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In Dungeons and Dragons lore, there exists a construct called the cosmos used to define a variety of planes of existence. One of these is the Material Plane, on which our normal reality is found.

Suppose there was an Earth-like planet on the Material Plane with a disturbed cosmos, such that other planes of existence passed close to the Material Plane, close enough to create a planar bridge. This happens periodically and fairly predictably and, when a strong enough bridge forms, the encroaching plane can steal one of the planet's moons and/or give the planet a moon.

What would be the physical impact on the planet if there can be anywhere between 0 and 4 moons in orbit at any given time?

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    $\begingroup$ I foresee very unstable tidal forces $\endgroup$
    – Jax
    Commented May 4, 2015 at 17:10
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    $\begingroup$ Are we assuming "Magic!" happens between one moon being stolen and another returned, such that your variable-number of moons are always in stable, largely-circular orbits? Or are you envisioning a moon being "stolen", subjected to whatever passes for "physics" in the other plane, and then returned with those changes? $\endgroup$
    – Kromey
    Commented May 4, 2015 at 19:05
  • $\begingroup$ @Kromey Moons abide by whatever laws govern the plane they are on. When they return to the Material Plane, they retain those changes placed by the thieving plane, where applicable, but, through magic, have a stable orbit when returned to the planet's care. $\endgroup$
    – Frostfyre
    Commented May 4, 2015 at 19:40
  • $\begingroup$ Any size variations between moons 1-4, or roughly the same size as our existing moon? $\endgroup$
    – Twelfth
    Commented May 5, 2015 at 0:10
  • $\begingroup$ @Twelfth Moons vary in size, ranging from Io to the Moon. There are effectively infinite possible moons, as planes may hold any number of alternative moons. $\endgroup$
    – Frostfyre
    Commented May 5, 2015 at 0:16

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Some givens, because "everyone dies" is boring: The moons stay in orbit, and don't hit each other.

First, the "mundane stuff":

Tides: Goes without saying, insane. Not catastrophic, just really unpredictable. Influence on sea life? Probably not that much.

Volcanism and tectonic activity: High. Volcanoes, lots of earthquakes(not big ones, mind, but lots and lots, probably each time you gain and lose a moon.)

Calendar: Almost worthless over long periods. The length of the day will change as new moons are added and subtracted. How much by? Depends on the size of the moon and its orbital distance. Not a lot a lot but perhaps a day or two over a few decades.

Tidal stresses from switching gravity on the moons themselves would probably make them erupt in their own volcanoes.(They'd almost certainly have liquid cores because tides) Rains of sulfuric acid, big rocks, lumps of ice, nearly anything is possible with just mundane moons made of regular moon stuffs.

Now, the exotics. I don't know much about what planes these moons visit, but coming back from the elemental plane of fire and orbiting close to the planet might have an interesting effect on local weather. Not setting-fires hot, but you know. Problematic ecologically. Also there would be nearly no night due to the giant scary red glow in the sky. A Europa-like moon might freeze and remelt every time it came to visit.

The less often a moon visited, the more likely it would be to have a catastrophically huge eruption and shower ejecta onto the world below. In general it's not if you receive orbital gifts but when and how often you get presents from your moon visitors. Most things would be killed on impact but you might get the occasionally durable houseguest, if there's life on the moons some of the time.

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  • $\begingroup$ Is there any particular reason you believe that moons colliding in orbit causes an extinction event on the planet? $\endgroup$
    – Frostfyre
    Commented May 7, 2015 at 12:05
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    $\begingroup$ They're moons. If a small moon hit a large moon quite far away, the ejecta(measured in thousands if not millions of tons) would still hit the earth at a reasonable fraction of escape velocity. A single 1 km impactor would cause all of the nastiness outlined here, dozens or hundreds would compound the effect. The best case scenario is many Tunguska-like airbursts, which would still qualify as apocalyptically bad for protagonists. $\endgroup$
    – Resonating
    Commented May 7, 2015 at 16:58
  • $\begingroup$ Hmmm... Good point. Maybe I could use some rule about the bridges and moons that mostly keeps impacts from happening. I still want to use one collision as the basis of sentient life, though. $\endgroup$
    – Frostfyre
    Commented May 7, 2015 at 17:06
  • $\begingroup$ I'm not sure things would be nearly as nasty as you describe. If the moons can be taken and returned predictably that implies they stay put. The easiest explanation for this is that there is a planet of basically identical mass in the same place in the other plane--the moon shifts planes but continues to orbit normally. $\endgroup$
    – Loren Pechtel
    Commented May 8, 2015 at 2:17
  • $\begingroup$ That sounds great for the moon, but unless you swap an identicallly massed moon for a different moon you're in for some abrupt earthquakes among other things. The earth-moon barycenter is about 5000 km from the center of the earth (~75% of the way to the surface) so there is going to be some recoil unless you swap in a moon with similar gravitational pull/orbital position. $\endgroup$
    – Resonating
    Commented May 8, 2015 at 9:07
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The physical impact of an Earth-Like planet going to one to two moons would be the might be the extinction of its inhabitants! It's doubtful that a moonless Earth could have ever developed life and the sudden introduction of a new moon would likely be destructive to everything on Earth from tides to tectonic activity to the speed of our rotation. Gravity is fun!

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    $\begingroup$ Note that the comments to the article do a pretty good job of refuting many of the author's points. While the comment section may not be the best place to get scientific fact (as opposed to opinion), it does encourage one thing: "Don't believe everything you read on the Internet." $\endgroup$
    – Frostfyre
    Commented May 4, 2015 at 18:32
  • $\begingroup$ @Frostfyre excellent point. I didn't even look at the comments. $\endgroup$
    – wposeyjr
    Commented May 4, 2015 at 18:46
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Here's an idea: the orbit passes through the overlapping zone and pops in and out as it orbits.

The idea is that coming back with the correct orbit is not a wild coincidence, since it always is in orbit. The planet and its doppelganger pin the fold between them so the fold stays around the planet, like cufflinks. But only one had a large moon, and now it's shared between them. Only a large mass can pop through the fold (sometimes), so the existance of the fold does not obstruct the view of normal space.

In this idea, the planets stay pinned together so the motion around the barycenter is consistent regardless of which side the moon is on. The moon affects one of the planets, and that planet affects the other through the fold. So the moon feels a planet with double the inertia for its mass, and the tides disappear with the moon.

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  • $\begingroup$ It took me several reads to understand this, but I see what you're suggesting: having the planes be overlapping. The problem with this is that I want the planes to move through the cosmos, so they won't be permanently overlapping. $\endgroup$
    – Frostfyre
    Commented May 5, 2015 at 12:17

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