Can Moons Have Their Own Moons?

By Ethan Siegel

The Saturnian system is known to have an incredible number of rings and moons, yet none of the moons we know of have moons of their own. NASA / JPL

In the Solar System, we have the central Sun, a great many planets, asteroids, Kuiper belt objects, and moons. While most of the planets have moons, and some of the Kuiper belt objects and even asteroids have natural satellites that orbit them, there are no known "moons of moons" out there. It may not be because we're just unlucky; there may be some fundamentally important rules of astrophysics that make it extraordinarily difficult for such an object to stably exist.

When all you've got is a single, massive object in space to consider, everything seems pretty straightforward. You'd intuit that gravitation would be the only force at work, and so you’d be able to place any object into a stable, elliptical-or-circular orbit around it. Under that setup, you'd expect that it would continue on that way forever. But there are other factors at play, including the fact that:
  • this object can have some sort of atmosphere, or a diffuse “halo” of particles around it,
  • this object isn’t necessarily stationary, but can spin — perhaps rapidly — about an axis,
  • and that this object isn’t necessarily as isolated as you initially imagined.

The first factor, an atmosphere, only matters in the most extreme of cases. Normally, an object orbiting a massive, solid world with no atmosphere would simply need to avoid the object’s surface, and it could remain revolving around it forever. But if you throw in the presence of an atmosphere, even an incredibly diffuse one, any orbiting bodies will have to contend with those atoms and particles surrounding the central mass.

Even though we normally think of our atmosphere as having an “end” and space beginning beyond a certain altitude, the reality is that atmospheres simply thin out as you go to higher and higher elevations. Earth’s atmosphere continues for many hundreds of kilometers; even the international space station will someday decay and meet a fiery doom unless we continually boost it. Over Solar System timescales of billions of years, the point is that orbiting bodies need to be a certain distance away from whatever mass they orbit in order to be “safe.”

Additionally, an object can spin. This applies to both the large mass and the smaller one orbiting it. There’s a “stable” point, where both masses are tidally locked to one another (where both of them always have the same side pointing to one another), but if you have any other configuration, there will be some “torquing” happening. This torque can work to either spiral the two masses inwards (if the rotation is too slow) or outwards (if the rotation is too fast) for the locking to occur. In other words, most satellites don’t start off in the ideal configuration! But there’s one more factor that we need to throw in to get to the issue of “moons of moons” and really see where the difficulty lies.

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