If you look at it logically rather than emotionally Pluto is not a planet. It's orbital charactaristics and hence most probably it's origins are different and it is something different.
Thats not logical, though its often the case thats put forward. Determing whether an object is a planet or not based on its orbit is untenable. Simply because it forces us to decide, along arbitrary lines, at what eccentricity and inclination does a body become a planet. This definition alone also makes all the asteroids planets.
Its a bias that we have because of our cosy position in a countable number of planets in a fairly simple system. If we lived on jupiter however would we insist that planets had to be 'big gassy things' and rule out the Earth because it was just a small rock near the Sun.
In another silly example, if the moon was knocked into its own orbit around the sun, would we include that as a planet.
The debate will probably go on in the silly way it has up to now until someone decides what a planet it. So far most definitions are useless. When Pluto was discovered it was all fairly simple, bigish things orbiting the sun were planets. So it should stay a planet until a better definition is made up.
Just for kicks lets try to come up with some definitions. This is not just for pluto but also to decide on extrasolar planets.
1. A planet must orbit the star. This is just to get rid of moons. In the pluto Charon system Charon is smaller so its the moon, and pluto the 'planet'.
2. The planet must not be internally heated by Hydrogen fusion. At that point you are a star. This may be important for the large giant extrasolar planets being discovered.
3. Planets are spherical. This takes care of asteroids. Once a planet has accreted enough mass it begins to heat up and internal heating occurs. This in turn causes the planet to become molten and so into a spherical shape. Even though they may have lost this internal heating since. In effect this is the size requirement also; if the object is big enough to develop then thats good enough.
Those are the absolutes. A planet _must_ satisfy those.
Next I would propose a set of criteria that are charecteristic of planets. Not all of these are required.
1. Planets have an atmosphere.
2. Planets have (or have had) a magnetic field.
Mars for example has very little atmosphere because it has lost its magnetic field. However we know it had one; those two points are related. Again this is a charecteristic of the size a planet needs to be.
Feel free to add other suggestions in there.
What I havent added in is orbit. Thats because I'm not prepared to say at exactly what inclination something goes from 'not a planet' to a planet. The further out we go, and with smaller bodies there is less to bind the objects to the flat orbital inclination that we are used to.
Discussing origins is also not helpful. Firstly we dont know the origin of planets until we get there with a shovel in most cases! Can we really say much more than stuff orbiting a Sun formed from the same nebula?
The problem most will have with the definitions there is that not only does it make pluto a planet, but makes any large object out there a planet. This will probably leave us with a solar system with tens or possibly hundreds of planets (that would confuse the astrologers!); if we are in a mindset of being able to count are planets then this may be daunting to put it mildly. Hoever unless someone can come up with rational way to argue that the trans neptunian objects are not planets, that is measure and unambiguous then we are stuck with lots of planets.
So where does this leave pluto. Well it leaves it as a planet. Back in the 'olden days' there was a simple definition of a planet and pluto met it. In order to say its not a planet we need a definition of a planet that excludes it. That definition of a planet needs to be broad enough to be applicable to future arguments, such as extrasolar planets. Bundling Pluto into a category with other objects that we dont call planets then saying that pluto isnt a planet because of that isnt logical.
Because of all that I' in favour of leaving pluto as a planet. However if the term 'planet' was given a proper definition then I've no problem reclassifying pluto. But as I said earlier, it doesnt really matter, a rose by any other name is still a rose.
Cheers,
~Albert
) emotions will run high with this one. So in the big wide world of Joe Public, I'd say there'll be an outcry (possibly small) if Pluto is relegated to an EKBO (even though it is!).
