Exoplanets Are Planets Too!

My first reaction was, “Oh no, not again!” I always maintained Pluto was a “planet,” no matter how the IAU redefined it in 2006. Are we going to revive that old trope again?

In the August 2013 Sky and Telescope, in an article of the same name, veteran writer David Grinspoon does just that, but with a new twist. “Recent discoveries have exposed the absurdity of the IAU’s planet definition.”

Well, of course! How did we miss the obvious? With recent discoveries of huge numbers of planets orbiting other suns, we are calling them “planets.” We can’t call them “dwarf planets” because those will be too small for current detection methods for quite some time to come.

But according to the IAU definition, the very first requirement of a celestial body be that it “(a) is in orbit around the sun.”

What the hell were they thinking?

An engineer, a physicist, and a mathematician are shown a pasture with a herd of sheep, and told to put them inside the smallest possible amount of fence. The engineer is first. He herds the sheep into a circle and then puts the fence around them, declaring, “A circle will use the least fence for a given area, so this is the best solution.” The physicist is next. She creates a circular fence of infinite radius around the sheep, and then draws the fence tight around the herd, declaring, “This will give the smallest circular fence around the herd.” The mathematician is last. After giving the problem a little thought, he puts a small fence around himself and then declares, “I define myself to be on the outside!”

Even this WordPress blog post is a wolf in sheep’s clothing, if you’ll pardon the terrible metaphor. Defining something so that it meets a predetermined selection criteria you need it to match is an ancient malady, and it’s not confined to religion and politics.

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