When is a Planet not a Planet ?

[Aquafire]

Wow: No sooner did the media herald the discovery of a "new Planet" that astronomers began to question whether it should be called a planet at all.

Along with Pluto, it looks like both are in for a sharp degrade of status from Planet to mere planetisimal.

Is this right ?

Or should we seriously rethink the parameters we use to call something a planetisimal.

If this is the case for Pluto and Sedna, why not Mercury?

After all, Mercury is only slightly larger than our own moon and Ganymede is certainly much bigger than Mercury, but isn't considered a planet.

So what is the most clear and sustainable definition of " Planet " ?

given these contradictory size examples ?

 

[Loren Booda]

Here's a try:

1. A planet has a comparably elliptical Keplerian orbit around its parent star (compounded for binaries, etc).

2. A planet forms under self-gravitation in relative isolation from other planets.

3. A planet could orbit a similar body in long-term stable configuration.

- "Booda's laws"

 

[Phobos]

Originally posted by Aquafire
Wow: No sooner did the media herald the discovery of a "new Planet" that astronomers began to question whether it should be called a planet at all.

Simply reviving an old debate that has been going on for Pluto. If/when they find a Kuiper Belt Object larger than Pluto, then the debate will heat up even more, I wager.

Along with Pluto, it looks like both are in for a sharp degrade of status from Planet to mere planetisimal.

Is this right ?

I'd be surprised. After the last round of debates a few years ago, the International Astronomer's Union decided to keep Pluto listed as a planet.

If this is the case for Pluto and Sedna, why not Mercury?

Good question, but Mercury is safely within the realm of the inner terrestrial planets and it's orbit fits nicely in the plane of the solar system with the other planets. Pluto is sitting out beyond the gas giants in the Kuiper Belt (that includes many large icy/rocky worlds) and it's orbit is off the plane more so than the other planets.

After all, Mercury is only slightly larger than our own moon and Ganymede is certainly much bigger than Mercury, but isn't considered a planet.

Ganymede orbits a planet, so it's a moon. Mercury orbits the sun. (Yeah, the Jupiter-Ganymede system orbits the sun, but that's just complicating the issue.)

So what is the most clear and sustainable definition of " Planet " ?
given these contradictory size examples ?

You know it when you see it. It's a bit of a gray area.

In general, a planet is big*, but not big enough such that there is a fusion reaction in its core (that would make it a star) and orbits a star(s).

* - This is part of the debate. Often, "big" is assumed to mean big enough to make the object roughly spherical (enough mass/gravity).