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Just an idea that had me doing a lot of navel gazing. Both planets are habitable to carbon based life forms, but intelligent life on each planet evolves very differently.

Eventually, the two life forms become aware of each other. It would be more of a sociological space opera than hard science. Perhaps each race will have the wisdom to leave the other alone and observe from afar. Providing that gravitational and orbital rules would allow this.

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Yes, and there already exists one in our solar system.

Pluto and Charon are of two very similar sizes and orbit each other somewhat stably. Although the both of them are considered dwarf planets/moons, they should be big enough to support some kind of life, even if it is dissimilar to that on earth. If they were located closer to the sun and had a source of liquid water, then it is possible for life to form on either -- or both -- planets.

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  • $\begingroup$ In this video showing teachers how to explain gravity with a prop, you can see an example of this phenomenon happening. youtube.com/watch?v=MTY1Kje0yLg $\endgroup$ – Keltari Oct 18 '18 at 21:53
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It is very much possible for two planets to orbit each other in the same habitable zone. These binary planet systems can occur in nature and we even have one here in our solar system with Pluto and its moon Charon. Granted Charon is only half the size of Pluto and considered a moon it is massive enough to effect Pluto's center of gravity. So yes two planets can orbit each other in the same habitable zone of a star. Actually some scientists say that a stars habitable can hold as much as 5 planets if the conditions are right and some say as much as 7 but the conditions for 7 planets in the same habitable zone are very rare. So in short, yes two planets can orbit each other in a stars habitable zone.

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    $\begingroup$ Technically, any body orbiting another body will shift the barycenter (the mutual center of mass between the two bodies). The Pluto/Charon system is a good example of one where the barycenter falls outside of either body, making it a good extreme example; but Pluto/Charon also doesn't orbit the center of the Sun, but a point slightly offset from the center of the Sun. Earth's moon doesn't orbit the center of the Earth, but rather a point offset (by a non-negligible amount) from the center of the Earth (but still within Earth). And so on. $\endgroup$ – a CVn Oct 17 '18 at 18:31
  • $\begingroup$ The Earth-Moon barycenter is about 1700km down. The centre of the Earth is about 6300km down. If Earth-Moon were a slightly larger double planet, that barycentre would surely be well outside both planets. Of interest is that the Sun-Jupiter barycenter is well outside the surface of the Sun. $\endgroup$ – elemtilas Oct 18 '18 at 7:01

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