Pluto: Dwarf Planet, or Just Small Thinking?

Posted on May 12, 2010


It’s been four years since the International Astronomical Union kicked Pluto out of the Planet Club. Most people may be too busy reading their horoscopes to care about actual planets, but for the rest of us, this Pluto thing isn’t over yet. Why do we care about a speck of light four billion miles away? Principle, mostly. The big and powerful are picking on the small and weak. Also, Pluto has a name. You don’t start calling your cow Millie and then send her to the slaughterhouse, do you? (Well, I wouldn’t.)

Feelings aside, the reasons for Pluto’s demotion to dwarf planet are unsound. The IAU issued three criteria for qualification as a planet: (1) The object must be massive enough to form a sphere. (2) It must circle the Sun. (3) It must have succeeded in “dominating the neighborhood” by clearing its orbit of debris. I’ll address these, but first let me state the obvious. Pluto is tiny and that’s really what this is about. Put Uranus into Pluto’s orbit and Uranus would still be a planet.

When astronomers try to demonstrate the absurdity of including Pluto, they invariably resort to visual comparisons, representing Jupiter as a beachball, for example, and Pluto as a poppy seed. But could we apply this logic to anything else? Alaska is 572,000 square miles. Rhode Island is 1,545. Should we decide, by a show of hands, that Rhode Island is now just a dwarf state? The largest mammal, the Blue Whale, is 110 feet long and weighs 190 tons. The smallest, the Hog-nosed Bat, is 1.3 inches and 0.07 ounces. The ratio of Jupiter’s diameter to that of Pluto is 63 to 1. The ratio of the whale’s length to the bat’s is much larger. Yet they’re in the same class and no one’s been expelled.

Some scientists argue that we can’t call Pluto a planet because it’s smaller than several of the Solar System’s moons, including ours. But this comparison, too, is weak. Vancouver Island, with an area of 12,000 square miles, isn’t a Canadian province, but rather a part of British Columbia. Prince Edward Island, at 2,200 square miles, is a province. To take the point to the ridiculous, Vatican City is a country, yet it measures less than one square mile. The bat previously mentioned is smaller than many insects.

What makes the decision especially irritating is that they changed the rules in the middle of the game. It seems sneaky. I had never thought of astronomers that way before. I bet Galileo wasn’t sneaky.

The rule that a planet should be spherical makes sense, but if something were to crash into Mercury, leaving it shaped like the letter D, would Mercury no longer be a planet? Besides, Pluto is spherical.

The second rule says the object must orbit the sun. Pluto orbits the sun. Okay, its orbit is a little wacky, but maybe it was involved in some sort of collision. Maybe whatever knocked Pluto off course was headed for Earth, and we were saved by the little guy. Is this how we show our appreciation?

Rule number three seems to have been the clincher. But I think they dreamed it up just so they could give Pluto the boot. A planet must dominate the neighborhood and clear its orbit of debris. There are street gangs that dominate their neighborhoods; that doesn’t make them good citizens. And what about Saturn? It hasn’t cleared its orbit. In fact, Saturn drags debris around with it. Now I like Saturn as much as anybody, but it shouldn’t get special treatment just because it’s big.

I think the IAU is really trying to say that Pluto is too odd. Well that’s not good enough. I know plenty of people (a few in my own family) who would be banished from the human race if that were a valid reason. Hawaii is the only state that’s an island; isn’t that odd? The nation of Lesotho is completely inside of South Africa. Pretty weird. And the city of Istanbul is on two different continents.

If you wanted to, you could come up with a rule that eliminates any planet. Mars is orange. Jupiter has no solid surface. Venus is too hot. Neptune is the only planet that was discovered on a Wednesday. Earth is the only one that allows furniture store owners to star in their own television commercials.

Astronomers respond by saying they have discovered objects similar to Pluto, some even larger, orbiting in its vicinity. If Pluto is allowed to stay, don’t we have to let those others in, too? No. They missed the deadline. Sometimes you show up late and lose your chance. Is it silly to describe objects that are billions of years old as being late? No sillier than mentioning a Blue Whale and a Hog-nosed Bat in the same sentence.

Posted in: In Over My Head