"I" Want More Planets

Today is another A-Z Wednesday, hosted by Reading at the Beach. Bloggers highlight a book that starts with the letter of the day; I read the book on that day but that is not required. This week we celebrate the letter "I." At least I do.


I've been plugging along at Is Pluto a Planet? by David A. Weintraub for a few weeks now. He writes animatedly about the history of our understanding of planets and the changing definitions as people refine their understanding of the universe. For example, the sun was considered a planet back when it revolved around the earth. The moons of Jupiter were called planets, but when the heliocentric model really took off, planets were defined as circling the sun, so moons were exiled from planetdom. Then more planets were discovered -- Uranus, Ceres, Pallas, a few others, then Neptune. And Vulcan. Oh, and Pluto. But along the way Ceres, Pallas, and their friends were converted into asteroids, and Vulcan disappeared (it was thought to orbit inside of Mercury, and accounted for the irregularities of the first planet's path. Turns out Einstein's special relativity accounts for it even better.). So the number of planets has always ballooned and deflated, depending on the power of our telescopes and the consensus of astronomers.

Strangely, it turns out that I know more about the ancient history of astronomy than modern stuff. The closer the book came to the present, the more interesting I found it. Suddenly there was drama and controversy -- Is Neptune's orbit explainable? Where would planet X be? Pluto was found because a rich astronomer paid someone to look for it, but he was looking based on flawed math -- it was only a coincidence that Pluto was there. In our huge sky, that seems amazing!

The final chapter looks at the modern definitions of a planet, especially ones that attempt to include or exclude Pluto. Weintraub does not have patience for "scientific" definitions based on utterly arbitrary definitions, so "things bigger or smaller than Pluto" do not cut the mustard as a definition. He prefers setting an upper bound as "not big enough for fusion" and a smaller bound as "big enough to be round." Unfortunately, his book was written just before the IAU's definition, which drags in the criteria that the planet clear its orbit, but I suspect he would not be impressed. Jupiter and Saturn have significant debris at their Lagrange points, in their orbits, and it is clearly easier to clear smaller sections of space nearer to the sun. Not to mention that the definition ignores orphan planets and other planet-looking objects. His definition gives us about 24 planets, with Ceres and Pallas and other hopefuls back in the front of the astronomy textbooks. I found it rather persuasive, although I'm not sure I can come up with a mnemonic for them all. That's what comes with replacing mythology with definitions. Loads of fun, but skip to the last few chapters if you want just the local news. B+

PS. I just looked up Weintraub's web page, so I can link him, and I was right about his distaste for the new definition of planet. He even mentions the Trojan asteroids in Jupiter's orbit! Also, since Pluto crosses Neptune's orbit, the "clear your orbit" rule seems to mean that Neptune isn't a planet either. Hmm. More fun in the skies!