For my current purposes, assume we have two planets identical to earth. So, could two of these planets form a stable binary system, and still be close enough to see the green foliage on the surface of the other planet? They would ideally not be tidal locked. If this is possible, how would it affect the day/night cycle and seasons? (If you don't mind taking it into account, I intended for these planets to rotate a binary star system, in which both stars are nearly identical.)

  • $\begingroup$ See worldbuilding.stackexchange.com/questions/21446/… and links therein. $\endgroup$
    – HDE 226868
    Commented Jan 4, 2016 at 0:52
  • $\begingroup$ That does help with the world's interactions, but I'm wondering more about how the ecology of both planets would be affected. $\endgroup$ Commented Jan 4, 2016 at 0:58
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    $\begingroup$ Pluto and Charon... $\endgroup$
    – SJuan76
    Commented Jan 4, 2016 at 1:17
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    $\begingroup$ Earth/moon is a binary system. Main difference between our system and yo urs imaginative one is the size of second planet too similar to size of the first to call any of them moon. But after all it is just definition. Earth could as well have habitable moon. The question remains how it could be formed but it is surely possible. $\endgroup$
    – jaboja
    Commented Jan 4, 2016 at 3:14

2 Answers 2


In Grimm's World the "moon" was a another world with visible continents and weather and even visible wildfires. Other than being very interesting to observe and affecting the culture of the people, it had no effect on the description of the planet in terms of having a dinural cycle, seasons, etc.

If you want to get more realistic, you can look at the Hill Sphere of a planet to see how far away a companion might be. If the distance is farther to compensate for the increased mass, so the tidal forces are no greater than what we have here, then you can postulate a day-length like ours rather than have tidal locking.

An alternative would be to have two planets in the same orbit around the sun, so they approach and receed in cycles. This would avoid tidal locking issues.


The way the moon formed was that something big collided with early Earth when things were still pretty gooey, and knocked a big chunk out, which then fell into orbit instead of drifting away or falling back to earth.

If you had a planetary mass that was somewhere between 2 and 3 earths, and hit it with a big rock so that it split fairly equally, then sure, why not.

As JDługosz says, you'd have to get the orbit distance right, and there are some formulas to determine the Hill Sphere.

  • $\begingroup$ 1+ for a plausible formation of a system $\endgroup$ Commented Jan 4, 2016 at 14:30

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