The recent discovery of another Pluto-sized object orbiting the Sun brings up the question: Is Pluto even a Real Planet, in which case this new one has a similar claim to planethood (planetosity?), or is it merely one of thousands of minor-planet poor stepchildren of the solar system? And this brings up one of my favorite arguments on the subject, which I noted in 2002:

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

[Attributed to Steven DeRose, Chief Scientist, Brown Univ. Scholarly Technology Group; if anyone has reason to doubt this attribution, and can suggest a more accurate one, please let me know.]

Guest (mail):
Seriously, how could Brian Leiter stay mad at you?
8.3.2005 4:57pm
joelw (mail):
There is definitely a Planned Planethood joke just sitting there.

Getting this planet it like getting an unexpected child, maybe the scientists should have gone to Planned Planethood, yada yada yada
8.3.2005 5:30pm
a fan (mail) (www):
Are you feeling okay, Eugene? You've had some mighty strange posts lately.
- a fan
8.3.2005 5:47pm
it's awesome! we encourage you to totally derail this blog in favor of geek entertainment! ENCORE...
8.3.2005 5:50pm
Eugene - You arn't supposed to be working are you? It looks like you are just extremely bored...
8.3.2005 5:56pm
Clayton E. Cramer (mail) (www):
The definition of "planet" is completely arbitrary. There is a school of thought that says anything with enough gravity to force itself into a spheroidal shape (roughly, 200 miles in diameter) and that circles (ellipses?) a primary that produces energy by fusion qualifies as a planet. Pluto therefore qualifies—but so does Sedna.
8.3.2005 6:10pm
Jeremy (mail):
There is actually a real song about this, and it's actually quite good. "Pluto" by 2 Skinnee Js
8.3.2005 6:16pm
Edward A. Hoffman (mail):
People usually try to mentally organize the world around them into categories, which often are defined arbitrarily. They then try to make everything they encounter fit into one of these categories, and the results are often nonsensical. It happens in academia, where the categories are disciplines like philosophy or law and where someone whose work straddles the artificial boundary between two fields is often seen as an anamoly even though her scholarship may be just as cohesive as that of professors who stay inside the boundaries of their disciplines.

The same thing happens in astronomy, where we insist that an object must be classified as a planet, a moon, a planetoid, an asteroid, a comet, etc. Two identical objects in two different orbits would be placed into different catgories even though the objects are indistinguishable from one another. Few people call Titan a planet, for example, but it is bigger than either Mercury or Pluto. The reason it isn't a planet has nothing to do with Titan itself; were it not in orbit of Saturn it would easily qualify.

Asking whether Pluto is a planet sounds like a question about the nature of Pluto, but it is really a question about the definition of "planet". Any ambiguity we may find is not the result of some unanswered scientific question but rather of our own imprecise terminology.
8.3.2005 6:38pm
Brian O'Connell (mail) (www):
The password is... taxonomy. It's pretty arbitrary in astronomy, whereas in biology, for instance, there can be real evidence supporting categories, like genetic drift.
8.3.2005 9:49pm
Warmongering Lunatic:
Even the definition of planet suggested by Mr. Cramer has serious ambiguities. For example, the Moon's orbit is more strongly influenced by the gravity of the Sun than of the Earth, and the orbit around the Sun forms a fully convex shape; is it thus a planet instead of a satellite?

The usual argument against that is that the Earth-Moon barycenter is in the Earth, and thus the Moon should be considered to orbit around the Earth. But in that case, Charon (Pluto's moon) is certainly a planet. And Jupiter is not a planet then, because it is not in orbit around the Sun -- the Solar-Jovian barycenter is outside of both bodies, just like the Charon-Pluto barycenter.

There is, of course, a much simpler standard -- declare anything that's spheroidal and not massive enough to fuse a planet, period. Bigger things are stars, smaller things are asteroids. Subtype as appropriate within the three classes. "Major planet" is then a non-scientific term like "major city" or "major river" or the like, to be defined culturally.
8.4.2005 1:51am