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thedoctorstatic t1_j4z1han wrote

The universe is essentially infinite, but I'd argue that's not enough for the chances of two identical mass and size bodies to form right beside each other and have a stable binary gravitational influence WHILE orbiting a star.

They would most likely collide(possibly like how proto earth collided with a similar proto planet which ejected the material that became the moon).

The problem is as they neutrally orbit each other, one would be closer to the star and feel a stronger gravitational tug, while the other planet would be pulling against(which would be in constant flux due to their changing position to each other relative to the star) and quickly lose stability

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Norwester77 t1_j4z3ui0 wrote

Other planets in the solar system would likely perturb their orbits, too.

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thedoctorstatic t1_j4zilbk wrote

Oh for sure they would, but it would just slowly influence it a tiny bit over huge time spans(unless it is jupiter size and closer to earth than mars or venus). The star is going to be a much more powerful pull than even that would be.

Hahaha maybe one of the planets woulf eventually find a home as a moon of it though

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Underhill42 t1_j53jknn wrote

So long as they're well within each other's Hill Spheres I don't think the sun should be an issue any more than it is with our moon. (The moon's orbit is about 1/4 of the way to Earth's Sphere) And Earth already orbits our combined center of mass with the moon, it's just that the size discrepancy (it's only ~1% of our mass) means that's still within Earth's volume, about 3/4 of the ways up from the core.

Forming would be a different question, but e.g. if Theia had been considerably bigger (or faster?) when it hit proto-Earth the "splash cloud" might have coalesced into two much more similarly-sized sister-planets.

Exactly the same size (to how many significant digits?) would indeed take crazy long odds. But within 10% or 20% is probably not too outlandish.

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