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As the 2006-2007 school year began, classrooms from kindergarten to college were abuzz with news from the frontiers of the Solar System. Only days earlier, on August 24, astronomers had kicked Pluto out of the family of planets.
At least that's the way most non-scientists were viewing the outcome of a vote taken at the meeting of the International Astronomical Union (IAU). Astronomers, like Vanderbilt University professor David A. Weintraub, knew better. As he writes in Is Pluto a Planet?, Pluto's status had been in doubt from almost the time it was discovered in 1930.
The doubts intensified early in the present century with the discovery of several bodies only slightly smaller than Pluto in the region of the Solar System known as the Kuiper Belt. Astronomers began thinking of Pluto as the first known and the largest Kuiper Belt Object discovered to date. With improved observational tools, most expected that an even larger KBO would soon be found.
In January 2005, astronomer Michael Brown announced the discovery of a large KBO in a photograph taken by his observing team at the Palomar Observatory on October 21, 2003. Its brightness suggested that it could be as larger or larger than Pluto. Designated 2003 UB313 by the IAU and nicknamed "Xena" by Dr. Brown and his colleagues, the object was quickly touted as "the tenth planet."
As Dr. Weintraub writes, it was not the first time a tenth planet had been found. The largest asteroids were considered planets for many years after their discovery early in the 19th century. As a result, Pluto was "the Fourth Ninth Planet."
It was also not the first time that astronomers had postulated the existence of yet unseen planet-sized bodies. Until Einstein's general theory of relativity explained deviations in the orbit of Mercury, astronomers searched avidly for Vulcan, a supposed planet so close to the Sun that it was hidden by glare. They hoped to repeat the mathematical tour-de-force that led to the discovery of Neptune. (Pluto was discovered after similar calculations, though we now know that was a fortunate but misleading accident.)
"Our quest to answer the question Is Pluto a planet? led us directly to a question about physics: What is a planet?" Dr. Weintraub writes. "Answering this second question, which was not simple or easy, has revealed that we live in a solar system that is quite different from the one we thought we lived in." That second question, and the author's surprising answer to the first, make this Historical Journey Through the Solar System fascinating and entertaining reading.
Yet the book suffers from an "elephant in the room" flaw that will hurt it in the marketplace. Nowhere does it discuss the 2006 IAU meeting and the tumultuous public reaction to it. It does not mention the nickname "Xena" for the latest tenth planet or its official name, Eris.
Weintraub would have preferred that the IAU adopt a broader definition of "planet" that includes subcategories, making Earth not only a planet but also a "rocky planet." Pluto, and Eris, plus several other large KBOs and asteroids would also make the cut.
He argues that having a short and memorable list serves neither astronomy nor education: "It is time to start teaching our youngsters something more complicated, with more depth of meaning, than the simple memorization rubric My Very Earthly Mother Just Served Us Nasty Pizza."
Too bad that he and the publisher did not delay the book long enough to put this
minority opinion in its full context.