A Planetary Classification Proposal
Apparently the American Astronomical Society is meeting today and is currently discussing “planethood” and the future of Pluto. I gave some thought to this last year and decided to post it here for comments. The idea is to move away from defining something as a planet or not and simply classify non-stellar objects using the Earth as a base planetary mass.
A planetary class is halfway between the mid point and the midpoint of the next class.
Planetclass begins at 5 planets and goes down to .5 planets
| Class | Magnitude | Mass | Example |
| milli-planet | 0.001 | 1.90 x 10^24 kg | Pluto,Ceres (.0021) are in the milliplanet class |
| centi-planet | 0.01 | 1.90 x 10^25 kg | Mercury at.055 is in the centiplanet class |
| deci-planet | 0.1 | 1.90 x 10^26 kg | Mars at .107 is in the deciplanet class |
| planet | 1.0 | 1.90 x 10^27 kg | Earth (1), Venus (.6) are in the planet class |
| deka-planet | 10 | 1.90 x 10^28 kg | Neptune (17.147) is in the dekaplanet class |
| hecto-planet | 100 | 1.90 x 10^29 kg | Saturn & Jupiter are in the hectoplanet class |
| kilo-planet | 1000 | 1.90 x 10^30 kg | Upsilon Andromedae d (1,248) is in the kiloplanet class |
The brown dwarf limit is 4,131 planets, or 4.131 kiloplanets. This would solve the entire debate about Pluto and the rest of the Keiper Belt Objects since they would simply range from milliplanets (Pluto) down to nanoplanets (embryonic comets).
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Technically speaking, what you are proposing is both a metric and a classification system. In keeping with tradition though, I would name the metric an astronomical mass (e.g. the Earth is 1 AM).
In the sense that you have defined it here, your classification system is really just a redundant way of restating the mass of non-stellar objects. (So what’s the difference between the Moon, Mars and Earth? The Moon is a centi-planet, Mars is a deci-planet, and the Earth is a planet. – Other than the mass, there’s not much here to distinguish between these three very different bodies.)
A more useful classification system would probably make use of more than one property and/or metric and would likely use more physically relevant criteria for distinguishing between the classes. In particular, I’m thinking of planets with or without: atmosphere, rocky surfaces, active vulcanism, liquid water, life, etc.
I do appreciate the simplicity of the system you propose, and it is superior to the current classification system of the IAU in one significant respect: the determination of whether or not something is a planetary body is dependent only on intrinsic properties of the body. The additional requirement of the body to have cleared out its orbit of significant debris was obviously a kludge added on specifically to exclude Pluto, Ceres, Eris, Sedna, and the numerous other small bodies from the classification of planet.
Meghar Scale of Planetary Mass Classification
Been there, done that.
The primary problem with the current classification system is the absurd claim that real planets clear their orbital neighborhoods. This is clearly false as anybody following the NEO detection programs can attest. By this definition, Neptune is not a planet because its orbit is crossed by Pluto.
Planethood should be:
a) hydrostatic equilibrium (i.e. its spherical due to its own gravity)
b) it orbits the sun and not another planetary object
Ceres and Pluto are planets. Vesta is not. Spherical KBOs likewise are planets.