Last week's compromise at the International Astronomical Union meeting in Prague over the definition of "planet" has been roundly mocked around the world by science writers, teachers, and bloggers alike. It has all the charm of a committee written product and that's exactly what it was. The vote on a definition is still two days away and several competing definitions are being discussed.
Traditionally, planets were lights in the sky that moved differently than the stars. With the introduction of the telescope and the heliocentric theory of the solar system, scientists figured out that the planets not only moved differently than stars, they were actually more like the Earth while the stars were like the Sun. With planets now thought of as Earth-like objects orbiting a star, the definition of planet had changed from one describing its movement to one describing its nature.
In 1781 William Herschel discovered a new planet and named it George after his king. Other astronomers ignored his name and called it Uranus, and started looking for planets of their own to name.
In 1801 Giuseppe Piazzi discovered an object orbiting between Mars and Jupiter, announced that he found a tiny planet, and named it Ceres Ferdinandea. The name seemed to cover all the bases, it had an element from classical mythology and it sucked up to his king. Unfortunately, Ferdinand of Sicily had recently been overthrown by Napoleon and no one went along with naming a celestial object after a powerless refugee. As astronomers began looking at the region in which Ceres had been found, they promptly found three more tiny planets. These were named Pallas, Juno, and Vesta. Naming planets after kings had proved to be a non-starter, so the astronomers stuck with classical mythology.
These tiny planets bothered astronomers. They were smaller than any of the known moons. Even with the best telescopes astronomers could see no details on them. They were just four tiny dots of light. Herschel, who by now had discovered four moons to go with his planet (he named them after characters in "A Midsummer Night's Dream"), suggested not letting these insignificant objects into the august club of planets. He suggested a new word, asteroid (star like), to describe them. The little planets remained in limbo until the 1840s when a new generation of more powerful telescopes led to the discovery of more tiny bodies between Mars and Jupiter. Facing the prospect of dozens or more new planets, the international astronomical community adopted Herschel's suggestion and demoted the asteroids into their own category apart from the planets.
At about this time, the search was on for another planet beyond Uranus. Based on a half century of observing Uranus' orbit some astronomers had come to believe that the gravity of another large body must be affecting it. By the 1840s they had an idea where to look. In 1846 Urbain Le Verrier calculated the exact location and observers had no problem finding the planet. They named it Neptune.
The same orbital mechanics that led to the discovery of Neptune led astronomers to expect another large planet further out. In 1930 Clyde Tombaugh discovered Pluto. Astronomers immediately knew that Pluto was too small to be the expected gravity source. Pluto, however, had an advantage that Ceres never did in becoming accepted as a planet: mass communication and mass literacy. The discovery of new planet was announced in newspapers and newsreels. The name was suggested by an eleven year old girl in Oxford. Walt Disney introduced a character named Pluto into his Mickey Mouse cartoons later that year. For three quarters of a century, children have learned that Pluto is a planet.
But astronomers never liked it. Pluto was too small to be a real planet. It's orbit is highly eccentric and at an angle to plane of all the other planets' orbits. In the very same year that Tombaugh discovered Pluto, Frederick C. Leonard predicted that there was a whole belt of tiny objects beyond Neptune. Sooner or later we would have good enough telescopes to find them and the astronomical community would be faced with the same problem that they had faced with the asteroids: too many and too small to be planets. That day came about fifteen years ago. Today, about 800 of these Kuiper objects, as they are called, have been discovered.
This brings us back to the present and the debate at the IAU meeting. Last week's proposed definition is that a planet must be big enough for its gravity to pull it into a spherical shape. This definition would include the inner planets, the outer planets, Pluto, about four of the asteroids, and about four other Kuiper objects. It also requires a planet to orbit a star and not another planet; that excludes all of the moons in the solar system. That definition gives us between 12 and 17 planets and the list will be sure to grow as other large Kuiper objects are discovered.
On Friday, a group of astronomers introduced a second definition at the IAU. This definition requires a planet to dominate its neighborhood and would eliminate all Kuiper objects, including Pluto, leaving us with only eight planets. Rather than send the two definitions up against each other, the executive committee of the IAU yesterday decided to hold separate votes on each element of the definitions. This has the possibility of going horribly wrong.
In today's New York Times David Overbye says that another definition will be proposed today. It's not clear from his mention whether this is an entirely new definition or an attempt at compromise between the various factions at the IAU.
Personally, I think the IAU should leave it alone. Planet is not an especially useful category. Based on size, composition, orbital characteristics, and probable origin there are at least five types of objects orbiting the sun (not counting moons, human made satellites, and that gigantic space cruiser full of super intelligent squid hiding behind Titan). Based on historical tradition, nine objects in three of those categories are called planets. Geologists have not been hampered by the fact that the traditional continents are a useless category. They don't insist that we give up on continent and start memorizing all twenty or so tectonic plates. Astronomers should learn from that. Let tradition call things what tradition will, and let science go its own way. Nerdy kids know that India is a better candidate for continent that Europe. The same kids will learn the differences between what most people call planets and the categories that matter to scientists.
By spending so much time on this, the IAU just make themselves look silly.