How I Killed Pluto and Why It Had It Coming - Mike Brown [81]
I began to quiz people about continents as much as I quizzed them about planets. I heard all sorts of interesting theories about how the word continent was defined, including a few from people who knew a little geology. I was told, emphatically: A continent is any island on its own continental plate. Greenland doesn’t qualify because it is on the same plate as the rest of North America and thus is not separate. I pointed out that continents have been around much longer than the plate tectonic theories of the 1970s. And I pointed out that by the “scientific” definition, we should really count the south island of New Zealand as a separate continent.
So how do we really define continents? Simply by tradition. The seven continents are the seven continents because that’s what people mean when they say the word continent.
But even that is not entirely true. Apparently some people mean different things. As I quizzed more and more people, I learned that, for example, many Europeans do not consider Australia to be a continent. Argentinians consider North and South America a single continent (the Panama Canal is not enough of a break for them, I guess). And rational people in many places believe that Europe is considered a separate continent only because, well, that’s where the people who defined the continents in the first place all came from.
Can it really be that the most important classification scheme for our understanding of landforms has no scientific basis whatsoever? Shouldn’t geologists get to work defining their terms more carefully?
And yet, when geologists talk about continental crust or continental shelves they know exactly what they are referring to. They never use the word continent by itself unless it is just to refer to one of those landmasses that we agree are called continents.
For the public, having a handful of continents whose names everyone can remember (even when everyone doesn’t always agree) is an important way to organize our understanding of the world around us. It is too difficult to make sense of the hundreds of countries on the earth without an organizing principle. The continents are a way to bring the vastness of the earth down to a human scale.
And so with planets. The planets are our way of organizing the universe beyond the earth. In fact, they are the grandest organizational scheme that most people know. Ask someone to describe what is around them and they’ll describe their neighborhood. Press further and they might talk about their town and region. If you keep pressing, perhaps they will mention their country, next their continent (that word again!), and finally the world. But if you don’t give up there and you ask for more, you will ultimately be led down the descriptive path of the solar system. You’ll be told about the planets. And after the planets? What next? More often than not, you will be left with blank stares.
When people describe their neighborhoods, they don’t care about the scientific meaning of the words they’re using; they care about recognizable landmarks to specify the points and the boundaries of their lives. The planets are these landmarks. That is what people mean when they say the word planet.
Is the word planet, then, specific or descriptive? When people say the word planet, do they mean precise places—Mercury and Venus and Earth and the others—or do they mean those places and anywhere else like them?
I found history to be a useful guide. When Uranus was accidentally discovered, it was quickly accepted as a planet; Neptune, likewise. Even Pluto, whose status was at stake in all of this, was accepted into the club with only a little grumbling. Sure, it was thought to be much bigger when it was originally accepted, much more like the other planets, but with its acceptance the bar was accidentally lowered, and