What is a planet?
The current definition, from Resolution B5 of the International Astronomical Union, is that
A planet is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
Pluto hasn’t cleared its orbit, so it’s not a planet. There’s actually some controversy about this (and as usual, we turn to Wikipedia to learn all about it), but essentially that’s how things stand. Pluto can at least rest assured that it’s not the first planet to be demoted.
But a lot of people don’t like that Pluto isn’t a planet, and last month the Illinois State Senate adopted a resolution that declared March 13, 2009 as “Pluto Day” and also included the line:
that as Pluto passes overhead through Illinois’ night skies, that it be reestablished with full planetary status
You can read the formal resultion here; it’s a short read, thought it packs no fewer than 10 “Whereas”s into just 1½ pages. Here’s what is bothering me: the bill’s sponsor, Senator Gary Dahl, apparently said:
I don’t think we are changing the status of the planet. We’re simply asking that March 13 be declared Pluto Day and that, for the day, Pluto is a planet.
But, if you add Pluto, aren’t you actually changing the definition of Planet? If not in broad terms, at least by adding “and Pluto”? I get the distinct impression that he doesn’t understand what a definition is. It reminds me of how our math majors often struggle with definitions when they’re first learning how to write proofs: starting from the definition of an even integer (twice an integer), it’s initially hard for them to prove that if n is even, then so is n+2.
And yes, I might be making too much of this. I don’t want to begrudge Pluto its special day. But still.