Twelve, then.

The IAU’s draft is out, defining a planet thusly:

“A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”

So Pluto remains a planet, 2003 UB313 becomes one, Ceres gets promoted, and–in what I consider a surprise move–Charon is upgraded from ‘moon’ to ‘companion planet’, and another dozen are moved onto a watchlist pending further data. Considering the mass ratio between Pluto and Charon, that’s not unreasonable. The center of gravity of the Pluto-Charon system is between them, not within Pluto, so one can say they orbit each other more than Charon orbits Pluto.

And a new class called ‘plutons’ (of which Pluto is the prototype) is proposed, defined thusly:

Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). Plutons typically have orbits that are highly tilted with respect to the classical planets (technically referred to as a large orbital inclination). Plutons also typically have orbits that are far from being perfectly circular (technically referred to as having a large orbital eccentricity). All of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets.

So, something for everybody, then. We have to re-number all our planets, now, too. Ceres is to be the fifth, not Jupiter.

Me, I can live with it. But now I have to get cracking–there are another three planets I haven’t seen yet. ;)

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6 comments so far

  1. drakegrey on

    I remember when first discovered, Ceres was a full planet so it’s nice to see her back in the game.

    Darn, this whole thing is like pro ball draft season, isn’t it? I think this needs full coverage over on ESPN. And I want a Charon rookie card. :D

    –Drake

  2. jamess_fox on

    The various reactions I’ve seen from the blog community are interesting. I’ve actually made my very first blog post about some I’ve seen.

    I’d like to say, however, that I think the IAU made a grave tactical error in announcing twelve planets right away. Now, you see 12 PLANETS! NOT NINE! everywhere. Only a tiny fraction of people seem to know that this number may grow, very soon. I wonder how they might react if the resolution is passed, and within 6 months the IAU announces that after anaylzing Hubble Space Telescope images, we know know enough about (some selection of large KBO’s) to induct them as planets. Chaos man.

    • The Rev Dr Sherwood Forrester on

      I have long since given up despairing of the media ever reporting science properly. Science isn’t sexy, it doesn’t sell newspapers (or advertising slots), and most people get the same sort of incomprehending eyeglaze that I get when the sports news starts.

      Joe and Joan Public just need a number and a few names that they can safely forget, if they even care that much.

      Not that I’m bitter. Cripes, there’s a daily horoscope column in damn near every newspaper, but you have to scratch to find even a weekly astronomy column. Ain’t fair, I tells ya.

  3. jayteeone on

    Pluto and those to follow are not full planets the way the media is making out. They are Plutons or sub-planets, but have significance. Why can’t the media just deal with scientific matters scientifically instead of throwing their typical media spin on things. Do they think the general public are simpletons who can’t figure it out?


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