Completed a fun book today. Is Pluto A Planet?, by David Weintraub
Pluto was recently demoted from its previous status as a planet to a Plutino following the recent discoveries of numerous Kuiper Belt Objects (KPO) and Scattered Disc Objects (SDO). While the debate continues and may be reopened to further discussion, David Weintraub, a Professor of Astronomy at Vanderbilt University, has written a fairly short and easy to read history of discovery of the objects in our solar system. That history includes the various ways in which science has classified those objects.
What were considered planets has varied through history. Weintraub continues the survey by discussing various means in which to define what a planet is or should be. In the process he takes a look at other bodies that fall on the edges of definition. These include brown dwarfs, pulsar planets, orphaned planets, and free roving planets that seemed to have formed not in stellar discs but out of material insufficient to create enough mass to ignite and form interior nuclear reaction to burn as a star. None of the definitions Weintraub proposes are satisfactory to solving the question, reasons Weintraub. As a science nerd they do not satisfy me either.
Weintraub concludes that Pluto should be reclassified once again a planet as well as reclassifying other objects as planets as well, including recently discovered 2003 UB213, 2002 AW197, 2002 TC302, 2003 EL61, 2005 FY9, Sedna, Orcus, Quanoar, Ixion, Varuna, Cybele, Hygeia, Vesta, Palla, and Ceres. Granted, he concludes, that they are also parts of object types such as Ceres would be a planet and also a large member of the asteroid belt, Quanoar, Sedna are planets and Kuiper Belt Objects. Pluto is a planet and the premier Plutino.
If you want to know why he concludes such, you have to read the book. I happily agree that the solar system contains more than the currently recognized 8 planets. Good book.