Reviewed by: Marianne Dyson, 2007
Many of us learned some version of the sentence, "My very eager/educated mother just served us nine pizzas/pumpkins," to help us remember the order of the planets. With the demotion of Pluto to dwarf in 2006, that sentence became obsolete. Many people are now confused about what it means to be a planet, but this book does a very disappointing job of answering its title question, "When is a planet not a planet?"
Author Elaine Scott provides young readers with a brief history of the planets as seen by ancient people. She then discusses early astronomers and their views of the solar system. She covers Ptolemy, Copernicus, the invention of the telescope, Galileo, Kepler, and Newton. The author states that Kepler's and Newton's laws have never been broken, and that "They are considered to be as true today as they were when the men discovered them." They may be as true now as then, but they were always "broken" in the sense that they did not completely account for orbital motions. Einstein's general theory of relativity is required to explain the precession of Mercury's orbit.
The third chapter opens with a discussion of how scientists develop hypotheses and theories to explain observations, though how this relates to the definition of planet is not clear. The author makes good use of analogy to explain the scientific method. But she applies it incorrectly by calling the limited observational knowledge of the planets a theory. I know of no theory that purported to explain why there were only six planets until the discovery of Uranus in 1781.
Only one sentence is given to Herschel's discovery of Uranus, not mentioning that he originally mistook it for a comet based on its fuzzy appearance. Also not mentioned is the discovery of Ceres as a test of Bode's law that indicated a planet should exist between Mars and Jupiter. Cere's reclassification from planet to asteroid is described, but not in the context of any test of theory. Neptune's discovery gets short shrift with no discussion of how perturbations in Uranus's orbit were used to hypothesize an unseen planet "tugging" on it. Instead, this idea is attributed to Lowell in his search for Planet X, though perturbations to Neptune's orbit were eventually shown to be the result of inaccurate calculations. Why Lowell failed to find Pluto is not explained.
Chapter 4 on "Pluto's Problems" opens with the formation of the solar system and the difference between rocky and gaseous planets. Though this information is correct, the text leaves the reader with the impression that Pluto being rocky is a problem, but this was not a factor in its reclassification. Its orbit was the most important factor, and that is explained in the text and with a drawing. However, the drawing is not to scale, and this fact is not included in the caption.
Most people assume that Pluto's reclassification to dwarf is a result of its small size, but that is a misconception. The text states correctly that Pluto's size was initially overestimated, and that after the discovery of Charon in 1976, it was reduced. But the text incorrectly implies that this led to its reclassification, and it did not.
There is currently no set amount of mass that defines a planet like there is for a star. (A star requires enough mass for its core to reach fusion temperature.) Instead, the body must have enough mass for its gravity to have pulled it into a round shape and for it to be the dominant body in its orbit (i.e. not a satellite of another body). This generally rules out asteroids (except Ceres) and comets.
Chapter 5 is a short profile of Mike Brown, the astronomer who found Eris in 2003.
The last chapter finally lays out the criteria for the definition of planet versus dwarf planet: a planet must; 1. orbit the Sun, 2. be round, and 3. have enough gravity to "clear the neighborhood" around its orbit. The text says that "A larger planet might clear its orbit by using its gravity to pull other, smaller, objects toward it and destroy them, the way asteroids are destroyed when they hit Earth." The illustration on page 35 is not of an asteroid being destroyed, but a popular artist's concept of a Mars-sized body colliding with the early Earth and forming the Moon. This unique collision between planets was not likely the result of Earth's gravity "clearing the neighborhood," and therefore a marginal choice to use to clarify this difficult concept.
The third criteria for dwarf planet is listed as "not be a moon or satellite of any another planet". Note that "another" is an unfortunate typo in this important line! The text then states that Charon is now also classified as a dwarf planet, and that Pluto and Charon make up a double-planet system. There is no explanation of why Charon is no longer a moon—the reason is that the center of mass of the system (the point around which both bodies revolve) lies outside the surface of either body. There is also no mention of Pluto and Charon's remaining moons: Nix and Hydra.
The book concludes by informing the reader that NASA's New Horizon's spacecraft will reach Pluto in 2015 and provide the first close-up views of this world, Charon, and their moons. A glossary, additional reading, and index are included.
I'm deducting a full point for not adequately answering the title question, especially in regards to the size misconception. Other data in the book was accurate (except for the bit about Newton's laws), but irrelevant to the issue of what makes a planet a planet. I'm also awarding no points for clarity of explanations or use of terms because of all the important information left out in chapters 2 and 3 and the misuse of theory. The book only gets a half point for current data for leaving out Einstein, Nix, and Hydra. I'm taking off a half point for readability because I think preteens will find the passive voice dull and the level of information more appropriate to 6-8-year-olds. Also, the chapter titles do not reflect their contents and there's that typo in the most important line in the whole book. Illustrations only get a half point because of the inaccurate orbital drawing and inappropriate planetary collision. Total is 2.5 points. Not recommended. When is a Planet Not a Planet? is a hodgepodge of solar system history and data that does not address the title's question.
Title: When Is a Planet Not a Planet: The Story of Pluto
Author: Elaine Scott
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