[X] My Assistant

[SFJCody]

http://groups.yahoo.com/group/mpml/message/15111

[Mongo]

I think that the problem we have is that the current classification may be too coarse-grained to be all that useful at grouping objects. There will be some classification system -- the objects themselves may be unaffected by what we call them, but their perception by most humans will be affected by their classification.

What are the primary physical break-points among various sizes of objects?

1) Deturium fusion at some point in the object's history would define the upper limit of planet-hood. This is usually put at about 13 Jupiter masses.

2) An obvious break-point is whether the object was almost entirely formed from planetisimals, or whether significant direct accretion of gas also occured. Not that all 'gas giants' are the same. Jupiter and Saturn are basically solar composition, while Uranus and Neptune are depleted in H and He compared to the sun. Other planetary systems are different again. One recently announced planet is about Saturn-mass, but otherwise apparently resembles a hot Neptune.

3) Another possible dividing line is whether the object has a significant secondary atmosphere, although this has a lot to do with the level of heating from its primary -- the colder the object, the easier to retain an atmosphere, unless it's too cold.

4) Sufficient self-heating to cause significant reprocessing of the surface might be a good indicator, but it would be difficult to determine from a great distance.

5) Probably the lowest dividing line of interest is whether the object self-gravitates into a spherical shape, although again, other factors enter into this, such as composition (silicates vs ices) and temperature. Another issue would be: how spherical is spherical enough? Objects at the lower limit might be only roughly spherical, so what degree of departure from the an ideal sphere (allowing for rotation) is acceptable?

My own list of known 'major planets' would include:

CLASS 1 PLANET: (significant direct acretion of gas, no deturium fusion) Jupiter, Saturn, Neptune, Uranus

CLASS 2 PLANET: (significant secondary atmosphere, no significant direct accretion of gas) Earth, Venus, Mars, Pluto

CLASS 3 PLANET: (self-gravitates into sphere, no significant secondary atmosphere) Mercury, 2003UB313, 2005FY9, 2003EL61, Sedna, Orcus, Quaoar, Ixion, Ceres, 2002TX300, 2002AW197, 2002UX25, Varuna, 2002MS4, 2003AZ84, 2004GV9, Vesta ...

While we're at it, here is a list of moons as well:

CLASS 2 MOON: Titan, Triton

CLASS 3 MOON: Ganymede, Callisto, Io, Luna, Europa, Titania, Oberon, Rhea, Charon, Iapetus, Ariel, Umbriel, Dione, Tethys, Enceladus, Mimas, Miranda

Bill