Stars are orbited by planets, and planets by moons. So why are there no moons with natural satellites of their own?
Matt Bothwell
University of Cambridge, UK
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The very short answer is that moons of moons (sometimes called 鈥渟ubmoons鈥�) aren鈥檛 normally stable. Every celestial body has an area around it that it gravitationally dominates, called its Hill sphere. Planets, which tend to be very far away from their stars, have big Hill spheres for stable moons to orbit inside. Moons, mostly being small and close to their planets, have much smaller Hill spheres. There just isn鈥檛 that much space around a moon where you can orbit long term: get too far away and the submoon will end up orbiting the planet; get too close, and it will crash into the moon.
In order for a moon to have a stable submoon, it has to have a big Hill sphere 鈥� which means the moon must be a) massive and b) far away from its planet.
Interestingly, Earth鈥檚 moon ticks both boxes: our moon could, in theory, have a moon of its own (as could Jupiter鈥檚 moon Callisto and Saturn鈥檚 moon Iapetus). Why these moons don鈥檛 have submoons, we don鈥檛 really know. One possibility is that it鈥檚 just hard for a moon to capture a smaller body, without it being stolen by its planet. Maybe our solar system is an outlier and there are exoplanets with exomoons with exosubmoons waiting to be discovered!
Moons generally don't have moons because the planet they orbit exerts a stronger gravitational influence than the moon itself
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Russell Sceats
Wivenhoe, Essex, UK
Moons generally don鈥檛 have moons because the planet they orbit exerts a much stronger gravitational influence than the moon itself. Any object trying to orbit a moon would be strongly perturbed by the planet鈥檚 gravity, making its orbit unstable over time. As a result, such objects would either fall onto the moon, escape into orbit around the planet or be ejected entirely, leaving no stable natural submoon systems.
Earth and its moon form a two-body gravitational system in which both bodies orbit their shared centre of mass, known as the barycentre. Earth doesn鈥檛 remain stationary while the moon circles it 鈥� both bodies move in response to each other鈥檚 gravitational pull.
The moon is unusually large relative to Earth, compared with most planet-moon systems. As a result, the barycentre is significantly displaced from Earth鈥檚 centre, and the motions of Earth and the moon are strongly coupled. The barycentre lies along a line from the centre of Earth to the moon, approximately 1700 kilometres beneath the planet鈥檚 surface.
Viewed dynamically, Earth and the moon aren鈥檛 simply a planet and its satellite, but a mutually orbiting pair bound to a common barycentre. Although the moon is conventionally classified as Earth鈥檚 natural satellite, there is no universally accepted definition of a 鈥渂inary planet鈥� established by the International Astronomical Union. For this reason, some scientists describe the Earth-moon system as being 鈥渂inary-like鈥� in nature.
Even if they aren鈥檛 officially classified as a binary planet system, they represent one of the closest examples of such a configuration in the solar system.
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