“You can spend too much time wondering which of identical twins is the more alike.” -Robert Brault
When we think of the idea of “Earth’s twin,” we inevitably think of a planet like ours orbiting a star like ours at the same distance and speed. Most planets are not rocky and Earth-sized; most stars are not like the Sun; most planetary orbits don’t have Earth’s orbital parameters. Yet the idea that a twin of our planet is out there, and that’s the most likely place to look for life, persists.
Why would that be the case? Of the billions and billions of Earth-sized planets in our galaxy alone, only a very small percentage meet our naive “Earth twin” criteria. Yet there are billions meeting other criteria that may well be habitable, meaning that unless complex life is heavily restricted to worlds around Sun-like stars for hitherto undiscovered reasons, we may be looking for all the wrong things by seeking a twin to our world.