This study aimed to determine students' knowledge levels related to specific astronomy concepts and the effect of a planetarium environment on teaching. The study sample included seventh-grade (12-13 years old) students. For this purpose, 240 students of various socioeconomic and cultural levels from six schools (two in the city center, two in the districts and two in the villages) were enrolled in the study. The pretest-posttest control group quasi-experimental design was used in the study. The experimental and control groups were generated by random assignment. The "Solar System and Beyond" unit was selected. In the experimental group, the unit was taught with the use of a planetarium environment, whereas the same unit was taught to the control group students in a classroom environment. A test consisting of 14 multiple-choice questions was used as the pretest and posttest at the beginning and end of the unit. The data obtained were evaluated using the SPSS 20.0 software package program. The study results showed that teaching astronomical concepts in a planetarium environment was more effective than in a classroom environment. The study also revealed that students in the planetarium-assisted group were more successful in comprehending subjects that require 3D thinking, a reference system, changing the time and observation of periodic motion than those in control group.