The Moon Test: What Qualifies as a Lunar Body?

Pop quiz: What do Europa, Titan, Phobos, and our own Moon have in common?

If you said, "They're all moons," you're right.

But here's the thing: these objects couldn't be more different from each other.

Europa is a smooth ice ball that might hide an entire ocean beneath its frozen surface. Titan has lakes of liquid methane and a thick atmosphere. Phobos is an ugly, potato-shaped chunk of rock that looks like someone forgot to finish sculpting it. Our Moon is a gray, crater-pocked sphere that's been orbiting Earth for 4.5 billion years.

So what makes all of these incredibly different objects "moons" instead of just random space rocks floating around?

The answer is simpler than you might think, but it gets complicated fast. Because as it turns out, there are regular moons, irregular moons, quasi-moons, mini-moons, trojan moons, and even double moons where it's not clear which one is the moon and which is the planet.

Welcome to the surprisingly messy world of lunar classification, where the definition of "moon" depends on who you ask and where the object happens to be hanging out in space.

The Simple Answer (That Isn't Quite Right)

Ask most people what a moon is, and they'll say something like: "A moon is a natural object that orbits a planet." That's a decent starting point. It captures the basic idea: moons are natural satellites (not artificial like the International Space Station), and they orbit something bigger than themselves (usually a planet).

But this simple definition immediately runs into problems:

Problem 1: What about moons that orbit dwarf planets? Pluto has five moons, but Pluto isn't classified as a planet. Does that make Charon (Pluto's largest moon) not a moon?

Problem 2: What about moons that orbit asteroids? The asteroid Didymos has a moon called Dimorphos. Is Dimorphos a moon even though it orbits a rock just 2,500 feet across?

Problem 3: What about objects that kind of act like moons but aren't really orbiting the planet? Earth has quasi-moons (like Zoozve) and mini-moons (like 2024 PT5) that look like moons from certain perspectives but are actually orbiting the Sun.

Problem 4: What if two objects orbit each other and they're roughly the same size? Is one the moon and one the planet, or are they a binary system?

See? It gets messy fast.

The Better Answer: Natural Satellites

Here's a more accurate definition used by astronomers:

A moon (also called a natural satellite) is a naturally occurring celestial body that orbits a planet, dwarf planet, or other larger body that itself orbits the Sun (or another star).

Let's break down what this means:

Requirement 1: It Must Be Natural

This rules out artificial satellites. The International Space Station orbits Earth, but it's not a moon because it's human-made. The Hubble Space Telescope? Satellite, not a moon. Communications satellites? Also, not moons.

All those thousands of objects humans have launched into orbit around Earth, other planets, and even asteroids are satellites, but they're not moons because they didn't form naturally.

Requirement 2: It Must Orbit Another Body

A moon has to go around something. That something is called its "primary" or "parent body."

Our Moon orbits Earth. Titan orbits Saturn. Europa orbits Jupiter. Phobos orbits Mars. Charon orbits Pluto. If an object is just floating through space on its own path around the Sun, it's not a moon. It might be an asteroid, a comet, or a planet, but it's not a moon.

Requirement 3: The Primary Must Orbit the Sun (Or Another Star)

This is what separates moons from planets. Planets orbit the Sun directly. Moons orbit planets (or dwarf planets or asteroids), which themselves orbit the Sun.

So there's a hierarchy:

• The Sun sits at the center

• Planets orbit the Sun

• Moons orbit the planets

• (And technically, the whole solar system orbits the center of the Milky Way, but that's another level entirely)

This means the Death Star from Star Wars, despite the name, is NOT a moon. It's an artificial space station, and even if it were natural, it would need to orbit a planet that orbits a star.

Requirement 4: There's No Minimum Size

Surprisingly, there's no official minimum size for a moon. The smallest confirmed moon in our solar system is barely a kilometer across. Some objects within Saturn's rings that might qualify as tiny moons are even smaller.

If it orbits a larger body naturally, it can be a moon, even if it's just a small rock.

How Moons Form: Three Origin Stories

Moons don't all form the same way. In fact, there are three main origin stories for how moons come to be:

Origin Story 1: Born Together

Many moons formed at the same time as their planet from the same cloud of gas and dust.

Imagine the early solar system as a huge rotating disk of material around the young Sun. Within that disk, smaller clumps of material started accumulating through gravity. The biggest clump became a planet. Smaller clumps nearby became moons orbiting that planet.

Jupiter's major moons (Io, Europa, Ganymede, and Callisto) probably formed this way, growing out of the same protoplanetary disk that formed Jupiter itself. They're called "regular moons" because they orbit in neat, predictable paths that match Jupiter's rotation.

Origin Story 2: Captured

Some moons started life as asteroids or Kuiper Belt objects minding their own business, orbiting the Sun. Then they wandered too close to a planet and got captured by that planet's gravity.

These "irregular moons" typically have weird orbits. They might orbit in the opposite direction from the planet's rotation (called retrograde), or at strange angles, or in elongated ellipses rather than circles.

Neptune's large moon Triton is probably a captured Kuiper Belt object (where Pluto lives). Mars's two tiny moons, Phobos and Deimos, were likely captured asteroids from the nearby asteroid belt.

Origin Story 3: Giant Impact

A few moons formed from catastrophic collisions.

The most famous example is our own Moon. Scientists believe that about 4.5 billion years ago, a Mars-sized object called Theia smashed into the young Earth. The impact was so violent that it blasted huge amounts of material from Earth's outer layers into orbit. Over time, this debris coalesced into the Moon.

This explains some weird things about the Moon:

• It's unusually large compared to Earth (most moons are much smaller relative to their planets)

• Its composition is similar to Earth's outer layers

• It's moving slowly away from Earth over time (currently drifting about 1.5 inches per year)

Pluto's largest moon, Charon, might have formed through a similar giant impact.

The Weird Moons: When the Rules Get Fuzzy

Now that we've established what a "normal" moon is, let's talk about the weird ones that break or bend the rules.

Quasi-Moons: The Pretenders

Remember Zoozve? It's an asteroid that orbits the Sun, not Venus. But from Venus's perspective, Zoozve kind of looks like it's orbiting Venus because their orbits are synchronized in a weird way.

Quasi-moons aren't real moons. They're asteroids that happen to follow orbits that mimic a moon's behavior for a while (sometimes hundreds or thousands of years) before moving on.

Earth has quasi-moons too. They're not orbiting Earth; they're orbiting the Sun in a pattern that makes them appear to circle Earth.

Mini-Moons: The Temporary Visitors

Sometimes a small asteroid gets temporarily captured in Earth's gravity for a few months or years before escaping back into its own orbit around the Sun.

Earth recently acquired a mini-moon called 2024 PT5. It's only 33 feet across and only hung around for 57 days before continuing on its cosmic journey.

Mini-moons are technically moons while they're captured (they're natural objects orbiting Earth), but they're not permanent fixtures like our Moon.

Trojan Moons: The Tag-Alongs

These are moons that share the same orbit as another, larger moon but stay 60 degrees ahead or behind it at special gravitational balance points called Lagrange points.

Saturn has trojan moons! Telesto and Calypso are trojan moons that follow the same orbit as Saturn's moon Tethys. They're all orbiting Saturn, but Telesto and Calypso are hitchhiking in Tethys's orbit.

So these are moons (orbiting Saturn) that are also trojans (sharing another moon's orbit). Trojan moons!

Binary Systems: Who's the Moon?

When two objects of similar size orbit each other, it gets philosophically tricky to decide which is the moon and which is the planet (or asteroid).

Technically, both objects orbit a point between them called the barycenter. If that barycenter is inside the larger object, we call the larger one the primary and the smaller one the moon. If the barycenter is in the space between them, some scientists argue they're a binary system, not a planet-moon system.

Pluto and Charon are a good example. Charon is big enough (about half Pluto's diameter) that the barycenter is above Pluto's surface. Some scientists call this a binary dwarf planet system rather than saying Charon is Pluto's moon.

But officially, Charon is still classified as a moon. The debate continues.

There's also an asteroid called 90 Antiope where both components are basically the same size. Is one the asteroid and one the moon? Or are they both asteroids in a binary system? Scientists can't quite agree.

The Numbers: How Many Moons Are There?

As of 2025, astronomers have identified 891 confirmed moons in our solar system.

That breaks down roughly as:

• 421 moons orbiting the eight major planets

• Over 470 moons orbiting dwarf planets, asteroids, and trans-Neptunian objects

Some highlights:

Jupiter holds the record with over 90 known moons (and the number keeps growing as we discover more tiny ones).

Saturn is close behind with over 80 confirmed moons, plus countless moonlets in its rings.

Earth, Mars, Neptune, and Uranus have much more modest moon counts (1, 2, 14, and 27 respectively).

Mercury and Venus have zero moons. Scientists think this is because they're too close to the Sun. Any potential moons would either get pulled into the Sun or pulled away from the planet.

Seven dwarf planets have known moons: Pluto (5), Haumea (2), Makemake (1), Eris (1), Gonggong (1), Quaoar (2), and Orcus (1).

Dozens of asteroids have their own moons, proving you don't need to be planet-sized to have a moon.

The Biggest, The Smallest, The Weirdest

Some notable moons worth knowing:

Biggest: Ganymede (Jupiter's moon) is 3,270 miles across, making it larger than the planet Mercury. It's so big it would be a planet if it orbited the Sun directly.

Smallest known: S/2009 S 1 (one of Saturn's moons) is estimated to be less than 1,000 feet across. It's basically a rock with pretensions.

Most volcanic: Io (Jupiter's moon) has over 400 active volcanoes, making it the most geologically active body in the solar system. Volcanic plumes shoot 190 miles into space.

Weirdest orbit: Triton (Neptune's moon) is the only large moon that orbits backward (retrograde) relative to its planet's rotation. It was definitely captured.

Most Earth-like: Titan (Saturn's moon) is the only moon with a thick atmosphere. It has lakes, rivers, and rain, except they're made of liquid methane instead of water.

Most likely to hide life: Europa (Jupiter's moon) probably has a liquid water ocean beneath its icy surface, making it one of the best places to look for extraterrestrial life.

Why It Matters What We Call Them

You might think, "Who cares whether we call something a moon or an asteroid or a quasi-moon? It's just a label." But classifications matter in science because they help us organize knowledge, make predictions, and communicate clearly.

If we call something a moon, we're saying:

• It formed or was captured in a certain way

• It's gravitationally bound to a larger body

• It probably has certain characteristics (composition, orbit, etc.)

• It can teach us about the history of its parent planet

Understanding what makes a moon a moon helps scientists figure out:

• How planetary systems form and evolve

• Where to look for resources (water ice, metals, etc.)

• Which moons might harbor life

• How gravity shapes the solar system

And honestly, it's just cool to know that when you look up at the Moon in the night sky, you're seeing an object that's part of a huge family of 891+ moons throughout our solar system, each with its own story of how it came to orbit its parent world.

The Bottom Line

So what makes a moon a moon and not just some random rock?

The simple answer: A moon is a natural object that orbits a planet, dwarf planet, or asteroid (which itself orbits the Sun).

The complete answer: It's a natural satellite formed through accretion alongside its planet, captured from another orbit, or created through a giant impact. It can be as small as a kilometer or larger than Mercury. It might have a regular circular orbit or a weird retrograde ellipse. It could be solid rock, an ice ball, or even have its own atmosphere and lakes. Some are barely distinguishable from asteroids, while others are worlds unto themselves.

But they all share one thing: they orbit something bigger than themselves, and in doing so, they become part of the gravitational dance that makes our solar system work.

Not just some random rock. A moon.

Sources

National Geographic Education. (2025). Moon. Retrieved from https://education.nationalgeographic.org/resource/moon/

National Air and Space Museum. (2024). What Makes a Moon a Moon? Retrieved from https://airandspace.si.edu/stories/editorial/what-makes-moon

NASA Solar System Exploration. (2025). Moons. Retrieved from https://solarsystem.nasa.gov/moons/overview/

Study.com. (2025). Solar System Moons | List, Names & Sizes. Retrieved from https://study.com/academy/lesson/solar-system-moons-list-names-sizes.html

The Solar System Wiki. (2026). Natural Satellite. Retrieved from https://thesolarsystem.fandom.com/wiki/Natural_Satellite

UCSB Science Line. What makes a moon a moon? Retrieved from http://scienceline.ucsb.edu/getkey.php?key=6863

Wikipedia. (2026). Minor-planet moon. Retrieved from https://en.wikipedia.org/wiki/Minor-planet_moon

Wikipedia. (2026). Natural satellite. Retrieved from https://en.wikipedia.org/wiki/Natural_satellite