“People who work every day are kind of scared of things they don’t understand.” -Young Jeezy
From Mercury out to Neptune, most of the worlds in the Solar System have moons, with a hitherto discovered population of around 200 known ones. Yet despite all of it, we don’t know of a single instance of a moon that has its own natural satellite: a moon with a moon of its own. But that doesn’t mean it couldn’t exist!
The major moons in our Solar System could contain some objects with candidates for potentially having orbiting moons of their own. If many of these moons were situated differently, astronomers would define them as planets. Image credit: Emily Lakdawalla, via http://www.planetary.org/multimedia/space-images/charts/the-not-planets.html. The Moon: Gari Arrillaga. Other data: NASA/JPL/JHUAPL/SwRI/UCLA/MPS/IDA. Processing by Ted Stryk, Gordan Ugarkovic, Emily Lakdawalla, and Jason Perry.
Within our own Solar System, we have many known candidates for moons that would be good fits for having their own moons, including worlds around each of the gas giants. But given the configuration of everything we see, what’s theoretically possible may not be physically realistic. There are a great many reasons why moons-with-moons wouldn’t be stable over the long term, and 4.5 billion years is certainly a long time to require survival!
Of course, a Kuiper belt object would need to have a moon with its own moon to be considered a moon having a moon. The distances at play would likely need to be very great; at some point, the gravitational binding energy becomes very small and the region you have for success is extremely narrow. Image credit: Robert Hurt (IPAC).
We haven’t found any so far, but it’s possible for moons to have moons of their own. What would it take to make them real? Come find out!