3rd Planet

News from Prague! Not that anyone really cares, becuase of the whole college-freak-out-fest that’s going on (not that I blame you), but there’s been some developments on the astrophysical front. The what? Never mind.

The International Astronomical Union has recently held a confrence in Prague, and they reached some sort of concensus as to what the actual definition of a planet is. Their [official definition] boils down to this:

The part of “IAU Resolution 5 for GA-XXVI” that describes the planet definition, states “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.”

What does this mean? Well their Resolution for General Assembly 26 boils down to two components that deal with planetorial qualifications: shape and orbit. The shape requirement is that it be round. How round? To be honest, I’ve no clue how to measure an object’s roundness, but they use the term “hyrdostatic equilibrium”.

This term has a lot of different defintions, but it essentially means that the forces acting upon a body (in this case, potential planets) must be in balance. The force pushing the body inward (gravity) must be equivalent to the force pushing it outwards (diffusion for gas planets, centripital force for rock-planets, I would assume). When the push-out and push-in forces are the same, nothing goes out or in. It just sits there, all equal and whatnot. The resulting shape is a sphere.

Secondly, the body must be orbiting a star. That’s a pretty sensible qualification. They also insert the comment that it can’t be a star (as in one star orbiting another does not make the former a planet), or a satellite (the moon is not a planet because it is spherical).

The point, they say, was to develop an objective definition of what a planet is. I think they accomplished that task. However, what remains to be seen is whether this will filter down culturally. Is anyone really going to care when they ‘officially’ classify Ceres, Charon, and Xena as planets? Or is it like when you get a promotion at work and nobody cares because it didn’t include a pay raise?

I’m not qutie sure;
I wish I classified as a planet though.


  1. Pluto is a planet. The definition is MORE inclusive, which means our solar system is bigger than we typically think of it.

    It also determines planet-status with a predominately objective criteria, rather than some arbitrary mass.

  2. Hey Tom!

    Long time no comment. Anyway, I only got to read your guestbook comment (thanks!) today because my guestbook seems to go into a coma whenever I try to access it.

    Congratulations on your new blog!

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