The Volokh Conspiracy

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Volokh Conspiracy

Maybe Pluto Is a Planet After All

Two planetary scientists so argue, and there's a legal hook, but it's really just an excuse for me to ....


David Grinspoon & Alan Stern (in the Washington Post) argue that Pluto is indeed a planet (thanks to Stephen Green at InstaPundit for the pointer); an excerpt:

We use "planet" to describe worlds with certain qualities. When we see one like Pluto, with its many familiar features — mountains of ice, glaciers of nitrogen, a blue sky with layers of smog — we and our colleagues quite naturally find ourselves using the word "planet" to describe it and compare it to other planets that we know and love….

We find ourselves using the word planet to describe the largest "moons" in the solar system. Moon refers to the fact that they orbit around other worlds which themselves orbit our star, but when we discuss a world like Saturn's Titan, which is larger than the planet Mercury, and has mountains, dunes and canyons, rivers, lakes and clouds, you will find us — in the literature and at our conferences — calling it a planet. This usage is not a mistake or a throwback. It is increasingly common in our profession and it is accurate.

[UPDATE: Reader Robert Woolley points to a similar recent statement by S. Alan Stern, the principal investigator of the New Horizons mission to Pluto, who condemns "the sad development of 2006 when a few hundred astronomers—mostly non-experts in the study of planets—declared that Pluto and the then burgeoning list of other small planets that had been discovered beyond Pluto were not planets, largely to prevent schoolchildren from having to memorize their names. (I wonder if those same astronomers—also non-experts in chemistry—believe that there are too many elements in the periodic table for the same reason.)" It is a small object, but there are those who love it.]

This is an interesting argument, and it helps illustrate how even scientific terms are often a matter of definition, with each definition yielding close cases along the edges, and with the definitions often chosen based partly on utility (what helps most with better understanding certain aspects of the world?) rather than simply based on objective scientific fact. That's a helpful lesson for incoming law students, and I've at times used the planet controversy as an analogy for them. Such definitions aren't completely arbitrary, because they are aimed at better categorizing real phenomena; but they aren't completely dictated by the real phenomena, either—likewise with many legal definitions.

But, really, this is all just an excuse for me to repost one of my favorite humorous songs, the author of which is regrettably shrouded in the mists of time:

ALWAYS A PLANET TO ME (to the tune of Billy Joel's "She's Always A Woman to Me")

He can orbit the sun, he can look like a moon
He can leave the ecliptic from April to June
He'll be just a faint smudge, magnitude twenty-three
He hides in the sky, but he's always a planet to me

Ohhh … a potato-shaped ball …
He can drift where he wants
He's a relic of time
Ohhh … if he's made of pure ice
Or of vapor and dust
It's the same to my mind

If he zooms in near us, would he show us a tail?
Was the Kuiper Belt once the great home whence he sailed?
And if he gets demoted, who'll be next, Mercury?
And the most he can do is cast shadows, it's true
But he's always a planet to me