OT: The solar system has 12 planets

[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