What is a planet?

[Perfection]

only if they have no atoms. Protons and electrons both emmit magnetic feilds.

Think nuetrinos

But seriously dude, answer the question. You're just dodging the question.

 

[Global Nexus]

Well, personally...I firstly classify "planets" into two groups: the Central and the Secondary. Central planets are anything orbiting the sun and nothing else [IE -- no moons, as they orbit planets] and go down to about Mercury in size. Secondary planets are anything smaller than that down until the object can no longer be capable of being spherical due to its gravity. I think it's a decent classification, but I dunno...

 

[Babbler]

A planet is a large, spherical object that revolves around a star and does not undergo nuclear reactions. Perhaps it can subdivided into terrestrials, jovians, and plutonians; or something like that.

Then the solar system would have over 20 planets

So? Compared to most other number in astronomy, it is not really that problematic. If it can deal with millions of lights-years, several billion stars, several billion galaxies, 13.7 billion years of history, etc, then 20 planet is not going to be a problem.

 

[FearlessLeader2]

Any object of mass sufficient to retain an atmosphere whose orbital path is around it's stellar primary. Any object that orbits such an object is a moon. Any object that orbits its stellar primary but is too small to retain an atmosphere is an asteroid.

In my book.

 

[Ultima Dragoon]

Ah, but Mercury does not have an atmosphere, and one of the moons of jupiter may (Ganymede? not sure.)

 

[Meleager]

Think nuetrinos

But seriously dude, answer the question. You're just dodging the question.

Yeah, Yeah, Yeah.... your right. Magantic feilds isn't that good a way to do it.

 

[FearlessLeader2]

Ah, but Mercury does not have an atmosphere, and one of the moons of jupiter may (Ganymede? not sure.)

Mercury has sufficient gravity to retain an atmosphere, but the sun cooked it off long ago. And moons may well have atmospheres, but their primary is not the sun. The definitons stand up to this challenge.

 

[Meleager]

Mercury has sufficient gravity to retain an atmosphere, but the sun cooked it off long ago. And moons may well have atmospheres, but their primary is not the sun. The definitons stand up to this challenge.

You need to define "atmosphere". An astroid could pull in a few gaseous particles easily enough.

 

[Perfection]

A planet is a large, spherical object that revolves around a star and does not undergo nuclear reactions. Perhaps it can subdivided into terrestrials, jovians, and plutonians; or something like that.


So? Compared to most other number in astronomy, it is not really that problematic. If it can deal with millions of lights-years, several billion stars, several billion galaxies, 13.7 billion years of history, etc, then 20 planet is not going to be a problem.

Well, if you can accept Ceres (the largest main-belt asteroid), Quaoar, Orcus, Sedna, 2003 UB313, Ixion, Varuna, 2005 FY9, 2003 EL61 and every other large hunk of ice we find lying around our sun. Be my guest. However, I think it is far to broad a discription. These small bodies are all members of large populations and they don't behave like traditional planets.

 

[ummmm........]

I'd go with anything that rounds a star in a unique, more or less circular (i.e. not wildly elliptical) orbit. But I'd also say that my definiton might vary based on why you're asking. It's just a word, it doesn't mean anything.

 

[Perfection]

Any object of mass sufficient to retain an atmosphere whose orbital path is around it's stellar primary.

See below

Any object that orbits such an object is a moon.

Even ring particles? Now, what about objects that orbit non-planets?

Any object that orbits its stellar primary but is too small to retain an atmosphere is an asteroid.

Even comets? KBOs are quite a bit unlike asteroids, it would be unacceptable to call them that.

In my book.

May I have a copy?

Mercury has sufficient gravity to retain an atmosphere, but the sun cooked it off long ago. And moons may well have atmospheres, but their primary is not the sun. The definitons stand up to this challenge.

The problem is any body has enough gravity to sustain an atmophere, the question is at what temperature for how long.

 

[Perfection]

Ah, but Mercury does not have an atmosphere, and one of the moons of jupiter may (Ganymede? not sure.)

None of Jupiter's moons have much of an atmophere. Saturn's moon Titan, though, as a nice and thick atmophere.

I'd go with anything that rounds a star in a unique, more or less circular (i.e. not wildly elliptical) orbit. But I'd also say that my definiton might vary based on why you're asking.

There are some gas giants in other star systems with wildly elliptical orbits. I don't think low eccentricity should be part of the definition.

It's just a word, it doesn't mean anything.

Very true. This really is a semantic debate only. Still, it does act as an interesting springboard to discuss astronomy

 

[Ultima Dragoon]

D'oh!

Oh well, I knew it was a large moon around a gas giant.

 

[Meleager]

Ok, I'll give this another try:

An object in a stable orbit around a star, which the International Astronomical Union agrees is a planet.

OR

An object in a stable orbit who's mass is greater then the sum mass of every other object in a similar orbit, and that is roughly sephirical due to its own gravity.

 

[Ultima Dragoon]

B - I - N - G - O!

 

[Perfection]

An object in a stable orbit who's mass is greater then the sum mass of every other object in a similar orbit, and that is roughly sephirical due to its own gravity.

That's actually not too bad. I vaguely prefer mine, as I would question some hypothetical borderline cases, but really I think it's pretty good.

Oh and lose the "stable orbit" bit. There's evidence that certain exoplanets have unstable orbits.

 

[Ultima Dragoon]

Like sedna?

What about the theory of "Nemesis"?

 

[Perfection]

Like sedna?

No, Sedna is not a planet. I'm talking about extrasolars.

What about the theory of "Nemesis"?

It's unsubstantiated, I don't buy it.

 

[Ultima Dragoon]

But even so, the thought of a brown dwarf rushing towards us is a unsettling prospect, no?

 

[Perfection]

But even so, the thought of a brown dwarf rushing towards us is a unsettling prospect, no?

Not really, it's not like it would go anywhere near the inner solar system. Besides, there's no evidence.