plan·e·tol·o·gy 
(plăn′ĭ-tŏlə-jē)
(plăn′ĭ-tŏlə-jē)n.
The branch of astronomy that deals with the composition and structure of the planets of the solar system and with the relationships, similarities, and differences between them.
plan′e·to·logi·cal (plăn′ĭ-tl-ŏjĭ-kəl) adj.
plan′e·tolo·gist n.