Publicly displaying personal tracking data can promote healthier lifestyles, with most existing research focusing on digital visualizations. However, public physicalizations have been shown to attract more attention and foster greater engagement. Despite this, few studies in this area have utilized personal physical activity data as input or explored its impact on motivating data generators’ physical activity. To address this gap, we designed PlanetWalker, a system that visualizes users’ walking steps on their phone and shows the visualization publicly through a digital display, as well as medium- and large-scale public physicalizations as probes. The probes were deployed following the Wizard of Oz method in sequence during a six-week in-the-wild study. Through evaluations with users generating data and passersby, our findings show how public physicalizations can improve motivation, facilitate social interaction, and increase engagement. We conclude by discussing the design directions for supporting the public presentation of personal data through public physicalization to motivate physical activity and provide design trade-offs and suggestions on the physicality and scale of the medium.