Quickly now: How many planets are there in our solar system?
If you were awake during any science class from the second grade on, you know the answer to that question is nine. More people are probably aware of the number of planets in our solar system than are aware of the number of continents on Earth. That number is seven.
But by Friday of this week, there is a very good chance that the number of officially-recognized planets will have changed. No, none of them are going to fly off into interplanetary space nor fall into the sun. And new ones are not going to be created via some Velikovskyan collision of worlds.
Rather, the International Astronomical Union has proposed a new definition for planet. The proposal will not really change the official definition, since there has never actually been an official astronomical definition of the word, only a popular one.
It seems rather unusual for scientists of any discipline to have never officially defined a term they have used for hundreds of years, but so it is with planet. But a committee of the IAU has now proposed a definition, one which the American Astronomical Society fully supports. Their recommendation will be debated this week at the IAU general assembly in Prague, with a vote expected on Thursday.
If the full body accepts the recommendation, as expected, then every new science textbook will need to be updated with a new count for the number of planets in the solar system. And every school child from now on will have to learn that there are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
That’s right; Pluto will likely be downgraded by the new planetary definition. It will, instead, become a pluton, a new term that defines an object that is larger than a comet or an asteroid, but not big enough to be a full-fledged planet.
Most students now learn that Pluto has a single moon, called Charon. But that will also change. Since Charon and Pluto are so similar in size and they both orbit a common center of gravity that is in space between the two bodies, the correct term for the pair will be a double pluton.
The asteroid Ceres will also be a pluton, as will the newly discovered planetary object orbiting far beyond Pluto, unofficially named Xena. At least a couple dozen other very large asteroids may be reclassified as plutons as well.
So what is the newly-proposed official definition of a planet? If the proposal is accepted, a planet will be defined as “a celestial body that has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape and is in orbit around a star and is neither a star, nor a satellite of a planet.” In other words, it has to be big enough to have enough gravity to hold itself together as a sphere.
It certainly is esoteric-sounding enough to be a scientific definition. I’m not sure most lexicographers would accept it as being in standard form, though, since it actually uses the word planet in the definition of the word planet. Otherwise, it seems accurate enough. It is the definition that I have supported for years even before I knew anyone was trying to define a planet, although I would have used fewer words.
So, with the crack of a gavel or the show of hands, or however they do votes at the IAU general assembly, the world view of the solar system will be all new. Our solar system will now be taught as the sun, around which orbit eight planets, four plutons, and hundreds of thousands of comets and asteroids.
It’s striking to contemplate that we Earthlings may be the only beings in the entire universe to know about the drastic change in the solar system, or to care.