Here's a good exercise for you math whizzes (no, I don't know the answer): How stable (and what shape) would the orbit (around the Sun) of the Earth be if the moon was equal to the Earth in size and mass, but correspondingly further away so that the tidal effect would be the same as it is now? 1: how far apart would that be? 2: would the orbit be disrupted by the Sun or Venus? (assume the rest of the solar system unchanged) 3: how likely for such a structure to develop somewhere? 4: would these twin planets be rotation-locked (same side always facing each other)? (This would make the "day" equal to their "month": how long would that be?) 5: considering the unusual size our moon actually is compared to its planet, how much of those effects are present as it is?