. The two moons of Mars, Phobos and Deimos, look more like small, irregularly shaped asteroids. In fact, they probably are captured asteroids or debris from an ancient collision between Mars and another body.

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. This raises an important question: are bits of material that make up the rings of the outer planets considered moons? It's a good one to ask and planetary scientists are working on coming up with a good definition to cover these objects. Currently, chunks of ice and rock and dust that make up rings are considered solely part of the rings, and are not individual moons. But, hidden within those rings are objects that really are moons, and they play a role in keeping the ring particles in line.

Are All Moons Really Moons?

Interestingly enough, not all moons orbit planets. Nearly 300 asteroids (or minor planets) are known to have moons of their own. There are also objects 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 apparently demoted from planet status in 2006 to dwarf planet status (still a topic of discussion among planetary scientists). 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. Charon is a weird case however, since it has 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 scientists 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. 

Do Moons Exist in Other Solar Systems?

As astronomers find planets around other stars, it's clear from the evidence in our own solar system that there are likely moons orbiting around those other worlds, too. The planets themselves are hard to find, so a moon would be pretty difficult to spot with our current technology. But that doesn't mean they're not there; just that we'll have to look extra hard and use innovative techniques to find them.

Edited and updated by Carolyn Collins Petersen.

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Millis, John P., Ph.D. "What Is the Definition of a Moon?" ThoughtCo, Jul. 14, 2017, thoughtco.com/what-is-a-moon-3073438. Millis, John P., Ph.D. (2017, July 14). What Is the Definition of a Moon? Retrieved from https://www.thoughtco.com/what-is-a-moon-3073438 Millis, John P., Ph.D. "What Is the Definition of a Moon?" ThoughtCo. https://www.thoughtco.com/what-is-a-moon-3073438 (accessed November 20, 2017).