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Review: The Case for Pluto

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The Case for Pluto: How a Little Planet Made a Big Difference
by Alan Boyle
Wiley, 2009
hardcover, 272 pp., illus.
ISBN 978-0-470-50544-1

How knew that such a small planet, er, dwarf planet, could cause such a ruckus? Over three years ago the International Astronomical Union (IAU) formally defined the term “planet” in such a way that it excluded Pluto, which previously had been considered one of the solar system’s nine planets, consigning it to the confusingly-named category of “dwarf planets” (which is not a category of planets, as the name might suggest, but a wholly separate one). The decision set off a flurry of media reaction at the time, from news articles to op-eds and cartoons. It has also had a delayed impact in the slower cycles of the book publishing world, as several books discussing the topic from various directions have hit the bookshelves in the last year or so. The latest, and certainly one of the better ones, is Alan Boyle’s The Case for Pluto.

With a title like that (echoing The Case for Mars, the series of Mars exploration conferences and, later, the Robert Zubrin book outlining the how and why of human Mars exploration), one might expect The Case for Pluto to be a tract arguing passionately in favor of reclassifying Pluto as a planet, just as Neil deGrasse Tyson made his case for Pluto’s demotion in The Pluto Files (see “Review: The Pluto Files”, The Space Review, April 27, 2009). Instead, Boyle, the science editor of, is far subtler, devoting much of the book to a history of planetary discovery, followed by Pluto’s discovery and later studies, which made it increasingly clear that Pluto was like neither the inner terrestrial planets nor the outer gas giants.

“Pluto isn’t the ninth of nine,” Boyle writes, “it’s the first of many.”

Boyle traces the debate about whether Pluto is a planet to a speech by Brian Marsden, then head of the Minor Planets Center, in a 1980 speech at a conference marking the 50th anniversary of Pluto’s discovery, where he suggested that Pluto should be reclassified as a minor planet. That argument raged off and on for a quarter-century before the fateful IAU meeting in 2006, covered in detail in the book. The “Battle of Prague” (as Boyle titles the chapter of the book that deals with the meeting) was less a conflict between friends and foes of Pluto than it was between geophysicists and dynamicists: the former wanted to use a body’s roundness as the primary criterion for planethood, which would include Pluto and several other worlds, while the latter favored the body’s ability to clear out its orbit, which would exclude those worlds. The dynamicists won the day, and the rest is history—at least for now.

In the book’s final chapter, also titled “The Case for Pluto”, Boyle makes his case for how Pluto should be classified. “Pluto isn’t the ninth of nine,” he writes, “it’s the first of many.” That is, it should be considered the first of a class of planets that stands alongside terrestrial and jovian planets. (This class is called “dwarf planets” later in the chapter, although it’s different from the IAU definition, where “planets” and “dwarf planets” are two distinct categories.) There are potentially hundreds of such dwarf planets in the solar system, a change that may be difficult for people used to a solar system of nine (or eight) planets. “Some people may find it difficult to handle a planetary paradigm shift, but shift happens, whether we like it or not.”

One of the biggest lessons from the book is a reminder that science is practiced by people, and thus how a debate like this “shows how politics and personalities can affect the scientific process”. The Case for Pluto is a good summary of the debate on Pluto’s identity, and a reminder that as our knowledge of the solar system and the universe changes, our structure for classifying that knowledge must change as well.