As an introduction to a theme issue on the politics and practices of geoengineering, this paper outlines a framework for thinking about the ‘geoengine’ that underpins concepts of planetary modification. It reviews some of the ways in which geoengineering has been framed as a technological, governance, and promissory discourse and examines the contradictions inherent within some of these framings. Questioning some of the motivations and forms of participation that are evident in the governance of geoengineering, and its current challenge to existing forms of global democracy, the paper speculates on the wider geopolitical implications of operating at the scale of the planet, and what this means for how we currently understand the ‘geo’ in geopolitics. While there is a big mismatch between the restricted decision-making processes around geoengineering and the potential scope and impact of those decisions, there are also opportunities for geographers to make incisive contributions to this debate to change what geoengineering is and becomes as a material–discursive practice. This paper suggests that only by attending to the geo-ontological formations inherent in the fabrication of an engineerable earth, can a new geopolitics be accommodated within political thought that takes account of a much more active ‘geoengine’ that is being opened to political and material modification.