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Is Pluto a Planet? A Historical Journey through the Solar System

David A. Weintraub
Princeton University Press
Publication Date: 
Number of Pages: 
[Reviewed by
Álvaro Lozano-Robledo
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In the summer of 2006, the celestial body that we know as Pluto was demoted from "planet" to "dwarf planet" by astronomers, due to size and gravitational pull considerations. The author of Is Pluto a Planet? uses this question as the starting point and motivation to go through the history of astronomy, from the ancient civilizations to the recent discovery of planets orbiting distant stars. The goal is to understand the definition of "planet" through history, its evolution and its meaning, as part of the bigger classification of astronomical objects.

The book covers, among many others, the following topics: early Greek astronomy; Aristotle's geocentric model of the universe; the contributions of Aristarchus, Eratosthenes and Hipparcus; an interesting discussion on epicycles and the mathematically accurate early models of the motion of the Sun and the Moon; Ptolemy's model; the Copernican universe with the Sun at its center; Kepler's laws; Galileo's telescope and the discovery of other "planets" orbiting Jupiter (its moons); Huygens and the moons of Saturn; Tycho's and Halley's comets and Halley's prediction; the Titius-Bode rule and the accidental discovery of Uranus by Herschel; the debate around the (dwarf planet) Ceres; the mathematical predicted existence of Neptune and its discovery; the search for intra-Mercurial planets (Vulcan); the discovery of Pluto and the determination of its orbit and mass; objects in the outer solar system, the Kuiper belt, etc; the "Plutinos", Charon, Ixion, Triton; objects in other solar systems; and finally a more detailed discussion of whether Pluto is a planet and the current definition(s) of planet and dwarf planet.

This "popular science" book is written in a very engaging style and I found the choice of topics and the flow of the text to be very entertaining. It does not assume any background on the part of the reader, but obviously some science background will help to get the most out of the text. I am an amateur astronomer, so I already knew many of the facts described in the book, but also found many other interesting parts that I didn't know about. Overall, it is a very enjoyable read.

Álvaro Lozano-Robledo is H. C. Wang Assistant Professor at Cornell University.

Chapter 1: What Is a Planet? 1
Chapter 2: Seven Perfect Planets Made of Aether 6
Chapter 3: The Earth Becomes a Planet 36
Chapter 4: Sixteen Planets 59
Chapter 5: Not Everything That Orbits the Sun Is a Planet 71
Chapter 6: Uranus! 82
Chapter 7: The Celestial Police 95
Chapter 8: Neptune, the Thirteenth Planet 107
Chapter 9: Easy Come, Easy Go 121
Chapter 10: Pluto, the Fourth Ninth Planet 130
Chapter 11: Hidden Secrets of the Outer Solar System 148
Chapter 12: The Plutinos 167
Chapter 13: Is Pluto a Planet? 179
Chapter 14: Goldilocks 185
POSTSCRIPT: Current Thoughts by Other Astronomers 222
APPENDIX: What We Know about Pluto 232