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I was wondering if a moon Is in the center of two planets If the gravity will rip It In half or If a moon can rotate around both planets or if the moon manipulates the planets rotation

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  • $\begingroup$ At present this is too broad and not well defined. Could you tell us about your planets? What sort of planets are they, where do they orbit relative to each other? Also we would need to know about your sun, what sort of star is it? I'd ask you that when you have the time you should check out the tour and explore our help center to figure out how we work. Welcome to the forum here. $\endgroup$ – Bitter dreggs. Sep 18 '19 at 18:08
  • $\begingroup$ Do you want your moon to hang in Lagrange point 1 between two planets? $\endgroup$ – Alexander Sep 18 '19 at 18:22
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    $\begingroup$ The smaller moons of the Pluto/Charon system (the closest we have to a double planet) orbit around their barycenter, which is outside Pluto's surface. I see no reason why that wouldn't scale up. en.wikipedia.org/wiki/Moons_of_Pluto But a moon in L1 wouldn't be stable, and I don't think figure-8 orbits are, either. $\endgroup$ – jamesqf Sep 18 '19 at 18:41
  • $\begingroup$ Came here to point out the Pluto/Charon binary dwarf planets (co-orbiting moons? The IAU definition of planets cleared some things up (who wants to memorize 30+ planets in second grade?), but made defining exactly what Pluto is a real mess), as well. That's the real world example of moons around double planets. -- To the other part of the question; in order to rip anything apart, a moon/asteroid/whatever has to be in the Roche Limit of something else. $\endgroup$ – Ghedipunk Sep 18 '19 at 19:15
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I'm afraid most of what you're looking for is impossible.

A moon can orbit a binary planet system by going around both planets, but never by going in-between them.

A "figure-8" orbit is highly unstable because for this to work it would need to be free of any outside interference - no stars, no planets, no other moons - nothing could be anywhere near this system, and even then it would just be a matter of time before the system collapses. Of course, life would be impossible in such a system, without a Sun.

An L1 orbit, which means the moon would lie between the two planets, is highly unstable because of the same reason...any kind of outside influence would cause the balance between the two planets to shift, causing the moon to either crash into one of the planets or begin orbiting one of them separately from the other. (which also might be unstable, depending on the difference in mass between the planets and the orbital distance of the moon)

If you want a moon going around two planets, it can do so by orbiting around both of them and always remaining on the outside.

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    $\begingroup$ "Just a matter of time before the system collapses." I think this answer could be greatly improved if you include how long the system could be in place before catastrophe hit. Even if the timeframe is only in the thousands of years (less than a blink of an eye, astronomically speaking), it may be workable for a story. $\endgroup$ – Michael W. Sep 18 '19 at 19:12
  • $\begingroup$ @MichaelW. It would need to be free of outside influence, including the outside influence of a sun. Which renders life impossible and thus won't work for a story. $\endgroup$ – overlord Sep 18 '19 at 19:20
  • $\begingroup$ An L-1 orbit is unstable in the long term, but can be periodically stable if the moon is in a halo orbit. $\endgroup$ – stix Sep 18 '19 at 19:29
  • $\begingroup$ @stix I suspect that the stability of that orbit depends the orbited body significantly outmassing the orbiting body. Fine for satellites, not so fine for big moons. $\endgroup$ – Starfish Prime Sep 18 '19 at 20:57
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    $\begingroup$ @MichaelW. having fiddled with gravity simulators, the initial state is sufficiently hard to set up that everything tends to be flung away after just a few orbits. More like years than millenia. That's not to say that more stable solutions could exist, of course, but it should tell you something about the fragility of the situation. $\endgroup$ – Starfish Prime Sep 18 '19 at 20:58
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In the center? Short answer no, long answer yes, but...

Orbiting around both? Yes.

If you have a binary planet, the two planets orbit a barycenter. A sufficient distance away, this is gravitationally equivalent to a single planet of the combined mass. So you can have a moon orbiting the barycenter of a binary planet system.

Another possibility depending on the setup is that the moon can orbit at a Lagrange point between the planets. The Lagrange points behave like little barycenters themselves, and some form stable-ish orbits.

This is the closest thing to what you describe: a moon could technically orbit at the L-1 Lagrange point, but such orbits are unstable. The moon could be in a halo orbit, which allows it to spend some time at the L-1 point, before drifting away. It is conceivable that your moon could be in a periodically stable L-1 orbit where it drifts in, orbits L-1 for a time, then drifts out, only to drift back in and repeat the process later.

If you're willing to put the moon at the L-4 or L-5 points, those orbits can be quite stable. This won't be directly "in between" the planets, but it would be at a vantage point where both planets would be very prominent in the moon's sky.

In any Lagrange orbit case, the mass of the moon needs to be much smaller than the planets in the binary, otherwise it will itself affect where the Lagrange points are and make the orbit much more unstable.

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  • $\begingroup$ It should be mentioned that anyone living on any of the planets in the L-1 case might want to put a fast track on their space programs. Such orbits are only stable over the course of tens of thousands of years, or (usually) significantly shorter. $\endgroup$ – Ghedipunk Sep 18 '19 at 21:02
  • $\begingroup$ Even at L4 and L5 your stability is limited by the ratio of your mass to the planet's mass... a trojan needs to have a mass below one hundredth of the planet's, and that's if your whole system is a nice simple three-body problem. Add more bodies, and you might need it to be smaller still. $\endgroup$ – Starfish Prime Sep 18 '19 at 21:07
  • $\begingroup$ @StarfishPrime That's fine. The moon is 1/100th the mass of the Earth. If we're talking about Jupiter sized planets then the Earth is 1/300th the mass of that of Jupiter. $\endgroup$ – stix Sep 18 '19 at 21:38
  • $\begingroup$ @Ghedipunk That's why I mentioned periodically stable orbits. It is possible to come up with an orbital scenario where the moon is only periodically at L1, then drifts away and comes back later. Earth has many of these temporary types of satellites that are in Solar orbit, then orbit an L-point, then go back into Solar orbit. $\endgroup$ – stix Sep 18 '19 at 21:38
  • $\begingroup$ In a 3 body system, though (not counting the star and other planets around that star), in order to be even periodically stable, it needs to enter L1 from an angle only a few arcminutes wide in a window only a few hours long each year... Otherwise it gets very unstable very quickly... The most likely result being that the smallest body gets ejected from the orbit, ending up in a highly elliptical orbit that probably crosses the orbits of other planets in the system... Unless it has thrusters (then that's no moon, it's a space station), the smaller body will never hit the keyhole twice. $\endgroup$ – Ghedipunk Sep 18 '19 at 21:47
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For a moon to orbit two planets and stay between them all the time, it would have to be placed in the L1 Lagrangian point, where the gravity and orbital centripetal forces of the two planets cancel out. Unfortunately, this Lagrangian is unstable; it is unlikely that any moon would stay there for very long.

You can, however, have a stable moon in the L4 or L5 Lagrangian points, which form equilateral triangles with the two planets and rotate with the planets as they rotate around each other.

It is not, I believe, possible to have a moon shift between orbiting one planet or the other in a figure-8 or any other configuration. It will have to either orbit one of them, or orbit both at a distance.

If, however, a moon has an orbit that intersects the L1 Lagrangian point between the planets, it might feasibly on occasion shifts its orbit from one of the planets to the other. It would orbit one planet, linger for a while in L1, and then go on to orbit the same planet OR the other. There might have to be some outside force that nudges the moon from orbiting the first planet to the second, such as another moon around one of the planets, or another planet exerting tidal forces on the system.

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