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So, I'm about to start a new worldbuilding project for a fantasy world, but I want to try to make the basic world at least somewhat plausible; the magic in the setting is going to be pretty low-key, and I don't wanna use it to handwave physics away.

Basically I'm wondering if my basic idea for the setting is scientifically plausible, since I've no science background myself and mostly just dreamt this up after watching some astronomy documentaries.

The idea is to have a double planet system where at least one planet is basically Earth's size and composition - I'd like the other to be as well, but if it has to be slightly off for this to work that's okay. I'd like them to be tidally locked to each other, and to orbit a Sun-like star in its habitable zone if that's at all possible. Lastly, I'd like them to have an orbital period of 1 day, so the day and the "month" are equally long.

Would this work at all, or would they collide or rip each other apart or something? Also curious in what ways a dual planet system like this would differ from the Earth-Moon system in terms of tides, geology, et cetera? I've gotten the idea that tidally locked worlds don't exert much tidal forces on each other, but I'm really unsure how that all works. Would there be no tides, or would there still be tides like on earth, or maybe even a lot stronger for some reason I don't grasp?

Anyway, if anyone has anything helpful or informative to say about this it would be wildly appreciated. Cheers!

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  • $\begingroup$ Don't forget to account for volcanism. Tidally locked planets with molten cores will have an absolute ton of volcanic activity. $\endgroup$
    – thanby
    Commented Dec 13, 2019 at 13:45

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I'm not certain, but I strongly suspect that a geosynchronous orbit (in this case, quite a bit further than the appr. 22,000 miles/40,000 km we usually think of, because of the extra mass of the second body) would be inside the Roche Limit -- and that the two bodies would tear each other apart with their tides, then the fragments coalesce into a single super-Earth (possibly with one or more moons).

Aside from the Roche limit, there's no other good reason such a close binary planet couldn't orbit a sun-like star in its habitable zone. There are, I think, Roche Limit calculators online. Plugging your planetary parameters into them would tell you how close your twin planets can be.

BTW, related reading: Rocheworld and sequel(s), by Robert L. Forward. He has a well-calculated pair of bodies near their Roche limits, close enough that they share an atmosphere and occasionally some sea water.

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  • $\begingroup$ Similar questions have been asked before. For equally sized planets (and that is important) the Roche limit is virtually on the surface, although such an orbit would obviously not work for other reasons. $\endgroup$
    – Slarty
    Commented Dec 13, 2019 at 13:45

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