This dissertation describes the construction of speculative planetology over the course of the 20th century. Speculative planetology is a conceptual toolkit for understanding planets as whole, interconnected systems. It has been negotiated through disciplinary exchanges among science, mass media, and public discourse. For example, climate scientists cite science fiction in their textbooks, while meteorologists take up computer graphics techniques designed for imaginary worlds. Across social media, in scientific publications, and at science fiction conventions, scientists and fans come together to speculate about planets like Mars, Venus, Arrakis, and Trappist-1e. Understanding our planet has required not only data collection on, and computation about, our world and others in our solar system, but also cultural work and impulses of the imagination. While the climate crisis has lent urgency and increasing importance to the scientific modeling of planets, the frames, metaphors, and forms used to limn a world were often borrowed from the arts and public culture before they became conceptual tools of science. These themes are elaborated roughly chronologically in four chapters, each of which traces interactions between science fiction and science about how to build worlds. My first chapter treats the origins of the interdisciplinary field of planetary science, showing how it emerged alongside genre changes in science fiction. I index these with Hal Clement’s Mission of Gravity and the journal Icarus, both of which privilege planets as sites for interdisciplinary convergence. The second chapter is about a discourse and set of assumptions shared between climate science and planetary science fiction. With Frank Herbert’s Dune as the main example, I show feedback loops between science fictional references by climate modelers and science fiction’s own use of scientific theory and rhetoric. The third is a study of how speculative planetology was picked up in the field of computer graphics. Here, science fictional narrativity in Star Trek II: The Wrath of Khan and 2010: The Year We Make Contact helped bridge between epistemic scales, allowing computer scientists to claim they were “simulating natural phenomena” despite drawing on decidedly unrealistic mechanisms to create their visualizations of planets. The fourth chapter explores the ethical and political concerns that emerge from a planetary perspective. Drawing on feminist science studies and examining cultural objects from the first successful general circulation model to the videogame SimEarth, it argues that speculative planetology—when done right—requires its practitioners to recognize that no perspective on any planet is complete, or comparable with a putatively universal perspective. This project makes the case for humanists to take up our part of the burden of planetary knowledge production by using the tools of critical analysis on a wider range of materials and contexts in order to reshape the modalities of speculative planetology.