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Moon Moons...

Could Earth's moon have its own moon? Science says: in theory. Credit: NASA/JPL/Space Science Institute

Topics: Astrophysics, Exoplanets, NASA, Planetary Science, Space Exploration

I couldn't compound the two words in the post title (as in the article) and keep a straight face. Although, someone will likely write fiction about double system moons (if they haven't already).

True to form, the Internet has endeavored to name an unnamed thing, and the results are hilarious. From the people who brought you Boaty McBoatface— the Arctic research drone that has already returned some very interesting discoveries from the world's coldest abysses — here come moonmoons: moons that orbit other moons.

Moonmoons — also known online as submoons, moonitos, grandmoons, moonettes and moooons — may not exist in our solar system or any other. However, according to a pair of astronomers writing in the preprint journal arXiv.org earlier this week, the concept of a moon hosting its own mini-moon is, at least, plausible.

Abstract
Each of the giant planets within the Solar System has large moons but none of these moons have their own moons (which we call submoons). By analogy with studies of moons around short-period exoplanets, we investigate the dynamical stability of submoons. We find that 10 km-scale submoons can only survive around large (1000 km-scale) moons on wide-separation orbits. Tidal dissipation destabilizes the orbits of submoons around moons that are small or too close to their host planet; this is the case for most of the Solar System’s moons. A handful of known moons are, however, capable of hosting long-lived submoons: Saturn’s moons Titan and Iapetus, Jupiter’s moon Callisto, and Earth’s Moon. Based on its inferred mass and orbital separation, the newly-discovered exomoon candidate Kepler-1625b-I can, in principle, host submoons, although its large orbital inclination may pose a difficulty for dynamical stability. The existence, or lack thereof, of submoons, may yield important constraints on satellite formation and evolution in planetary systems.

Moonmoons (Moons That Orbit Other Moons) Could Exist, Scientists Say

Brandon Specktor, Live Science

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