Discussion in 'Off-Topic' started by Gogf, Oct 5, 2005.
But seriously dude, answer the question. You're just dodging the question.
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...
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.
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.
Ah, but Mercury does not have an atmosphere, and one of the moons of jupiter may (Ganymede? not sure.)
Yeah, Yeah, Yeah.... your right. Magantic feilds isn't that good a way to do it.
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.
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.
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.
Even ring particles? Now, what about objects that orbit non-planets?
Even comets? KBOs are quite a bit unlike asteroids, it would be unacceptable to call them that.
May I have a copy?
The problem is any body has enough gravity to sustain an atmophere, the question is at what temperature for how long.
None of Jupiter's moons have much of an atmophere. Saturn's moon Titan, though, as a nice and thick atmophere.
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.
Very true. This really is a semantic debate only. Still, it does act as an interesting springboard to discuss astronomy
Oh well, I knew it was a large moon around a gas giant.
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.
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.
B - I - N - G - O!
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.
What about the theory of "Nemesis"?
No, Sedna is not a planet. I'm talking about extrasolars.
It's unsubstantiated, I don't buy it.
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.
Separate names with a comma.