As I had expected, the recent discovery of two trans-Neptunian objects similar in size to Pluto has prompted a definition of “planet” that demotes Pluto to the new class of dwarf planet. This restores the number of planets, now called “classical planets” to 8, and ensures a distinction between objects that orbit the Sun with only their moons as companions and objects that orbit in a belt of similar objects. The definition chosen by the International Astronomical Union (IAU) is more satisfying than that first proposed by an IAU committee; that definition would have immediately increased the number of planets to 12, with estimates that the number could rise as high as 30. Among the objects that would be considered planets under the previously proposed definition are large asteroids such as Ceres, which was discovered in 1801, and the largest of the Kuiper Belt objects, whose numbers are expected to grow rapidly in this century. The current definition is a better reflection of how astronomers approach as objects of study the planets, the asteroids, and the objects in the Kuiper Belt.
For astronomers, this definition is just a technicality. For school children, it may be a blessing, as they only need to learn 8 planets rather than 12-plus planets. Books, of course, would need to change. Just this weekend, my wife bought a Dr. Seuss book on the planets that of course includes Pluto. On the other hand, the compulsion felt by some composers to extend Gustauff Holt's the Planets to include Pluto is now gone, a result that in itself justifies Pluto's demotion.
Not too surprisingly, the only real impact of calling Pluto a dwarf planet is within a community where science plays no role: the astrology community. According to a Wall Street Journal article, many astrologers use Pluto, but not the other dwarf planets, in their forecasts. With a single vote, astronomers have changed astrological predictions across the globe, which, with the exception of a forward-looking few who include the minor planets in their calculations, has angered the astrologers. Too bad such fun is a one-time event.
The evolution of a definition in astrophysics is not unusual. Astronomy starts with phenomenology, classifying objects by their observed characteristics. Later, as theories are developed and tested against improved observations, the terminology shifts to reflect this better understanding. This usually means creating subclasses of the original class.
The term nova was coined to describe the sudden appearance of a star. As these objects became better understood, the terminology was expanded to include the supernova, which is much brighter, but much more rare, than the nova. These two names for stars that suddenly appear in the sky reflect very different physics. The nova is an outburst by a cataclysmic variable star, while the supernova is created by the implosion of a star. In time, the supernova itself was split into several classes, such as the type 1a supernova, which is the implosion of a degenerate dwarf star in a binary star system, and the type 2 supernova, which is the collapse of the core of a very massive fusion-powered star.
The planet is the earliest example of the evolution of a term. The word was coined by the ancient Greek astronomers to describe the star-like objects that moved across the sky. The nature of the planets was totally unknown to these astronomers; they were planets by virtue of their motion rather than by their physical traits. With time, however, the idea that the planets were physically different from the stars began to take hold. This was driven by the change in brightness of Venus and Mercury as they change their position in the sky. Copernicus himself pointed out that the way they change their brightness suggests a body with phases similar to the Moon's. This suggests that the planets do not emit light, but reflect Sun light. On the other hand, given the great distance of the stars in Copernicus's solar system, the stars must emit their own light. Planet and star then came to represent two physically different objects. The idea of the planet continued to change in modern times with the discovery of asteroids at the beginning of the 19th century. These bodies were called minor planets to distinguish them from the larger bodies of the Solar System.
The naming of classes in astronomy is a mixture of the unimaginative and the overly imaginative. Most classes are named after a prototypical object within that class. For example, a class of variable stars is named after RR Lyra. One of the recent IAU resolutions defines a plutonian object as an object similar to Pluto. But in other cases, the name of a class reflects an observed property, such as the name nova, reflecting the sudden appearance of a star-like object, or the starburst galaxy, which is a galaxy containing many newly-formed stars. Sometimes classes have two names, often because someone with an ego has invented a clever new name. So the AM Herculis binary star, which is a class of cataclysmic variable system, was rechristened as the “polar” by some astronomers to reflect the strongly-polarized light emitted by these systems; to me, however, this new name brings to mind polar bears and penguins, which doesn't fit with an object hot enough to emit x-rays, so I still call them AM Herculis binaries.
The names of the stars give astronomy a romantic allure. These names are descended from the ancient astronomers, which reinforces the sense that modern astronomy is part of an ancient tradition. Some objects discovered in modern times have modern nicknames, such as the Whirlpool Nebula. But the objects with names are exceptional. Only the brightest objects have names. Most objects in the sky are simply known by a letter or a number. This may be a letter plus the name of a constellation for a visible star, or it may be a series of letters followed by a number for a member of a catalog of objects. You may be familiar with this from Star Trek, where a character may talk about investigating NGC 1234; this is a number from the New General Catalog of Nebulae and Clusters of Stars, which was new in 1888. Many such catalogs exist, each produced by a particular research group working on a particular research project. A modern list of stars or galaxies therefore has all the allure and romance of a car part catalog.
New planets, however, beg to be given names similar to the classic names of the original five planets of ancient astronomy. So Uranus was given without controversy the name of the Greek god of the sky. The naming of Neptune and Pluto, however, came up against the egos of the discoverers of these planets. In the case of Neptune, Leverrier, who was one of the two astronomers who correctly predict its existence and location from its gravitational perturbations of Uranus, pushed unsuccessfully to have the planet named after himself. In the case of Pluto, astronomers at the Lowell observatory pushed to have the planet named after the deceased Percival Lowell, who incorrectly predicted the existence of a large planet at the position occupied by Pluto. While they did not succeed in naming the dwarf planet Lowell, they did succeed in giving it a symbol containing Lowell's initials.
Now we meet not just ego, but irony, in the naming of the most recently discovered dwarf planets. What particularly seems to have caught the public's attention is the nickname of the Kuiper Belt object that forced a formal definition for “planet;” Zena. The name is an ironic comment on the Greek habit of naming stars, planets, and constellations after mythological characters. If the Greeks can name constellations after heroes such as Perseus and Hercules, why shouldn't we name characters after our current television and movie heroes. But I doubt this irony was intended; the person who came up with this nickname is more likely just infatuated with an attractive woman in leather. At least the choice shows some small cultural depth: the nickname is not after a character from Star Trek, Star Wars, or the Lord of the Rings.