Pluto isn't a planet. Does that make you angry? Do you want to shake your fist at me, or call me stupid? I once volunteered at the Hayden Planetarium in Manhattan, giving a presentation about why Pluto was no longer considered a planet; in The Planets, Dava Sobel has a chapter similar to the presentation I used to give that describes Pluto's history, including its ultimate demotion from planet to renegade object of the Kuiper Belt.

The transition into the Pluto chapter involves an anecdote of Sobel's grandfather and Ellis Island, her emotional reponse to the story, and therefore her predisposition of creating false memories "such as the recollection... of having interacted with visitors from another planet." Eventually she explains what this has to do with Pluto—a scientist embarrassed his observatory by publishing a book about Martians and, to save their reputation, came up with a formula that determined Pluto's existence—but by the time she gets there, it feels like she's already spent more time on Pluto than Uranus and Neptune put together (both of which are actual planets).

One difficulty, Sobel points out, is that the word "planet" was coined before a clear definition was established. (An asterisk leads to a discussion of the word "life," which "poses similar difficulties" for scientists.) I really wanted her to explain, maybe, that the original definition classified both the moon and the sun as planets, or that more recent designations agree that planets are heavenly bodies that orbit around a star, but differ in their body size and type. Instead I found that information in the dictionary.

Sobel talks at Town Hall (Eighth Ave and Seneca St) on Thurs Oct 27, 7:30 pm, $5.