On Wednesday, the 2006 meeting of the International Astronomical Union begins in Prague. I will NOT be there, as I have to leave for the telescopes in Chile tomorrow, and a week after that I am moving from Arizona to Texas. But several thousand astronomers will be in Prague over the next two weeks to talk about research and discuss important scientific and procedural issues.
One of the biggest topics that will come up is the definition of a planet. The issue of whether Pluto will continue to be called a planet has captured the public's attention, but this will also deal with what to call objects the size of Jupiter floating through space, and what to call objects ten times the size of Jupiter orbiting other stars.
The gossip I've heard (which may be wrong) is that there is no good definition of a planet. Many of us would claim to know a planet if we saw it, but crafting a working definition is hard! Perhaps the best definition would include how planets are formed, but we can't be certain how any specific object formed.
As for Pluto, it has historically been called a planet, but now we know of dozens of objects that are very similar to Pluto in almost every way -- size, shape, where they are in the Solar System, what they are made of, having moons, and so on, and so on. So, if you want to keep calling Pluto a planet, we will end up with a couple of dozen planets in our Solar System. Or, we could set a size limit, and then we would have 8 planets in the solar system (but what about planet moons that are larger than Mercury?). And what is the rational for a size criterion?
In 1801, astronomer Giuseppe Piazzi discovered a moving object in the sky. It's orbit was between Mars and Jupiter, and it was thought this could be a new planet. The object was named Ceres, and for almost a year was known as the 8th planet (Neptune and Pluto were still unknown). Then, another, almost identical object was found between Mars and Jupiter. Within a few years, several of these were known. They were much smaller than the other planets, and so soon became known as "asteroids," and Ceres was demoted from planetary status.
Pluto has been a planet for over 70 years, but my hunch is that soon it, too, will have to be demoted to the king (or maybe just prince) of the "Kuiper Belt Objects." (Kuiper is pronounced KOI-pur).
My other hunch is that, no matter what the definition of a planet is, many people will be unhappy.