OT: The solar system has 12 planets

[henk brouwer]

Hmm the barycentre is indeed defined by the *average* distance between the two objects. Still, if they are in eleptical orbits the distance to the barycentre can change, if I understand correctly, actually it would have to because the objects move closer to each other, so one or both of them have to get closer to the barycentre. the wiki page on 'center of mass'( http://en.wikipedia.org/wiki/Center_of_mass )has an animation of two objects orbiting around their barycentre in an eleptical orbit when you scroll down a bit. in this animation the distance between the objects and the barycentre changes. If the radius of one of them had been a bit over twice as large the barycentre would disapear in it for part of each orbit. (I really should do the math, but that scares me a bit actually, I just hope I'm right )

[Cipher7071]

It would seem that the barycenter for two perfectly spherical bodies would never move. But, if either or both were oddly shaped and rotating, then the barycenter could shift somewhat.

I find the definition of a pluton that depends on the object's origin compelling. Otherwise, how to decide according to size or orbit seems abitrary to me. They shouldn't spend too much time arguing over it, as much of the argument is semantic.

[ToddT]

Nah, i have a very cynical take on this. Given the perodic discussion as to whether pluto is a planet or not, and now the discovery of other objects almost as big, the ones pushing for planet status of the planets and ensuring pluto's status, or looking make a name for themselves to find other planets in the solar system, which is more glamorous, finding a large rock or a small planet. From what i remember, if pluto had been found several years later it would most likely not have been classified as a planet.
pluto was found while ppl where looking for a 9th planet that had been mathmatically calculated to exist, when it was found it was for smaller than expected, hence part of the reason to found the everelusive tenth planet. I think that was before they new the exist of the ort cloaud and kiper belt.

[Cipher7071]

You're right about one thing: Where egos are involved, what's logical will often take a back seat.

[Glyn]

It doesn’t matter how many objects are classified as planets. What real matters are the questions we study.

Teacher:
“Today class we are studding the inter planets and planetary systems. A planetary system is a planet with one or more moons. How many inter planets and planetary systems are there? Nine, right.”

[DeadZone]

Why cant it simply be

A planet is a solar body orbiting a star on its own stationary orbit, having some sort of atmosphere (remember, all nine "current" planets are believed to have one)

With a moon being a solar body that orbits a planet

This is how I always understood it growing up, and it makes complete sense to me

I suppose a size classification would make sense

[Phoenix-D]

Pluto doesn't have an atmosphere, so that wouldn't count it either.

[Renegade 13]

DeadZoneMDx said:
Why cant it simply be

A planet is a solar body orbiting a star on its own stationary orbit, having some sort of atmosphere (remember, all nine "current" planets are believed to have one)

With a moon being a solar body that orbits a planet

There's a few reasons why this definition wouldn't work.

1: What do you do when you find a Kuiper Belt or Oort Cloud object the size of, say, Mars? Given current theories and models of solar system formation, it is possible, even likely that such objects exist at extreme distances from the sun. At such distances, there's no way for it to have an atmosphere...yet if it's as large as Mars, why shouldn't it be classified as a planet?

2: This classification does nothing to address the upper range of planetary sizes, those that are approaching sufficient mass to sustain deuterium fusion, at least for a time. Admittedly, the current definition that has been proposed does nothing to define an upper limit on planetary size, but I think they're planning to reveal one at the upcoming IAU conference in Prague. Your definition doesn't address the boundary between brown dwarf stars and extremely large gas giants.

3: It is theoretically possible to have two similarly sized planets orbitting around a common barycenter, both possessing an atmosphere and orbitting in tandem around their parent star. Your definition technically would exclude these as planets, since they do not each have their own stationary orbit around the star, so they'd essentially end up having to be classified each as a moon of the other, which wouldn't make much sense

4: Under your definition, would Mercury qualify as a planet? According to wikipedia's entry on Mercury, its atmosphere is described as below:

Mercury is much too small for its gravity to retain any significant atmosphere over long periods of time, but it does have a very tenuous atmosphere containing hydrogen, helium, oxygen, sodium, calcium and potassium. The atmosphere is not stable—atoms are continuously lost and replenished, from a variety of sources. The hydrogen and helium atoms probably come from the solar wind, diffusing into Mercury's magnetosphere before later escaping back into space.

This brings up the problem of defining what qualifies as an "atmosphere" and brings problems of its own, such as measuring said atmosphere on Kuiper Belt objects that are so far away as to render atmospheric sampling at our current level of technology impossible.

Whew, that ended up longer than planned. Please feel free to refute my arguments, since they probably have gaping holes in them