Warm slippery facts about cold hard Pluto

A teacher wrote a letter to one of my favourite magazines this month, expressing a wish that the International Astronomical Union would make up its mind on Pluto once and for all, because it’s hard to teach kids about the solar system when this stuff keeps changing. Is it a planet or not? Do we tell them there are nine planets, or eight (or even twelve or thirteen, given that a couple of main belt asteroids were called planets when they were first discovered)? The cold hard facts are lukewarm and slippery right now.

Here’s a relevant quote from Richard Feynman, a rare bird in that he was both an effective teacher and a brilliant scientist:

“You can know the name of a bird in all the languages of the world, but when you’re finished, you’ll know absolutely nothing whatever about the bird … So let’s look at the bird and see what it’s doing — that’s what counts. I learned very early the difference between knowing the name of something and knowing something.”

When you teach that we have this many planets and that many asteroids and some other number of dwarf planets, your students are just learning about an arbitrary classification system. They aren’t learning anything about what these objects are. It doesn’t matter whether we call Pluto a planet, a dwarf planet, or a strawberry — its physical properties do not change. Why not teach about the objects first, and the classification second?

*  *  *

Our solar system consists of a star — the Sun — and many objects orbiting it.

Close to the Sun, there are four objects mostly made of rock and large enough that their gravity pulls them into roughly spherical shapes. One of these is the Earth.

Farther out, there’s a zone littered with smaller rocky bodies, each less than a thousand kilometres across. Most of them are too small to be spherical. There’s a large concentration of them here, but this isn’t the only place you can find such objects — some of them orbit closer to the Sun or farther from it, and others have stretched elliptical orbits that let them swing very close in and then very far out.

Beyond that there are four very large bodies made mostly of gas. They’re so huge that their gravity, over time, has allowed each of them to pull many smaller bodies into orbit around them. The largest one is over 300 times more massive than the Earth.

Finally there is a vast belt of objects made of ice and rock, orbiting out beyond the gas giants. These objects, like the inner belt objects, are on average a couple of hundred kilometres across, but the largest is over a thousand kilometres. Again, with a few exceptions, these objects tend not to be spherical in shape because they’re too small.

*  *  *

If we’re not sure whether eight or nine or twelve of these objects should be called planets, maybe that just shows that it’s not a relevant number to make your students memorize and write on a test. Have them draw a map instead — where are the four rocky planets? Where are the gas giants? What kind of objects make up the belt between Mars and Jupiter?

If you’re worried about teaching them something different this year than last year, even though the actual number of things in the solar system hasn’t changed, maybe it’s time to teach them why we classify things the way we do. You don’t need to know all the details of the IAU decisions, but it’s important to understand that these classifications are arbitrary, and that they can change even when the objects concerned stay the same. You’ll be teaching them more about reality (as we know it) and less about bookkeeping.


1 Comment

  1. Ann said,

    28 December 2008 at 3:16 pm

    This is a really good post.

    As an artsy-fartsy type, I’m all for teaching more understanding, and less bookkeeping.

    The problem is (at least, here in the States), our education system is headed in the opposite direction, with more and more emphesis on Standards tests, which are all based on fill-in-the-blank-with-the-correctly-spelled-name, and there’s just less time in the day for anything requiring creative thought.

    But then again, “No Child Left Behind” was Bush’s baby. Maybe things will get better with someone who actually likes looking at things from several directions in the Bully Pulpit (a girl can dream).

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