Wikipedia:Reference desk/Archives/Science/2018 April 14

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April 14

Moon's moons?

Are there any moons that have "moons" of their own? -- If not, why not? It seems that given enough time, there would be bits of debris with just the right trajectory to enter into orbit around a moon. —2606:A000:4C0C:E200:0:0:0:3 (talk) 05:29, 14 April 2018 (UTC)

None are known, and they would likely be unstable. See subsatellite. However, there are moons that do not orbit each other, but which make repeated close approaches to each other. See co-orbital configuration. --69.159.62.113 (talk) 06:00, 14 April 2018 (UTC)

Thanks for the links. Similarly, are there any other natural satellites orbiting Earth (e.g. a meteor or cometary debris)? Again, it seems there should be. —2606:A000:4C0C:E200:0:0:0:3 (talk) 06:57, 14 April 2018 (UTC) ... I found Claimed moons of Earth which sort-of answers this.

According to this answer, from the University of Cornell's Astronomy department, to a similar question, a moon of a moon is theoretically possible. As, you'll have gathered, though, we haven't actually found one yet. {the poster formerly known as 87.81.230.195} 2.218.14.51 (talk) 11:54, 14 April 2018 (UTC)

But as it notes, their orbits are generally not stable over long periods, which is why they're rare. The primary yanks on it and gradually perturbs its orbit, eventually either capturing it itself, colliding with it, or ejecting it from the system. All the stable many-body planetary systems we see are large planets with small moons relative to the primary, as with Jupiter and Saturn. The moons formed around the same time as the planet, and fell into resonances with each other, which is what keeps them stable. Everything else in the proto-system either got swept up by the proto-moons or proto-planet, herded into the rings, or ejected. Jupiter's and Saturn's rings are also in stable resonances with the primary and its moons, and in fact at least part of Saturn's rings may be a moon that got destabilized and ripped apart.

The same processes are why Earth doesn't have any other satellites. The moons usually need to form with the planet, so they can stabilize each other. None of the terrestrial planets, it looks like, even formed moons, because they aren't massive enough. Earth's moon came from a collision with another protoplanet. Some of the ejecta got blasted into orbit around Earth, and then all got swept up into the Moon. The same may have happened to Venus, but it later lost its moon. Mars's moons are captured asteroids which are unstable; Phobos will eventually crash into Mars, while Deimos will escape. The dwarf planet systems look to all be the result of cosmic bumper cars, which, along with the low mass, explains the large number of bodies. --47.146.63.87 (talk) 01:54, 15 April 2018 (UTC)

The closest detected so far are the rings of Rhea. If they exist, it would be largish particles orbiting a moon. It is a temporary orbit as they are eventually pulled into orbit around Saturn. But, theoretically, a largish rock could maintain an orbit around Rhea for a very long time. 209.149.113.5 (talk) 11:35, 17 April 2018 (UTC)

Sorry, I forgot to thank y'all for your replies. Interesting! —Same OP, new IP=2606:A000:1126:4CA:0:98F2:CFF6:1782 (talk) 01:40, 21 April 2018 (UTC)