What is a planet?

What is a planet? In my opinion any astronomical body that's essentially massive enough to maintain a spherical shape, but not massive enough to trigger a self-sustaining fusion reaction (becoming a star) should be considered a planet.

Looking over the known bodies in our solar system, it seems that a reasonable cut-off at the low end would be 10e20 (one with twenty zeros) kilograms. Some solar system bodies with a mass in the 10e19 range are reasonably spherical, others are not. Since it's desirable to have an unambiguous definition (based solely on mass - which is one of the most easily determined properties of solar system bodies) it seems reasonable to fix 10e20 kilograms as the lower limit for planetary status.

The upper limit would be more flexible (as far as mass goes). The primary criteria here is that the body in question is not a star - that is, it does not generate a self-sustaining fusion reaction. For objects otherwise similar to those in our solar system, this limit would likely lie within the 10e28 kilogram mass range.

Planet Classes can now be designated as:

Class 0 -- with masses in the 10e20 kilogram range

Class 1 -- with masses in the 10e21 kilogram range

Class 2

Class 3

Class 4

Class 5

Class 6

Class 7 -- with masses in the 10e27 kilogram range

Class 8 -- with masses in the 10e28 kilogram range

Jupiter would be a Class 7 planet. Earth would be a Class 4 planet. Pluto would be a Class 2 planet. Tethys (one of Saturn's moons) would be a Class 0 planet.

Bodies with masses less than 10e20 kilograms could be referred to as "subplanetary" and designated as lying within Subplanetary Class -1, Subplanetary Class -2, etc. for masses in the 10e19 and 10e18 kilogram ranges respectively.

Note that the location of an object is not relevant in this system. The object is classified the same regardless of whether it orbits a star or a planet. A Class 5 planet, for example, could even be adrift in interstellar space.