Planets have moons because gravity gives them the ability to collect and hold onto smaller objects. But there’s no single reason every moon exists. Some formed alongside their planet from the same swirling cloud of debris, some were captured from elsewhere in space, and at least one (our own Moon) was born from a catastrophic collision. The story of why planets have moons is really three different stories, each with its own physics.
Three Ways a Planet Gets a Moon
Every moon in the solar system arrived by one of three routes: it grew in place, it was captured, or it was blasted into existence by an impact.
The gas giants, Jupiter and Saturn, built most of their moons the same way the Sun built its planets. As these massive worlds formed, they were surrounded by disks of leftover gas and dust. Within those disks, particles collided, stuck together, and gradually grew into the moons we see today. These are called regular moons, and they share a telltale signature: they orbit close to their planet, follow nearly circular paths, and travel in the same direction the planet spins. Saturn alone has 274 confirmed moons as of 2025, many of them small objects that formed or were swept up from this process.
Capture works differently. A stray object drifting through space passes close enough to a planet that gravity snags it into orbit. The best example is Neptune’s largest moon, Triton. Triton orbits Neptune backward (retrograde), tilted 157 degrees from Neptune’s equator. That reversed orbit rules out the possibility that it formed around Neptune. Instead, Triton was likely a Kuiper Belt object that Neptune pulled in. After capture, tidal forces slowly reshaped its wild, elongated orbit into the nearly perfect circle it traces today, a process that took roughly 200 million years.
Then there’s the violent route. Earth’s Moon almost certainly formed when a Mars-sized body slammed into the young Earth at a roughly 45-degree angle. The collision ejected a massive cloud of molten and vaporized rock into orbit, heated to between 3,000 and 4,000 degrees. That orbiting debris disk gradually clumped together just beyond the point where Earth’s tidal forces would have torn it apart. Simulations suggest the Moon’s assembly took about a hundred years after the impact, and nearly all the disk material ended up either in the Moon or pulled back down to Earth, with less than 5% escaping into space.
What Makes a Planet Capable of Holding Moons
Having a moon isn’t just about forming or capturing one. A planet also has to keep it. Every planet has a region of gravitational dominance called its Hill sphere, the zone where its gravity wins out over the Sun’s pull. The Hill sphere depends on the planet’s mass and its distance from the Sun. A moon’s orbit needs to stay well within this zone, typically inside the inner half, or the Sun will eventually tug it away.
This is why location matters enormously. Mercury and Venus have zero moons, and the primary reason is their closeness to the Sun. The Sun’s gravitational influence shrinks their Hill spheres so severely that holding a moon in a stable long-term orbit is essentially impossible. Even if either planet somehow captured a small body, the Sun would strip it away over time. Their proximity to the Sun also makes capture events far less likely in the first place.
On the other end of the spectrum, Jupiter and Saturn sit far from the Sun with enormous masses, giving them vast Hill spheres. They can hold moons at tremendous distances. Many of their outermost moons, the irregular ones, follow large, tilted, elliptical orbits and often travel backward. These are captured objects, pulled in from the debris of the early solar system, and the giant planets’ huge gravitational reach is what allows them to keep such loosely bound companions.
The Roche Limit: Where Moons Can’t Exist
There’s also a minimum distance. Get too close to a planet and tidal forces will rip a moon apart or prevent one from forming at all. This boundary is called the Roche limit, and it varies depending on the density of the moon. Inside the Roche limit, a planet’s gravity pulls harder on the near side of an orbiting body than the far side, stretching it until it breaks. This is exactly why Saturn’s rings exist where they do: the dense ring material sits inside Saturn’s Roche limit, where it can never clump into a moon.
Outside the Roche limit, the opposite happens. Loose material tends to accrete into solid bodies. So moons form and survive in a specific band: outside the Roche limit but inside the Hill sphere. Mars illustrates this nicely. Its moon Phobos orbits just outside the Martian Roche limit and is slowly spiraling inward. Eventually, tidal forces will either shatter it into a ring or drag it into Mars.
Why Our Moon Is Unusual
Earth’s Moon is strange compared to every other moon in the solar system. It’s about 1% the mass of Earth, which sounds small until you compare it to the giant planets, whose combined moons total roughly 0.01% of the planet’s mass. That’s a hundred-fold difference in the moon-to-planet ratio, and it’s a direct consequence of how the Moon formed. A giant impact can concentrate far more material into orbit than a circumplanetary disk or a capture event.
The Moon also carries chemical fingerprints of its violent birth. Its iron content is only about 8% by mass, much lower than Earth’s, because the impact mostly ejected lighter mantle rock rather than the iron-rich cores of either body. And its stable isotope ratios closely match Earth’s mantle, which makes sense if the Moon assembled from material that was thoroughly mixed and vaporized during the collision.
Today, the Moon is still drifting away from Earth at about 4 centimeters per year. This happens because Earth’s ocean tides, raised by the Moon’s gravity, create a gravitational tug that adds energy to the Moon’s orbit. That same interaction slows Earth’s rotation. Billions of years ago, days were significantly shorter. The Moon was closer, and tides were stronger. This ongoing exchange of energy is also why the Moon is tidally locked, always showing us the same face. Early in its history, tidal friction within the Moon burned off its rotational energy until its spin synchronized with its orbit.
Even Asteroids Can Have Moons
You don’t need to be a planet to have a moon. Over 400 asteroids are known to have small companions. These tiny systems form when an asteroid spins fast enough that it starts shedding material from its surface. The ejected debris forms a disk around the asteroid, and chunks within that disk gradually merge through collisions and gravitational attraction, building a small satellite. The resulting moons tend to be elongated rather than round, shaped by the tidal pull of the larger asteroid as they grow. They typically settle into synchronous orbits, keeping one face toward the primary body, much like our Moon does with Earth.
Dwarf planets get moons too. Pluto has five. Eris has one. The same basic physics applies at every scale: if an object has enough mass relative to its surroundings and enough gravitational room to work with, smaller bodies can end up orbiting it.
Why Some Planets Have Hundreds and Others Have None
The number of moons a planet holds comes down to three factors working together: mass, distance from the Sun, and history. Jupiter and Saturn are massive and far from the Sun, giving them enormous gravitational reach and billions of years of opportunities to capture passing objects. Their early circumplanetary disks also produced large regular moons. Mars, smaller and closer to the Sun, managed to hold onto only two tiny moons, likely captured asteroids. Mercury and Venus, hugging the Sun, kept none.
Earth’s single large Moon required a specific, low-probability event: a giant impact at the right angle and speed. Without that collision 4.5 billion years ago, Earth might be moonless too. The existence of moons isn’t inevitable. It depends on the right combination of gravity, geometry, and chance.