How I Killed Pluto and Why It Had It Coming - Mike Brown [90]
The secret committee had its reasons, which were passionately stated. First: The word planet should have a scientific basis. Who was I to argue against that point? I had been willing to go along with a cultural definition instead of a scientific one, but if the astronomers were going to insist on science, I could hardly say no. And second, they suggested that in deciding whether an object is a planet or not, you should be able to tell just by looking at it—in other words, you shouldn’t have to know anything about where it is and what it is doing and what else is around it. The committee didn’t buy the idea that planets should be the small number of unique important dominant objects in the solar system.
And then they discussed the newly proposed twelfth planet, Charon.
Charon is the biggest of Pluto’s three moons. It was discovered, accidentally, in 1978 by James Christy, an astronomer at the United States Naval Observatory who was examining old photographs of Pluto and noticed a slight bulge coming and going, first on one side and then the other. Though Charon is smaller than our moon, than four of Jupiter’s moons, than one of Saturn’s moons, and than one of Neptune’s moons, making it only the eighth largest moon in the solar system, it is big proportionally to Pluto. And because it is big in proportion to the planet around which it orbits, it alone of all of the moons in the solar system deserved to be a planet.
What?
In the proposal from the committee, Charon was considered to be a planet for two reasons. First, it was big enough to be round, which was in itself a good enough reason to be considered a planet if you’re inclined to think of planets that way. But there are many round objects in the solar system that no one considers a planet. My nemesis the moon, for example. In fact, the proposal from the committee specifically excluded moons from being called planets. But it made a special exception for Charon—smaller than our own moon by a factor of about sixty—for one reason: Pluto and Charon go around a center of mass that is a bit outside Pluto.
Here a bit of quick physics is necessary (and I would like to point out that the fact that we need a physics lesson to explain the proposed definition of the word planet is already a bad sign). Whenever an object orbits another object (the moon about the earth, the earth about the sun, for example), it is not that the bigger object is stationary while the smaller object goes in circles. Instead, both objects go in circles around what is called the center of mass. You can find the center of mass of the earth and moon, for example, by finding a really large seesaw, putting the earth on one end and the moon, which has only 1 percent the mass of the earth, on the other end, and trying to make them balance. In the case of the earth and moon, you would have to move the pivot point to a location about a quarter of the way inside the earth. The seesaw is now balanced, and you have found the center of mass. In the twenty-nine days it takes for the moon to go in its big circle around the earth, the earth, too, in addition to traveling around the sun, goes in a tiny circle that is smaller than the earth itself. Rather than the moon circling the earth, both objects really circle around the point inside the earth that is the common center of mass.
There is nothing particularly special about the location of the center of mass. If you were to find yourself at the precise spot that is the center of mass of the earth-moon system, the only thing unusual that you would notice is that there would be one thousand miles of rock on top of your head.
Pluto is only about twice the size of Charon, so if you put Pluto and Charon on the cosmic seesaw you would find that the balance point is a little bit outside Pluto, rather than inside it. Again,