Why Don’t Moons Have Moons?
Planets orbit stars. Moons orbit planets. So why doesn’t anything orbit a moon? The answer is a fun cosmic tug-of-war — and it turns out a moon could have a “submoon,” even if none has ever been found. Here’s the science, simplified.
A moon usually can’t keep its own moon because the planet’s gravity is far stronger than the moon’s. Anything trying to orbit a moon gets pulled around by the planet until its orbit becomes unstable — so it crashes into the moon, escapes to orbit the planet, or is flung away. A moon of a moon is called a “submoon.” Surprisingly, a few large, far-out moons (like our own) could theoretically hold a small submoon — but none has ever been found.
It’s the kind of question a curious child asks — and in fact, this exact question was sparked by a scientist’s four-year-old son. If planets orbit stars and moons orbit planets, why don’t moons have their own little moons? The simple answer comes down to gravity and stability. Let’s unpack it step by step, in plain language.
In space, gravity is a game of “who pulls hardest.” A moon trying to hold onto its own moon is like a child trying to keep a balloon while standing next to a giant fan — the bigger force almost always wins. — Legacy IAS Faculty
The Simple Reason — The Planet’s Gravity Wins
Every object in space pulls on every other object with gravity. A moon does have its own gravity — but the planet it orbits is much bigger and much stronger. So if a small object tries to settle into orbit around a moon, the planet keeps tugging at it. Over time, that constant interference makes the orbit wobble and break down. The little object ends up doing one of three things:
Crashes
It falls onto the moon’s surface.
Switches Bosses
It escapes the moon and starts orbiting the planet instead.
Gets Ejected
It’s flung out of the system entirely.
The result: no stable moon-of-a-moon. That’s the short version — but the full story is even more interesting.
First, the Name: What Is a “Submoon”?
A moon that orbits another moon has a few playful nicknames — submoon, “moonmoon,” or “mini-moon” — but scientists settled on submoon. The whole question shot to fame in 2018 when astronomer Juna Kollmeier’s young son asked her, simply, “Can moons have moons?” She and fellow scientist Sean Raymond decided to actually work out the answer — and published it in a respected astronomy journal.
The Two Invisible Boundaries — A Cosmic “Goldilocks Zone”
For a submoon to survive, it has to orbit inside a very narrow safe band around its moon — not too far, not too close:
The Outer Edge: Hill Sphere
The Hill sphere is the bubble around a moon where its gravity beats the planet’s. A submoon must stay inside this bubble. For most moons, this bubble is tiny — sometimes barely bigger than the moon itself.
The Inner Edge: Roche Limit
Get too close to the moon and tidal forces tear the submoon apart — this danger line is the Roche limit. A submoon must orbit beyond it.
So a submoon must orbit outside the Roche limit but inside the Hill sphere — a “just right” zone. For a small moon, or a moon close to its planet, this safe zone is squeezed almost to nothing — leaving no room for a stable submoon. That’s the case for most moons in our solar system.
Why Tides Slowly Ruin Everything
Even when a submoon fits in the safe zone, tides work against it over millions of years. Here’s the simple idea: the side of a moon facing its submoon feels slightly stronger gravity than the far side, creating a small “stretch” or bulge. This tidal tug-of-war steadily drains energy from the submoon’s orbit — gradually pulling it off course until it crashes or escapes. To survive billions of years, a submoon needs a big, roomy safe zone that tides can’t easily destabilise — which only the largest, most distant moons have.
The Surprise: A Few Moons COULD Have Submoons
Here’s the twist. The 2019 study found that large moons orbiting far from their planet actually could hold a small submoon (around 10-20 km wide) for billions of years. A handful of real moons make the cut:
| Moon | Planet | Could Host a Submoon? |
|---|---|---|
| The Moon | Earth | Yes — could hold a submoon ~20 km or larger. |
| Callisto | Jupiter | Yes — large and far enough out. |
| Titan | Saturn | Yes — one of the best candidates. |
| Iapetus | Saturn | Yes — distant orbit helps. |
| Triton | Neptune | Only a tiny one (under ~5 km). |
Despite being theoretically possible, not a single submoon has ever been found — not even around the moons that could support one. That mystery is actually useful to scientists: the absence of submoons offers clues about how planets and moons formed in the first place.
Why Earth’s Moon Never Got One
Our Moon could theoretically hold a submoon today — so why doesn’t it? The likely reason is timing. Billions of years ago, the Moon formed much closer to Earth. Back then, its safe zone (Hill sphere) was so tiny it was practically inside the Moon — no room for a submoon to form. By the time the Moon slowly drifted far enough out to have a roomy safe zone, it was simply too late — there was nothing left nearby to capture.
Bonus: Earth & the Moon Are Almost a “Double Planet”
Here’s a beautiful detail. Earth doesn’t sit still while the Moon circles it. Both bodies actually orbit a shared centre of mass called the barycentre — like two dancers spinning around a common point. Because our Moon is unusually large (about a quarter of Earth’s width), this point is noticeably off-centre:
The Earth-Moon barycentre lies about 1,700 km beneath Earth’s surface — still inside the Earth. Because it stays inside our planet, the pair isn’t officially called a “binary planet” — but it’s one of the closest examples in the solar system. Contrast this with Pluto and its moon Charon: their barycentre lies in open space between them, so they genuinely qualify as a “double” (binary) system — the two wobble around a point out in the void.
Did You Know? — Fun Cosmic Facts
Temporary “Mini-Moons”
Earth occasionally captures tiny asteroids as short-lived extra moons for a few months or years before they drift away.
The Moon Is Drifting Away
Our Moon moves about 3.8 cm farther from Earth every year — roughly the speed your fingernails grow.
Tidal Locking
Tides are also why we only ever see one face of the Moon — it’s “locked” to spin once per orbit.
An Oversized Moon
Our Moon is unusually big for its planet — which is exactly why the Earth-Moon dance is so strongly coupled.
Three keywords worth remembering: Barycentre (shared centre of mass two bodies orbit), Hill sphere (zone where a body’s gravity dominates for holding satellites), and Roche limit (distance within which tidal forces tear a body apart). These concepts also explain planetary rings, tidal locking, and binary systems like Pluto-Charon.
Frequently Asked Questions
Q1. Can a moon have its own moon?
In theory, yes — it’s called a submoon. But it can only survive around a large moon orbiting far from its planet, where the moon’s gravity is strong enough to hold it. In practice, no submoon has ever been discovered in our solar system.
Q2. What is a submoon called?
A moon orbiting another moon is most commonly called a “submoon.” Playful alternatives like “moonmoon” and “mini-moon” were suggested, but submoon is the term scientists generally use.
Q3. Why can’t most moons hold a submoon?
Because the planet’s gravity dominates, the “safe zone” (Hill sphere) around most moons is too small, and tidal forces gradually destabilise any submoon’s orbit. Only large, distant moons — like Earth’s Moon, Titan, Callisto and Iapetus — have a big enough safe zone in principle.
Q4. What is the barycentre of the Earth and Moon?
It’s the shared centre of mass that both Earth and the Moon orbit. For the Earth-Moon system it lies about 1,700 km below Earth’s surface — inside the Earth — which is why the pair isn’t officially a binary planet, unlike Pluto and Charon whose barycentre lies in space between them.
Key Takeaways
- The simple reason: a planet’s gravity is far stronger than a moon’s, so anything orbiting a moon gets destabilised — it crashes, escapes, or is ejected.
- A moon’s moon is a “submoon” — a question famously sparked by a scientist’s young son and studied in 2019.
- Two boundaries matter: a submoon must orbit inside the moon’s Hill sphere (gravity safe zone) but beyond the Roche limit (tidal break-up line) — a tight “Goldilocks” band.
- Tides slowly drain a submoon’s orbital energy, so only large, distant moons could keep one for billions of years.
- A few moons could host submoons in theory — Earth’s Moon, Titan, Callisto, Iapetus — yet none has ever been found.
- Earth & the Moon orbit a shared barycentre ~1,700 km below Earth’s surface, making them almost a “double planet” (unlike Pluto-Charon, which truly are).
