Is Pluto a Planet : A Historical Journey Through the Solar System by David Weintraub
[amazon
Author David Weintraub is Professor of Astronomy at Vanderbilt.
There has long been a debate about whether or not Pluto is really a planet. Is it or isn’t it? In order to answer that question, one must first decide exactly what a planet is. A recent discovery, 2003 UB313, 
The book traces the definition of “planet” throughout western history. From the orignal seven “planets”: sun, mercury, venus, moon, mars, jupiter, saturn in the earth-centered universe, through the period of 12 planets (which included Jupiter’s moons) to ten (when Jupiter’s moons were rejected but Ceres was discovered) to the recent 9. But Pluto was discovered by accident and should never have been considered a planet in the thinking of the time. Had smaller bodies been discovered first, it never would have been.
So Weintraub tries to come up with a astrophysical definition of what a planet is, rather than the philosophical definition in common use. That definition (it’s not really a spoilier) is a body, bound by gravity to a star which is smaller in mass capable of sustaining nuclear fusion, but large enough that its own gravity gives it a spherical shape. By that definition, there are about 24 planets in the solar system (it would, for example, include Ceres and Sedna). But philosophical arguments, so far, have overruled that and there really is no consensus on the issue. In fact, the entire final chapter is a survey of contemporary astronomer’s opinions on the question.
It’s a fascinating read, well written and accessible, although the way new discoveries are named makes it slow going in some paragraphs.
