If we’re gonna nominate names of fictional gods for planets, how about Unicron, who qualifies as both? 
Urd, Skuld, and Verdandi would be perfect if they discovered a triplet of very close plutons.
You’re not going to find any unused Greek or Roman mythological names. They’ve been used up on all sorts of minor bodies. Completely used up. Which is why you see names like “Sedna” nowadays. You’ve got to go further afield. I suppose Morgoth or Manwe is out of the question? Personally, I don’t have a problem with Xena. And we haven’t used up Shakespeare characters, or can those only be applied to objects orbiting Uranus?
These mnemonic phrases are easy to come up with with, there’s nothing particularly special about them.
That said, I’m disappointed. Pluto is not a planet, it’s a tiny little ball of ice smaller than our moon. Ceres is not a planet, it’s an asteroid. Bah, humbug!
Does this mean we can start making plans to go to the world Ceres?
Yeah, I stole that one from Asimov.
It’s not a bad criteria. If Pluto and Charon were of near-identical mass, neither of them could be considered the “primary”.
No, Vespa.
Funny, she doesn’t look Drewish…
[Conan the Barbarian]By Crom![/CtB] I think it is time then to consider other Mythologies:
Like the Celts (Funny, they look drewish!):
The Maya and Aztec:
If it holds, the name of Sedna then looks like a precedent that other Mythologies (Sedna comes from the Inuit Eskimos) will be considered from now on.
What the fuck are these people talking about? A guy on TV said that my anus is a planet because it is round and it goes around the sun. He was right about it being round, but how does this geek know if my asshold goes around the sun or not?
A common misconception, an anus is a rectum not a planet. However, if we are talking about the Gluteus Maximus, that indeed sounds like a planet, yet everybody knows they make a moon…
… What?
<d&r>
Of course it goes around the sun. It’s gravitationally-coupled to the Earth. 
I personally think the term planet should only refer to the 8 classical planets in our solar system, and all the other objects that fits their new definition of planet should be called planetoids. That way Pluto will be something more than just a chunk of ice and maybe the pro-planetary-Pluto people won’t be too pissed off, while planets will still be special when we find dozens or hundreds of large objects in the Kuiper belt. It could get complicated when we consider bodies orbiting other stars, but it’ll be a long time before we can detect anything smaller than Mercury anyway.
That would be awesome, but what kind of a planet could live up to that name?
It would have to be a world where copyright doesn’t exist.
Even with the extra limitations, I still don’t like these definitions. Here’s why:
What makes Mercury a planet, but not Io? If Pluto is a planet, why isn’t Europa? Why are the gas giants lumped as planets? What is the one characteristic that all planets have in common?
Because, because, because, and there isn’t one.
There once was a perfectly workable definition of planet: a wandering star. I.e. all planets are objects bright enough to return light to earth at night and move against the background of the stars. I know, I know. That takes us back down to only Mercury, Venus, Mars, Jupiter, and Saturn. But you know what? They’re kinda important, being the namesakes (along with Sol and Luna) for the days of the week.
No definition of “planet” will have a useful astronomical meaning. Any time you say “planet,” you gotta then clarify. Rocky? Gaseous? Icy? What?
My proposal:
[ul][li]Planets: objects bright enough to return light to earth at night and move against the background of the stars, but are present every year. (The above named FIVE, count 'em FIVE.)[/li][li]Asteroids: rocky objects small enough such their own gravity does not pull them into a spheircal shape.[/li][li]Terrestrials: Rocky objects large enough such that their own gravity has pulled them into a spherical shape. (Note, this includes Ceres, and many of the moons)[/li][li]Jovians: Bodies made up of gasses, pulled into spherical from their own gravity, but not massive enough to turn into stars (i.e. not completely engulfed in fusion)[/li][li]Plutons: Objects made of ices, pulled into spherical shapes from their own gravity, originating from the Kuiper Belt.[/li][li]Comets: Icy bodies orginating from the Oort cloud.[/li][/ul]
No ambiguity. No discussion. No need for the solar system to contain dozens of “planets.” No more need to clarify *what kind of *planet. And people might now understand a little better the relevance of “Planets” to our ancestors.
I think your idea is great for our Solar System, but it means that all non-stellar bodies orbiting other stars cannot be called “planets”.
To be fair, there aren’t that many terms in astronomy that have useful meanings without clarification.
You say galaxy; what type of galaxy? Spiral? Elliptical? Peculiar? Dwarf?
You say star; what type of star? Young? Old? O-type? M-type? What exactly do you mean?
Sure, you can say in general terms, galaxy=amalgamation of stars, but a dwarf galaxy behaves very differently to a spiral galaxy which behaves very differently to an elliptical.
A star can be defined as a gravitationally bound spheroid of gas emitting energy via nulear fusion, but an old star behaves in a different way to a young star, and O-type stars have different properties and behave differently to M-type stars.
That in mind, what is wrong with defining planet as “a natural object directly orbiting a star, that is sufficiently massive so that it forms an approximately spherical shape because of its own gravity”, and then sub-dividing the class of planet?
Earth is not a planet? :dubious:
Good point.
Hrmph. The more I think about it, the more I must concede any definition of “planet” that relies fully upon historical convention or the structure of our own Solar System is too parochial to be of scientific use in a universe with confirmed extrasolar non-stellar bodies orbiting the other stars. Any mass cutoff that is linked to a completely ambiguous physical characteristic (like, say, “must be as big or bigger than Pluto”) also fails to say anything useful beyond our neighborhood. “Hydrostatic equilibrium” is, of course, tied to what may be an overly-human fascination with “roundness”, but it’s also a legitimately universally-applicable attribute, and one I suppose extraterrestrials might conceivably take a special note of. I’m not thrilled, but durned if I can do any better.
[QUOTE=JustAnotherGeek]
[list][li]Planets: objects bright enough to return light to earth at night and move against the background of the stars, but are present every year. (The above named FIVE, count 'em FIVE.)[/li][/QUOTE]
What do you mean, five? ALL of the bodies you list return light to earth at night. All of them. Oh you mean visible to the naked human eye? Whose eye? Under what conditions? You’ll have to pardon me, but this is just a silly definition. Anyways, you forgot to include Luna as a planet, since it clearly meets the definition, so that’d be six.
In that case, Uranus would fit: The brightness of Uranus is between magnitude +5.5 and +6.0, so it can be seen with the naked eye as a faint star under dark sky conditions, and occasionally even Vesta.
Nope. It’s Terrestrial body. See my above definitions. You could certainly call it a world, but not a planet.