Baby boomers like me who came of age in the 1970s and date their first reading of science books to the 1960s learned that Mercury is in a tidally-locked orbit around the Sun, so that one side is always in ferocious sunlight while the other is exposed to the near-absolute zero of space. By the mid-60s, that was known to be untrue, but it took years before the truth percolated into popular culture. We learned that Jupiter has twelve moons (over sixty have now been counted) and Saturn nine (we now count 61, in addition to the matter making up the sixth planet's rings).
The nine planets were Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto - that had been the situation since Pluto's discovery in 1930, when our parents were young. But in 1978, the astronomical dot of Pluto was resolved into the binary system of Pluto and Charon, two relatively small bodies in orbit around a common center. Pluto itself has turned out to be far smaller than we baby boomers were taught. By the early 1970s, initial estimates had been scaled down, and it was thought to be comparable in mass to Mars. Certainly, we thought, it was larger than Mercury.
In fact, it is far smaller than Earth's moon and only about a twentieth of the mass of Mercury - the next smallest of the "nine planets" we were taught about.
Until very recently, astronomy needed no formal definition of a planet, but this has changed as our knowledge of the Solar System has increased. During the 1990s we discovered a toroidal region of space known as the Kuiper Belt, which contains not only Pluto but many other objects of similar composition and with similarly unusual orbits when compared to those of the eight larger planets. With a better understanding of the Solar System, astronomers came to understand Pluto as the largest of these Kuiper Belt objects, all of which are very different from any of the other eight planets, and much smaller. Astronomers began to find large objects even beyond the Kuiper Belt, all contributing to what I call the Grand Opening Up of the Solar System.
Against this background, astronomers sought a more precise method of classifying the various large objects that had been identified in orbit around the Sun. Any definition of a planet that included Pluto but definitively excluded the smaller, but similar, Charon, and disqualified such bodies as Quaoar, Sedna, and Eris, would seem arbitrary. Eris, discovered in early 2005, lies beyond the Kuiper Belt in a region of space referred to as the Scattered Disc. It is significantly larger than Pluto, and would surely qualify as a planet if Pluto continued to qualify.
There is an exciting story to be told about the Grand Opening Up of the Solar System, how it led to efforts in 2006 to develop definitions of such categories as "planet", and how it still goes on. A well-informed science journalist with good publishing connections could get a wonderful book out of this story, in the process telling the public much about contemporary astronomy and why the study of our own Solar System is currently in such a wonderful ferment. I'd like to read that book. It could explain how the category "dwarf planet" was invented for bodies such as Pluto and Eris, why some large bodies have not yet been classified until we know more about them (Charon is one of these, as is Vesta, the second largest body in the Asteroid Belt between Mars and Jupiter), what plans are afoot to investigate these bodies, and much else.
Unfortunately, I can't imagine Chris Mooney and Sheril Kirshenbaum writing that book. Where I see excited astronomers responding rationally and reasonably to the Grand Opening Up of the Solar System - refining the categories and definitions that they use in their work - they see a bunch of mean scientists taking an opportunity to give the public a poke in the eye by taking away its beloved ninth planet. This is a pity. They could have done some positive communication here, in the opening chapter of Unscientific America. Instead, they produced a dull and inaccurate narrative that is meant to support their theory that out-of-touch (or even mean-natured and anti-populist) scientists are largely to blame for America's alarming degree of scientific illiteracy.
What a waste of a great opportunity to practice what they preach, and improve the public's understanding of what is really going on in science.