# Question about binary planets and a moon

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Say you have two planets orbiting each other, one of them slightly smaller than the other.

Is it possible for a moon to orbit in a figure-eight pattern around and between the two planets? Would this system even be stable?

Edited by JMBuilder
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Correct me if I am wrong, and knowing this forum somebody certainly will, but I had heard that two bodies of the same size, or nearly the same size, cannot have a stable orbit,  One of the bodies has to be a lot smaller than the other for a stable orbit to persist.

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53 minutes ago, benzman said:

Correct me if I am wrong, and knowing this forum somebody certainly will, but I had heard that two bodies of the same size, or nearly the same size, cannot have a stable orbit,  One of the bodies has to be a lot smaller than the other for a stable orbit to persist.

Actually,  no. Binary planets, though rare, do exist. However, a figure-eight orbit moon is probably unstable.

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If you had three bodies of equal mass, they can orbit each other in figure eights. But each one would follow the pattern.

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1 hour ago, Bill Phil said:

If you had three bodies of equal mass, they can orbit each other in figure eights. But each one would follow the pattern.

Yeah, I'm pretty sure you could do this in Universe Sandbox, but it would be really rare to happen in real life. But knowing how many planets there are in the universe, there may be a few out there.

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It would be VERY unstable, to the point that its almost impossible that it would form, and if it did, probably wouldn't last more than a single orbit.

Also relevant:

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Binary planets are fine. They'll gradually spiral out until they're both tidally locked, but can be otherwise stable.

The Earth and Moon probably isn't too far off being a binary. The moon isn't that much smaller than Mercury after all, and is actually influenced more strongly by the sun than by earth (Uniquely amongst moons and their parent planet).

Another definition is that the centre of mass of a binary should lie outside both bodies, which it will in the earth-moon system in another few hundred million years.

One issue with that definition is that unless the two bodies are exceptionally close in mass it generally leads to very distantly orbiting binaries. You're unlikely to find a third significant mass orbiting such a pair, though you could get an insignificant mass orbiting just one of the parent bodies, or very far out around both.

Another interesting factoid is that the barycenter of Jupiter/Sun is actually outside the sun. Does that make the solar system a binary star/planet? Jupiter doesn't meet any of the criteria to even be considered a brown dwarf. Food for thought.

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The figure-eight shown for binaries are always of their iso-potential energy line (meaning to remove something out of the binary's influence off that line you need to remove that muh energy). No hint or point for where the thing will actually move, which is based on force and velocity vector. So your wish can't happen, an object stably traveling in figure-eight around two bodies.

Regarding Sun-Jupiter, the position of the barycenter is indeed outside the Sun; but the distance from the barycenter to one of the component is much closer than the other, so applying the term "binary" isn't very sounding. Had Jupiter been nearer to the Sun, retaing it's mass, under a limit the barycenter would be inside the Sun. You can also make it for Sun-Earth : just move Earth so far away.

Edited by YNM
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Another question:

Is it possible to have a black hole with a small enough mass and size to orbit a star instead of everything else orbiting it or being pulled into it?

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20 hours ago, JMBuilder said:

Another question:

Is it possible to have a black hole with a small enough mass and size to orbit a star instead of everything else orbiting it or being pulled into it?

Certainly. Although black holes tend to have masses on the order of stellar masses, so its very likely to be more of a binary relationship than "one thing orbiting another".

There are some theories which expect the existence of "micro" black holes with masses on the order of planetary mass and less, but these would be extremely small (1-0.1mm and less) and very, very difficult to detect.

Its important to remember that a black hole only has the exact same properties of any other similar mass. Black holes do not "suck in" things any more than anything else just as massive does. There is no reason why things cannot orbit a black hole forever. The fun/weird stuff only starts when you  get close to it, where things can be dragged around by weird phenomena, the only reason you are able to get so close is because all the mass is so concentrated into a small area.

But, say you were 1 AU away from a 1 solar mass black hole (not sure if 1 stellar mass is enough to form a black hole by the usual mechanisms, not sure, just an example), the gravitational effects would be indistinguishable from orbiting the sun.

You often hear about black holes sucking material from nearby stars because (well, mostly because this is a dynamic situation that has many visible effects, so they are easily spotted) there are common mechanisms whereby this situation can arise (stars formed close together, one prone to black hole/neutron star formation). And close to a black hole (within "weirdness range", tidal forces [amongst other effects] are extreme, so you do get "extra suckiness").

That is a very "off the top of the head" explanation though, so expect errors - I'd encourage further reading, there is a TON of it!

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I appreciate all the responses. Very informative. Coming up with wierd star systems is one of my odd hobbies.

Edited by JMBuilder

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