pluto_hubble1What 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.

5 Responses to “Definitions”

  1. Laurel Kornfeld Says:

    A definition does not suddenly become fact because a tiny group of people decreed it so. Pluto not being a planet is NOT how things stand. It’s how they stand for some. Only four percent of the IAU approved this, most of whom are not planetary scientists. They violated their own bylaws in adopting a resolution at the last minute without vetting it before the proper committee as required. Their decision was immediately opposed by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Stern and like-minded scientists are working to overturn the demotion, and many are ignoring it altogether.

    The resolution you cite above is actually 5a. A second resolution, 5b, that would have established dwarf planets as a subclass of planets, failed by a vote of 33-91. This resulted in the absurdity of dwarf planets being defined as not planets at all. Additionally, the requirement for an object to clear its orbit is extremely vague. None of the planets in our solar system has fully cleared its orbit of asteroids, so if applied literally, this criterion could leave us with zero planets.

    What we are talking about here is not a definition but one interpretation in an ongoing debate. There is another legitimate side, which defines planet as any non-self-luminous spheroidal body orbiting a star. This is because objects become spherical only when they are large enough to be pulled by their own gravity into a round shape. This is a characteristic of planets and not of asteroids. Yes, that means Ceres is a planet too since it is spherical.

  2. Ξ Says:

    Thanks Laurel — I appreciate getting insight by someone who knows more about the current situation than I do!

    (It reminds me of a line that someone once shared, “You can tell that an area of math is dead when everybody agrees on all the definitions.”)

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  4. jd2718 Says:

    And so defining “planet” is hard. Why?

    “Star” is relatively simple. Why not just “things that orbit stars?”

    But, aha, that’s not good enough. Because this is all anthrocentric.

    Planet = “an object somewhat like Earth” – and now we have something that people will fight over.

  5. Ξ Says:

    I don’t know that it’s just anthrocentric — it strikes me as a simple sorting problem, because there are a LOT of things that orbit stars (including, indirectly, moons). In a sense, it’s no different from wanting to group integers, rationals, and irrationals instead of just calling everything a number.

    What seems to be true is that there isn’t a natural grouping that gives us 9 planets in the solar system. There’s one that gives us 8, and another one that gives us (at least) 11, and the choice of which to use is where the disagreement lies.

    [It reminds me a bit of the description of breaking down the motor cycle in Zen and the Art of Motorcycle Maintenance.]

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