I propose that a federal system of government, able to implement the principle of subsidiarity on interplanetary scales, will be the most appropriate form of political organisation to guide the future colonisation of the Solar System. Only a federal system will simultaneously satisfy the three key criteria of (1) accommodating and protecting social and cultural diversity among Solar System colonies; (2) minimising the risk of conflict between these diverse colonies and with the Earth; and (3) protecting the rights and liberties of individual human beings throughout the Solar System. No other form of political organisation is likely to leave humanity in a better position to exploit the opportunities, and minimise the risks, associated with building a Solar System-wide civilisation.