March 20, 20255 min readWhat Makes a Moon?Defining the word moon is harder than you might thinkBy Phil Plait edited by Lee BillingsA quintet of Saturns moons come together in this image from NASAs Cassini spacecraft. NASA/JPL-Caltech/Space Science InstituteHeres a simple question: Whats a moon?As with so many questions in science, it may seem straightforward but truly isnt. Why, a moon is a celestial body that orbits a planet, youre probably thinking. Well, sureif you squint your eyes and dont look too closely, thats a pretty decent description.But rigidly defining the term moon isnt so easy.On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.The canonical example is of course our own moon, a decently big chunk of rock that orbits Earth. But centuries ago the first telescopic observations of other planets revealed that many have moons as well; Jupiter has four giant, easily seen satellites, and Saturn has several that are visible by modest means as well. So at that point in time, our definition of moon seemed safe enough.Then, of course, things got complicatedbecause they always do. As telescopes got bigger and better, more moons were found. Mars has two, and poor Mercury and Venus have none, but in contrast, moons seemingly kept sprouting on Jupiter like mushrooms after a rainstorm. For the first half of the 20th century, Jupiter was known to have an even dozen. A handful more were found telescopically in the 1970s, and the numbers jumped a bit when we started sending spacecraft to the outer planets. Then, in the 2000s, the numbers leaped upward as more exacting techniques were used to scrutinize Jupiters environs.As of this writing Jupiter has 95 confirmed moons. They range in size from mighty Ganymede, the largest in the solar system at more than 5,200 kilometers acrosswider than the planet Mercury!to the tiniest that were able to see from Earth, at roughly 1 km in diameter. Saturn is more distant from us than Jupiter, so its moons are harder to see, yet we now know it boasts at least 274 moons, a staggering number! Of these, 128 were just announced this month by scientists who had used an advanced searching technique that allows extremely faint satellites to be spotted in telescopic observations. Most of these new additions are only a few kilometers across.Its clear that with ever more powerful equipment, well find that many planets have orbital companions of arbitrarily small dimensions. Is something the size of a football stadium a moon? Sure! But what about something the size of a car, a basketball or a grape? What about a grain of dust?Saturns rings are composed of trillions of small icy particles. Is each of these a moon?At some lower limit, that term just doesnt seem to fit.The problem is further complicated by the fact that many asteroids have moons. More than 430 asteroids are known or suspected to be orbited by smaller asteroids. Its possible that those satellites were formed from low-speed collisions that either ejected material that subsequently coalesced as moons or slowed two asteroids enough to put them in orbit around each other. In some cases an asteroid and its moons may have even formed together.Out past Neptune are countless small icy and rocky bodies called Trans-Neptunian Objects (TNOs), and many of these have moons as well. While some TNOs could be called dwarf planets because of their size, many more are tiny and dont even come close to falling into that category.And, although I hate to complicate things even more, I should note that if we broaden our moon definition to any object that orbits something bigger, then planets are moons. Even small stars that orbit big stars would be moons!Clearly were running into trouble trying to hang the word moon on these objects.Theres also the problem that the arguments for what makes a moon can change when viewed differently. For example, the suns gravity tugs harder on the moon than Earths does! So does the moon orbit the sun or our planet? Well, the trick here is that both Earth and the moon orbit the sun together. The suns effect on both is equal, so in a sense it cancels out, and therefore the moon orbits our own planet more than it orbits the sun.Theres actually mathematical support for this. Its possible to balance out the equations of gravity for a planet and star, including the centrifugal force created by the orbiting body, to see how far a planets sphere of influence stretches. This area, where the planets gravity locally dominates over the star, is also called the Hill sphere, after American astronomer George Hill, who first derived it. The Hill sphere for Earth, given its distance from the sun, is about 1.5 million km. The moon is only about 385,000 km from Earth, so its well inside our Hill sphere and therefore more under Earths influence than the suns.This region of space grows larger the farther a planet is from its star. Jupiter is more massive than Saturn, but Saturn is farther from the sun, so its Hill sphere is nearly twice the volume of Jupiters. That may be why weve found so many more moons orbiting Saturn even though its more distant from Earth and thus harder to search for companions.Neptune is so far from the sun that its Hill sphere is the largest of all the planets. Its possible that Neptune has far more moons than Jupiter or Saturn, and we just havent found them yet because theyre too faint to easily see from Earth.This still leaves us with some interesting edge cases. Pluto has five known moons. The largest, Charon, was discovered in 1978. Its roughly half the diameter of Pluto and has about one eighth its mass. Because of this, Charon doesnt so much orbit Pluto as they both orbit their barycenter, a mutual center of mass. This is like two people on a seesaw; the balance point is closer the person who weighs more. The Pluto-Charon barycenter is actually outside the body of Pluto itself! So is Charon a moon? Or is it more that they both comprise a binary planet?Even trickier, its possible for moons to have moons! Similar to the Hill sphere argument for planets, some moons can have a large enough sphere of influence to potentially possess moons of their own. What do we call these? Some people argue for the term moonmoon, which is delightful but somewhat imprecise. I prefer submoon.Dont even get me started on quasi-moons.In the end, the problem lies in our preference for straightforward simplicity rather than complex nuance; rigidly defining the term moon is hopeless because its not definable. Its a concept more than a definition, much like the term planet. Perhaps thats why the International Astronomical Union, the official keeper of celestial names and definitions, doesnt have a definition for what makes a moon.Humans like to put things in distinctive bins, but nature is not so prejudiced. Whenever objects fall into a range, a spectrum of characteristics, the transition along that spectrum tends to be smooth, and trying to wedge them into defined borders winds up generating more exceptions than rule-followers.Sometimes its best to accept something for what it is and not how it falls into our narrow classifications. You can understand it better that way, and isnt that the point?