D Tibbets wrote:
The recent decision of the criteria for a planet is arbitrary and foolish. The requirement for the body to be large enough to gravitationally form a sphere is reasonable, but how perfect of a sphere do you need? The Earth is egg shaped, and what about mountains?
1 - The object must be in orbit around the Sun.
2 - The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
3 - It must have cleared the neighborhood around its orbit.
Going in depth in two:
Hydrostatic equilibrium is the current distinguishing criterion between dwarf planets and small Solar System bodies, and has other roles in astrophysics and planetary geology. This qualification typically means that the object is symmetrically rounded into a spheroid or ellipsoid shape, where any irregular surface features are due to a relatively thin solid crust. There are 31 observationally confirmed such objects (apart from the Sun), sometimes called planemos, in the Solar System, seven more that are virtually certain, and a hundred or so more that are likely.
Clearing out everything in it's orbit is the other criteria. Again, it is a matter of scale. Jupiter is certainly good at collecting or dispersing dust and other objects from nearby. But it still hasn't cleared out its orbit, mostly because it is a continuous process, even after billions of years things are still colliding with Jupiter. There is always new stuff entering the domain where the gravity of a body perturbes it . Only the scale is different. Ceres has cleared it's orbit just as well as Jupiter, as has Vesta, or any other rock that has survived to the current age of the Solar system.
I think you are not aware of what exactly "clearing it's neighborhood means".
"Pluto fails to meet the third condition, because its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit). The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified."
In other words, clearing it´s neighborhood means that the particular object must be by far the largest mass in it's orbit.
Thus, Jupiter has cleared it´s orbit LOOOOONG ago, during the formation of the solar system, when it was already millions of times larger in mass than any other object in it´s orbit. It doesn´t matter there are 1 trillion objects on Jupiter's orbit. What matters is the % of the mass in that orbit those objects sum.
You might argue that Pluto is not a planet because it is in a resonant orbit with Neptune, but you could reverse that argument and say that Neptune is not a planet.
no, you cannot, because Neptune is by far, VERY FAR, the largest mass in it´s orbit.
You could say that only the larger body could be a planet, but what about other star systems? If you have two bodies like Neptune and Uranus that is in a resonate orbit, which do you reject as a planet? Note that a resonant orbit is not the same as being a trogen. In that case it would be difficult to justify a planet designation. But, what if a Earth like object was in a trojan orbit with a super Jupiter. Would it be a planet?
I am sure that when we are confronted with such a case, we will create a definition.
Fact is, we need a threshold, otherwise we would have hundreds (not 30) plants on the solar system. It seems you people are not searching for a good scientific definition of a planet... the definition you guys want is "what I studied at school".