We previously proposed a hypothetical learning progression around the disciplinary core idea of the Solar System and its formation as a first step in a research program to begin to fill this gap and address questions of student learning in this domain. In this study, we evaluate the effectiveness of two dimensions within the learning progression, dynamical properties and gravity, in describing change in how student reason in the domain across the course of their 14-week astronomy unit. A sample of sixth-grade students (N = 24) were interviewed before and after instruction. We compared changes in how students explained the dynamic properties of planets and the role of gravity in the Solar System to their experiences during instruction. Our findings provide evidence for the usefulness of this learning progression in describing how students' explanations may progress, offer insight into how instruction may support that progress, and highlight the challenges in drawing conclusions on how students' explanations may progress when limitations are identified in instructional experiences. We also discuss the connection between these two construct maps but also point out what appears to be a missing element in our original definition of the learning progression: inertia.