I was helping a friend in making a game world, and he described a binary planet to me inhabited by people. that made me question the feasibility of that even existing, but I’m barely knowledgeable, so i found this forum and wanted to see if anyone could help me get a better grasp on how that would affect the said civilization.
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A “binary planet” usually means two planets of similar size orbiting each other while also orbiting a star. This is possible in theory, but it would be very rare and a bit unstable unless everything is perfectly balanced.
What about a nonbinary planet ? 
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Interesting question—binary planet setups are actually a lot more “physics-constrained” than most worldbuilding settings make them seem.
If you imagine two Earth-like planets orbiting each other closely, the biggest immediate effects would be extreme tidal forces. You’d likely get massive ocean tides (or even global crustal stress if they’re close enough), which could seriously shape geography and even long-term geology. Day/night cycles would also get weird depending on rotation lock—some regions might see the other planet constantly hanging in the sky.
From a civilization standpoint, that kind of environment could go either way: it might make navigation, calendars, and mythology really complex but also culturally rich, or it could be harsh enough that stable development is harder unless the planets are far enough apart to reduce the worst tidal effects.
Also, if the bodies are too close, they might not even remain stable long-term due to Roche limit issues (they could eventually deform or even start exchanging material).
If you’re into these kinds of speculative science/worldbuilding questions, I’ve seen some fun discussions and quizzes around related astronomy topics too, like this one: telenor quiz
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An example might be Pluto and Caronte.
Regards
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