In this article we build on recent ‘geo-’ themed work in human geography to move beyond the Earth and consider what our own planet shares with other astronomical bodies. Mobilising ideas from the planetary sciences, we sketch a speculative philosophy of planetary evolution in three steps. First, we develop an idea of ‘the planetary’ that sees planets as ongoing processes and sets
of relations stretched over space and time. We then consider ‘the intraplanetary’: how planets develop internal self-difference and evolve their own unique identities. From there, we turn to ‘the multiplanetary’, exploring in the broadest way how planets might come to forge new kinds of interplanetary relations. We conclude by asking how the idea of a planetary becoming ‘without return’ might help us think more expansively about human interactions with other astronomical bodies – and with our home planet.