[♪ INTRO].

The basic science behind orbits is simple enough: lighter things orbit more massive things, and more massive things orbit even more massive things. For example, our moon orbits the Earth, the Earth orbits the Sun, and the Sun orbits the center of our galaxy.

But what if you extend the logic in the other direction? All objects with mass exert some amount of gravitational force. Probably your stapler isn’t going to orbit your desk, but it stands to reason that anything with enough gravity could then have something orbit it — including a moon having its own moon.

But it is a lot less likely. The main problem is that there are already a bunch of other massive things around, exerting their own gravitational pulls. At the very least, there’s the host planet and a star that planet orbits.

Because of the complex gravity going on here, the most likely way a moon could get its own moon is by capturing it — for example, if an asteroid took just the right path to fall into orbit around it, despite the strong pulls from the host planet and central star. But even if a moon obtained its own little moon, the planet could destabilize its orbit enough to make it collide with the main moon or the planet. That, or it could send the little guy off into space.

This is probably why we don’t see small moons around any normal moons in our solar system. It’s not impossible; it’s just unlikely that the sub-moon’s orbit would last very long. But!

With enough distance between a moon and its host planet, a large enough moon, and a small enough mini-moon, there’s no theoretical reason why this can’t happen somewhere. When a moon is far enough away, the planet no longer has such a strong effect on what’s happening around that moon. And if the moon’s moon is small, while the parent moon is large, it’s basically only affected by the massive thing it’s actually close to.

That’s why the artificial satellites we put into orbit around our moon don’t get thrown off by the Earth — there’s enough distance, and the satellites are relatively tiny compared to the moon. In fact, Jupiter’s moon Callisto, Saturn’s moons Titan and Iapetus, and, even our own moon — all fit the conditions to have maybe had little moons of their own once upon a time. More promising still, researchers recently discovered that a planet outside the solar system, called Kepler-1625b, might have a moon.

And that moon may be a perfect candidate for a moonception we could observe from Earth, although we’ll probably need better telescopes to be able to see if it has one. Discovering sub-moons could tell us more about the formation of planets and planetary systems in general, particularly how close orbiting bodies affect one another. Most importantly of all, these moons of moons would need a name.

Scientists have considered sub-moons and even possibly moonmoons, but personally, I vote for “li’l moonlets.” And if you have your own ideas for names, let us know in the comments below! Thanks for watching this episode of SciShow! If you’re interested in exploring more fascinating questions about space, you might want to check out SciShow Space at youtube.com/scishowspace.

We wouldn’t be able to make these videos, or the ones over on SciShow Space, without the support of our Patreon community. If you want to learn more about becoming a part of that, just go to patreon.com/scishow. [♪ OUTRO].