Jiri Hojda/Alamy
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
The very short answer is that moons of moons (sometimes called “submoons”) aren’t 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’t 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 – which means the moon must be a) massive and b) far away from its planet.
Interestingly, Earth’s moon ticks both boxes: our moon could, in theory, have a moon of its own (as could Jupiter’s moon Callisto and Saturn’s moon Iapetus). Why these moons don’t have submoons, we don’t really know. One possibility is that it’s 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!
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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’t 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’s 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’t remain stationary while the moon circles it – both bodies move in response to each other’s 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’s 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’s surface.
Viewed dynamically, Earth and the moon aren’t simply a planet and its satellite, but a mutually orbiting pair bound to a common barycentre. Although the moon is conventionally classified as Earth’s natural satellite, there is no universally accepted definition of a “binary planet” established by the International Astronomical Union. For this reason, some scientists describe the Earth-moon system as being “binary-like” in nature.
Even if they aren’t 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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