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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·log′i·cal (plăn′ĭ-tl-ŏj′ĭ-kəl) adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Astronomy) astronomy someone who studies or specializes in planetology
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014