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For the purpose of a story, I would like to imagine a system of two Earth-like planets orbiting like Earth and Moon.

I imagine that these planets were created by a gigantic collision at the beginning of this stellar system between two planets.

One of these planets traps quite all the iron and metals, and the other stays with a small iron nucleus.

Both of the planets have Earth-like atmosphere and have developed life.

Is this kind of system realistic?

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  • $\begingroup$ Your English wasn't that bad. It was understandable, all right. I did a little cleanup. $\endgroup$ – Mołot Apr 12 '17 at 13:59
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    $\begingroup$ They wouldn't have the same orbit as our Earth-moon system unless one of the planets were as small as our moon. Two planets of Earth-size would orbit a common point called the barycenter. $\endgroup$ – Feyre Apr 12 '17 at 14:02
  • $\begingroup$ @Feyre I believe that for the OP's story this wouldn't really be much of an issue. But surely, this would need to be a part of good answer. $\endgroup$ – Mołot Apr 12 '17 at 14:03
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    $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Apr 12 '17 at 14:16
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It's not entirely implausible for two earth like planets to form this way, The giant impact hypothisis is after all almost exactly what you described. The one limiting fact that I see is that were one of the two bodies to end up with most of the iron (and by extension nickel) then the planet may not be able to generate a magnetic field. The movement of the molten metal core forms a geodynamo that produces the magnetic field which protects the atmosphere from the solar wind. Without the magnetic field, the solar wind will strip the atmosphere from a planet. So if the world with the small iron core did develop earth like conditions they may not last long.

All other things being equal, the planet lacking the large iron core will also have a lower mass and by extension gravity than the other. Earth's core is about 1/3 of the mass of the planet. While not that big of an issue it does pose problems for gaining and holding onto the atmosphere especially when being blasted by the solar wind.

As for their orbit, they would orbit like the earth-moon system. This means they orbit a Barycenter, the center of the masses of the two planets. For the earth-moon system that point is beneath the crust of the earth so from a distance the moon orbits the earth while the planet wobbles. Given the likely dissimilar masses the orbits may look like the pluto system:

In summary, the system is not unrealistic. The two planets would have large differences in their masses and one would likely have a thinner atmosphere. But they could have earth like conditions and evolve life.

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    $\begingroup$ Indeed, the Pluto/Charon system seems to be a much better example of two quite similar worlds orbiting each other than the Earth/Moon system. Just up the sizes a bit, and move them closer to the sun. $\endgroup$ – jamesqf Apr 12 '17 at 17:55
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A "binary planet" setup is totally plausible. The Moon is already not all that much smaller than the Earth, and the Pluto-Charon system is even closer in size. Simulations of giant impacts sometimes find close to equal-sized planets orbiting each other.

It would look something like this:

enter image description here

Wikipedia even has a page on "double planets" here. And I used them to build systems with lots and lots of planets in the habitable zone: see here and here.

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  • $\begingroup$ Though I think they'd have to be considerably further apart. Just eyeballing that image looks like the smaller planet is well within the Roche limit. $\endgroup$ – jamesqf Apr 13 '17 at 18:33
  • $\begingroup$ You are right -- that is just an illustration. Tides would be so strong at that separation that the planets would be deformed and would be pushed apart very fast. $\endgroup$ – Sean Raymond Apr 15 '17 at 14:39
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https://www.space.com/27832-binary-earth-size-alien-planets.html THEY SIMULATED IT

"These binary planets would loom extraordinarily close to one another, separated by a distance of about half the diameter of each of the worlds. Over time, the rate at which both planets spin would fall into lockstep, with each world only turning one face toward its partner."

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