Planets that orbit at very similar distances from their stars could jockey for position while still remaining in a broadly stable configuration for billions of years – long enough to be spotted by astronomers. Such star systems might be so unlikely that they could only be created artificially, by advanced alien civilisations.
Most planets have their orbits to themselves, but orbital mechanics allows for worlds to be close enough to effectively share an orbit, forcing them into a helter-skelter dance where they …