A moon is a natural object that orbits a planet, a dwarf planet, or even an asteroid. Astronomers also call them natural satellites, and they’re held in their orbits by the gravity of the larger body they circle. Our own Moon is the most familiar example, but it’s far from alone. Our solar system is home to more than 890 known moons, ranging from tiny lumpy rocks to worlds larger than the planet Mercury.
When most people hear “moon,” they picture our single silvery companion. But moons are a wildly varied and surprising bunch, and some of them are among the most fascinating places in the entire solar system. Let’s take a look at what a moon really is, how many there are, and where they all came from.
The Simple Definition
At its core, a moon is any natural object that orbits something bigger than itself, as long as that bigger thing isn’t a star. If an object orbits the Sun directly, we call it a planet, dwarf planet, or asteroid. But if it orbits one of those, it’s a moon.
The key word is “natural.” Spacecraft and other human-made objects circling Earth are also technically satellites, but we call those artificial satellites to tell them apart from the real thing. A moon is the work of nature, held in place by the same force that keeps you on the ground: gravity.
How Many Moons Are There?
Far more than you’d think. As of recent counts, our solar system has over 890 confirmed moons, and the number keeps climbing as telescopes spot ever-smaller ones. Here’s how they’re spread across the planets.
| Planet | Known moons |
|---|---|
| Mercury | 0 |
| Venus | 0 |
| Earth | 1 |
| Mars | 2 |
| Jupiter | 95+ |
| Saturn | 270+ (the most of any planet) |
| Uranus | 28+ |
| Neptune | 16+ |
Saturn currently holds the crown with the most moons of any planet, as our guide to how many moons Saturn has explains, though Jupiter isn’t far behind, as you can see in our Jupiter moon count. And it’s not just planets: even some asteroids and dwarf planets have their own tiny moons. Pluto alone has five.
Moons Come in Astonishing Variety
This is where moons get truly interesting, because “moon” covers an enormous range of worlds.
At the small end, some moons are just lumpy chunks of rock only a few kilometers across, like Mars’s two little moons, Phobos and Deimos. At the large end, Jupiter’s moon Ganymede and Saturn’s moon Titan are each bigger than the planet Mercury. If they orbited the Sun instead of a planet, we’d probably call them planets in their own right.
And they’re not just dead balls of rock. Jupiter’s moon Io is the most volcanically active world in the solar system, erupting constantly. Several moons, including Europa and Enceladus, are thought to hide vast oceans of liquid water beneath their icy crusts, which makes them some of the most promising places to search for alien life. Titan even has a thick atmosphere and lakes of liquid methane on its surface. Moons, it turns out, can be worlds every bit as complex and dramatic as the planets they circle.
Where Do Moons Come From?
Moons are born in a few different ways, and scientists have identified three main origin stories.
Most large moons formed right alongside their planet, condensing out of the same disk of gas and dust that built the planet itself in the early solar system. A second group are captured objects: rogue asteroids or icy bodies that wandered too close to a planet and got snared by its gravity. Neptune’s big moon Triton and Mars’s two small moons are likely captives like this. And then there’s the most dramatic origin of all, a giant impact, which is exactly how our own Moon is thought to have formed.
Our Own Moon
Our Moon is a special case. It probably formed about 4.5 billion years ago when a Mars-sized world slammed into the young Earth, blasting debris into orbit that gradually clumped together into the Moon we see today. It’s unusually large compared to its planet, and like many moons, it’s tidally locked, meaning it always shows us the same face. Our guide to what makes our Moon so special digs into why our companion is such a standout. For the full solar-system tally, NASA’s moon facts page keeps a running count.
So the next time you look up at the Moon, remember it’s just one member of a huge and diverse family. Across the solar system, hundreds of moons are circling their planets right now, some frozen, some volcanic, some hiding oceans, each one a world with its own story.
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