Definition of "planet"

Yes, I’m already aware of the working definition used by the IAU, which is as close to an official answer as one is going to get on this question. In any event, that definition is geared mostly towards distinguishing between planets and brown dwarfs, while dodging the question of a lower bound. I’m interested in seeing some alternate definitions, which is why I put this here rather than in GQ.

Incidentally, this question is actually not inspired by the recent discovery of Sedna, although that discovery is fortuitous for me. The reason I’m asking is that I’m giving a presentation to the local astronomy club next month on the planets of the solar system, and I want to talk about just what should be considered a planet.

I’ve thought about this a lot as the Pluto controversy has raged. Here’s my proposed definition:

  1. The body should orbit a star.
  2. The body should have enough mass so that its gravity forms it into a sphere.
  3. The body should oribit within the plane of the eclipitc. Since I kind of want to grandfather in Pluto, I propose that the maximum allowable eccentrictiy should be 18 degrees.

I’m pretty sure my criteron will make Ceres and Vesta planets. I’m OK with that.

  1. The body should have a rank of no lower than Captain.

I don’t see planet as a particularly useful term for astronomers, kinda like constellation. I think it should be given to the public to use as they wont. Go ahead and define the term as the 9 ones they use now. And then come up with a new term to cover all bodies that orbit the sun.

Or group them. Rocky bodies, gaseous bodies, icy bodies. Then you could classify them like you do stars. Earth and Venus would be Sol-R4s. Jupiter could be Sol-G8. Pluto might be Sol-I1 and Charon could be Pluto-I0.5. Then you could talk about all the Sol-Rs or Sol-Rs over 1. Something like that.

hee!
:smiley:

Lower bound? Well, what’s the functional difference between an asteroid and a planet? I’d say a planet orbits a star. I’m not sure if the requirement of some sort of atmosphere doesn’t eliminate Pluto while allowing for massive comets. A planet should have a predictable, stable orbital such that it repeats. A planet (not its life system, etc, but the planet itself) should be able to withstand hits from other space debris.

The people who discovered Sedna have a Pluto-excluding definition of “planet” that I tend to agree with (caltech.edu):

I like that Caltech definition, as it’s mostly non-arbitrary and mostly consistent with the common notion. But explicit grandfatherings like vibrotronica proposes don’t sit too well with me… If we want to grandfather in Pluto, then why not grandfather in Ceres, as well?

One also has to be careful about definitions of other terms in one’s definition. For instance, what does it mean for an object to orbit the Sun? One can make a strong case that Luna orbits the Sun to a greater extent than it orbits the Earth. And how does one define the ecliptic? In the Sol system, it’s pretty straigtforward, but what if we’re looking at another star with only one major object orbiting it? Is it by definition in the ecliptic? Or what about two planet-like objects in a system, with radically different orbital planes… Which is the planet?

I get what you mean but none of them are perfect spheres. Spherelike object maybe? I’m sorry I’m being anal but that’s what this whole debate is.