Can a Natural Satellite Have Its Own Natural Satellite?
In all known planetary systems, natural satellites occur in a restricted dynamical phase space: planets orbit stars and moons orbit planets. It is natural to ask, can submoons orbit moons? If so, why don’t any of the known moons of the Solar System have their own submoons? One possibility is that the formation mechanism of planetmoon systems precludes their formation. Another possibility, is that these bodies are dynamically unstable and are rapidly scoured from their system after formation. Here, we investigate the latter hypothesis.
What are the requirements for stability of a submoon? To ensure dynamical stability, the host moon must have a Hill sphere that is larger than its physical radius as well as its Roche limit. The submoon must also survive any long-term dynamical effects such as tidal evolution.
Only large moons on wide-separation orbits can host long-lived submoons. This is mainly because massive, distant moons have larger Hill radii that provide more stable real estate for submoons
The researchers calculated that in the Solar System, 4 natural satellites could have their own natural satellite.
Our Moon, Callisto of Jupiter, and Titan and Iapetus of Saturn.
These satellites are relatively large, are relatively distant from their planets, ie there is a small area around them that could have a natural satellite and that satellite would not be stolen by the planet.