Pluto - Not a Planet?

[Boerseun]

What's the big idea behind classifying something as a 'planet', in any case? If you think about it, Pluto is even smaller than our moon, yet is considered a planet because it revolves primarily around the sun. In my view, every object of any size that is the product of accretion over kazillions of years around a specific star could very well be termed a 'planet'. That, of course, will imply that we have well over a trillion planets. Inventing a mnemonic to remember their sequence will be a bit tough, though.

But any case - what is the fundamental difference between a rock in the asteroid belt and Jupiter, for instance? I'd say that Earth is better classified with the rock than with Jupiter. Earth and the rock have similar densities, similar composition, etc. Yet we are classified with Jupiter as 'planets', whilst Jupiter might very well be termed a 'failed star', or the sun's binary counterpart which failed to accrete enough hydrogen to light up.

Most stars in our galaxy seem to be in binary systems. This alone indicates that planets should be fairly common, seeing as more than one object can clearly form from the same accretion disk. So, what are planets, then? The residue of what would have formed the core of a star had only enough hydrogen been added in the forming stages?

This suggests a relation between pebbles and stars - or even as high as black holes. You start with two grains of dust that collide and stick. You add material, and before you know it, you have an asteroid. Any bigger, and you'll end up with a moon-sized object, big enough so that its gravity will self-regulate the debris from any susequent collisions to form a sphere. Any bigger than this, and you'll end up with an Earth-sized planet. And bigger still, you'll end up with a Jovian planet and if you keep adding mass, the whole thing will self-ignite under its own gravity and you'll have a star. Keep on adding mass and eventually the thing will collapse under its own weight and you'll end up with a black hole.

And the only change to your initial speck of dust that grew into a mighty black hole was simply adding mass.

So, does the classification of these objects as asteroids, moons, planets, and stars make any sense, apart from the obvious (the star is the burning one, the planet revolves around the star, the moon revolves around the planet) make any sense? Seeing as they're trying to blast Pluto as a planet, they should do it according to its orbit, and not its size. Size is immaterial. But in doing this, be have defined planets as sticking to a certain orbital quality. Then, one can ask, why is Venus the only planet that spins from East to West? What happened there? Should we now declassify Venus as a planet as well?

I think everything in orbit around our Sun, from the inner rocky planets to the outer Kuiper belt objects and even the Oort cloud, are deserving of the name "planet" to some degree. Because the borders between our classifications are very weak and full of holes, as the Pluto debacle clearly shows.

[ronthepon]

Really well said, Boerseun.

[Turtle]

I respectively respectably disagree with you last two gentlemen posters. Did you read the entire reasoning the committee gave for the decision?

Originally Posted by news.scottsman.com

The IAU, responsible for the naming of planets and moons since 1919, set up a Planet Definition Committee (PDC) to consider the problem. Committee member Professor Richard Binzel said: "Our goal was to find a scientific basis for a new definition of planet, and we chose gravity as the determining factor.

"Nature decides whether or not an object is a planet."

http://news.scotsman.com/scitech.cfm?id=1194292006

[Jay-qu]

do you really think nature decides?

Planet is a word, it is associated with objects, we choose what those objects may be by physical properties of said objects.

I think its just all caught up in semantics..

[Turtle]

Originally Posted by Jay-qu

do you really think nature decides?

Planet is a word, it is associated with objects, we choose what those objects may be by physical properties of said objects.

Oh good grief! It is simply an expression there, the use of the word nature. The natural physical property chosen for the planet definition is gravity and when a "space body" has enough mass so that its gravitational forces pull it into a sphere then that is the definition division of planet or pluton. It is not arbitrary at all as you imply, it is a progression of taxonomy.

[Boerseun]

Originally Posted by Turtle

...and when a "space body" has enough mass so that its gravitational forces pull it into a sphere then that is the definition division of planet or pluton...

In other words, all moons save those of Mars have now officially ceased to exist as 'moons', and are all now bona fide planets.

[Turtle]

Originally Posted by Boerseun

In other words, all moons save those of Mars have now officially ceased to exist as 'moons', and are all now bona fide planets.

No no no... this has nothing to due with moons changing in the inner solar system. Moons orbit planets, & only Pluto is losing planet status to pluton status & so its so-called moon Charon raises to pluton status. Again...did ya read the whole article? Or are you guys just yankin' an old man's chain late at night>

[ronthepon]

Hey, relax!

We merely were not aware of that hard definition for a planet.

Or... maybe I just speak for myself... because I had not seen the article at all.

[Turtle]

Originally Posted by ronthepon

Hey, relax!

We merely were not aware of that hard definition for a planet.

Or... maybe I just speak for myself... because I had not seen the article at all.

No worries mate; I have been operating under the principle of reading what I reply to is all.

[Boerseun]

Sorry Turtle - the page crashes whenever I try to open it, so I haven't actually read the whole thing. But what I can understand from the whole issue is that in order to clarify Pluto's status, they must redefine what are planets, and what are not. They can't simply say "okay - beyond Neptune, there's a new classification system", can they? They have to be consistent, no?

Clearly, we can't say that a moon is a moon because it revolves around a planet. That won't do. In a planet/moon system, they both revolve around their common center of mass, and the planet will then simply be the bigger of the two 'moons' revolving around the same point. There are no fundamental difference between the Earth and the Moon, except for the little bit of activity on the surface of the Earth. Take that away, and the Earth will simply be a bigger version of the moon, orbiting the same point. The fact that the point around which they orbit is below Earth's surface, is moot - as long as it's not the exact center of Earth, both Earth and the moon will orbit around it.
So, are planets simply the biggest moons in any given planetary system? And are 'stars' simply the biggest planets in any solar system? Forget about the sun's nuclear fire - as far as composition is concerned, there's not a lot of difference between the sun and Jupiter. Yet, Jupiter is a planet, same as the Earth...