If I understand correctly, it's highly likely that the planets in a twin-Earth binary planet system would be tidally locked. But is it totally impossible for them to be much farther apart—say, at several times the distance between the Earth and the moon—and rotate independently? (As a cheat, if necessary, could such a system at least be stable for several million years if it were created by an incredibly advanced civilization with god-like technologies and resources?)

A few bonus questions, assuming that their co-orbital period is several months:

  • What would this mean for their day/night cycles? Obviously the combination of rotating independently and orbiting each other would cause some eccentricities here, but I'm trying to puzzle it out and it's making my brain hurt.
  • Tidal effects would be pretty severe, yes? What's the least severe possible tidal effect each planet could expect to experience?
  • Is it possible for both planets to have seasons, e.g. if their axes and/or co-orbital plane was offset to that of their solar orbit?

1 Answer 1


It's certainly possible for them to be further apart and still rotating, there are any number of scenarios that would allow that. We've no way to know how likely it is but nothing prevents it. You don't even need it to be artificially created, just have them far enough apart or have them form recently enough.

Think of tidal forces as acting like a break. They start off spinning fast and the tidal force gradually slows them down until they stop relevant to each other, however they are really really massive and spinning really really fast and tides aren't actually that strong so they slow down really really really slowly.

Day/Night cycles would not really change at all. The speed of the individual planet's rotation does not depend on where it is in the binary planet pair.

Seasons again are going to depend upon the axial tilt of the individual planets so are not drastically effected.

Tides could range from severe to mild, it all depends on how far apart you put the two planets. This is the one area where you could get a big difference. The moon has 1% of the mass of the earth, if this sister earth had 100% of the mass then you would need to be 10 times as far away to get the same level of gravity since gravity decreases based on the square of the distance.

However you are actually looking at the tidal forces, which is the difference between the gravitational pull on the center of the planet and the surface. That decreases by the cube of the distance, so in fact you only need to be 4.6 times as far away as the moon is to get the same tidal effects as our moon from a sister earth.

  • 2
    $\begingroup$ Tidal forces follow an inverse cube law, not inverse square. If they didn't then Earth's tides would be dominated by the sun rather than the moon. So a planet with 100x the mass of the moon that was ten times as far away as the moon would actually have 1/10th of the tidal effect that the moon has, 100/1000. $\endgroup$
    – Mike Scott
    Mar 17, 2016 at 19:45
  • $\begingroup$ @MikeScott Thanks, just finished looking that up myself. I know it was greater than squared but wasn't sure the amount off hand. :) $\endgroup$
    – Tim B
    Mar 17, 2016 at 19:50
  • $\begingroup$ Thanks! Are you sure about the day/night cycle? Since each planet is rotating in relation to the sun both axially and in its binary orbit, wouldn't day length fluctuate (even if very slightly) in some sort of increasing-and-decreasing cycle? Or am I overthinking this? $\endgroup$ Mar 17, 2016 at 20:25
  • $\begingroup$ You are overthinking it. Only the planet's rotation affects the duration of the day-night cycle. $\endgroup$
    – Jim2B
    Mar 17, 2016 at 20:55
  • $\begingroup$ There will be eclipses, though, which make for unusually long nights. $\endgroup$ Mar 18, 2016 at 2:49

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