The discovery and characterisation of planets orbiting distant stars has shed light on the origin of our own Solar System. It is important that college-level introductory astronomy students have a general understanding of the planet formation process before they are able to draw parallels between extrasolar systems and our own Solar System. In this work, we introduce the Planet Formation Concept Inventory (PFCI), an educational research tool used to assess student learning on the topic of planet formation. The PFCI Version 3 was administered to <i>N</i> = 561 students pre-instruction and <i>N</i> = 374 students post-instruction. Here, we present a Classical Test Theory (CTT) analysis of the PFCI Version 3. Ultimately, we conclude that the PFCI is a reliable and valid instrument that can differentiate experts from novices, and can be used to assess college-level introductory astronomy students' learning on the topic of planet formation. Initial findings on class normalised gain scores indicate that the PFCI may be capable of assessing the effectiveness of different instructional models. In the future, we recommend a national study of the PFCI to discern its ability to provide insight regarding the ascribed characteristics of learners and the effectiveness of different instructional strategies being used to teach this topic.