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How close could two earth sized celestial bodies get in their nearest approach either as planets or moons around a larger planet?

Close enough for atmospheric interaction in a stable orbit at nearest approach, or for non-space-fairing species to somehow make the jump without cosmological assistance (aka asteroid impact)?

It seems just from using the roche limit of an earth to earth sized bodies any approach under ~20,000 km would be unstable in the long term, which is orders of magnitude beyond a normal atmosphere size.

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    $\begingroup$ Just as a point to make, 2 earth sized planets near each other's Roche limits would cause catastrophic earthquakes. Also, you might want to define the type of orbit. Would they orbit each other in a kind of duel planet orbit or would they each have independent orbits around the same star? Independent orbits would need, probably a few million miles apart for stability. Orbiting each other you could work them to perhaps, earth-Moon distance or even closer assuming they were tidally locked to each other to avoid too much disruptive tidal forces. $\endgroup$ – userLTK Sep 30 '16 at 0:23
  • $\begingroup$ This question may be too open ended, but I'm wondering what system (if any) could make this happen? I figured something like earth sized moons orbiting a larger planet like jupiter may be more stable, where they had very similar orbital radii so they periodically became very close? $\endgroup$ – joeyfb Sep 30 '16 at 0:28
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    $\begingroup$ Just an extremely simple 8th-grade answer, no, you can't have orbits anything like that close. Nor can you have a large moon anything like as close as you are thinking. BUT if this is for a story: don't forget the other direction. One could say it is perhaps generally agreed today that "panspermia" can happen: life - microbes - via asteroids/etc can travel large astronomical distances. If we learned today that Mars has life - and indeed it's the "same" as Earth life - we'd assume that it travelled via ejecta - the Mars-Earth distance is no problem for such a putative story. $\endgroup$ – Fattie Sep 30 '16 at 13:19
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    $\begingroup$ (Just BTW there's an old-time sci-fi book called something like "star ships under sail" where the idea is that a twin planet system is so close the oceans kind of rise up and touch every few years, and you can sail back and fore!) $\endgroup$ – Fattie Sep 30 '16 at 13:21
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    $\begingroup$ Getting the planets close enough to within the ~10 kms range of their atmospheres seems far out of plausibility. But! If two planets came within thousands of kilometers of each other could the mutual gravitational interactions actually extend their atmospheres in a way that they'd interact? This brings up earthquake problem of close interactions though and tectonic instability unless they're tidally locked... $\endgroup$ – joeyfb Sep 30 '16 at 20:18
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From comments

no, you can't have orbits anything like that close. Nor can you have a large moon anything like as close as you are thinking. 2 earth sized planets near each other's Roche limits would cause catastrophic earthquakes. Independent orbits would need, probably a few million miles apart for stability. Orbiting each other you could work them to perhaps, earth-Moon distance or even closer assuming they were tidally locked to each other to avoid too much disruptive tidal forces.

BUT if this is for a story: don't forget the other direction. One could say it is perhaps generally agreed today that "panspermia" can happen: life - microbes - via asteroids/etc can travel large astronomical distances. If we learned today that Mars has life - and indeed it's the "same" as Earth life - we'd assume that it travelled via ejecta - the Mars-Earth distance is no problem for such a putative story

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If it was a one time pass and the life exchanged was bacterial, then yes, it's possible. That close, both planets would experience enormous tidal flexing and probably extinction level events, but the good news is that if both orbited the sun, passing that close they would change each other's orbits, so it could be a once time thing.

Events like that would be rare and potentially dangerous as a collision is also possible. But in theory, a near pass that exchanges some primitive bacterial life is just possible, making it possible for one planet to give life to the other. There's easier ways for that to happen like a large asteroid impact and debris knocked into space.

It's not possible for the two planets to do this periodically. That would be too unstable and it's not possible for birds to jump from planet to planet, unless you don't mind them dying in the process. But a one time pass and bacteria. A very rare occurrence but possible.

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If you're thinking of occasional close encounters, this is very unlikely to work. The approach would be catastrophical for both planets. Their plate tectonics would get completely screwed up. It would kill a whole lot of life on both planets.

But it could be possible that two planet-size bodies might coexist very close to each other, so that the tops of their atmospheres would not be separated by a very large gap. It's not clear whether this is stable long term from a gravitational p.o.v. Fluids would definitely migrate towards the stronger gravitational field over the very long term, unless there's some active mechanism (perhaps powered somehow by heat from the star?) pumping them back occasionally.

The last scenario is explored in the sci-fi book Rocheworld by Robert Forward. It's sci-fi but it does not stray too far from rigorous science.

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