I personally find this definition confusing and overly complicated. I agree with the 1st criteria, which defines the lower boundary of a planet, as a body which has sufficient mass that its gravity overcomes its internal structure, producing a spheroid. Beyond that, I am perplexed by the definition. They didn't even attempt to define a precise upper boundary of a planet, by just obliquely saying it is not a star. As I understand it, the precise boundary between a brown dwarf star and a giant planet may still in question. The IAU, unfortunately to my mind, chose to mix roles with entities in their definition, which just complicates the definition. Why burden the definition of a planet beyond describing its intrinsic, or inherent characteristics. A star is a star, regardless of where it is found, or how it is moving. It may be orbiting a galaxy. Two stars may be orbiting each other, still they each are stars, whether the center of the orbit is positioned within, or outside the sphere of one or both stars. A star is a star even if it has been kicked out of the galaxy and is wandering in inter-galactic space. We might call such stars rogue stars, but they are still stars. We sub-classify stars by certain intrinsic characteristics, ie., main stream, red giants, white dwarfs, red dwarfs, brown, dwarfs, neutron stars, and so on. Does a planet suddenly change its intrinsic characteristics if it too is kicked out of the star system in which it was formed. Does it then cease to be a spheroid that does not generate internal heat from nuclear fusion. Does it cease to be rocky, or gaseous. Why should we require a planet to be defined by where it is, or how it moves, any more than we do of stars? If we want to talk about roles, we might say planets may be found in 3 different roles: 1) Major planet - a planet which orbits around a star 2) Minor planet - a planet which is a natural satellite, orbiting around another planet. This would allow moons to be planets and to orbit other moons as well. 3) Wandering planet - a planet not orbiting any particular star or other planet. Such a planet could be orbiting in intra-galactic space, or wandering in inter-galactic space. Like stars, planets might be sub-classed by their inherent characteristics, such by relative size and composition, as in giant and gas or rocky planets. All other smaller objects might be classed as sub-planets, or perhaps planetoids (exapting a term for this definition), which, like stars and planets, might, in turn, be sub-classified by their inherent characteristics, such as asteroid, comets, and so on. They may or may not be in orbit around any star, planet or other planetoid. Perhaps a more precise definition of a planet might be: 1) A planet must not be so massive that it can support any fusion reactions, even briefly. 2) A planet must not contain degenerate matter, such as a core of solid neutronium that might be left over from the death of a star. 3) A planet must have sufficient mass to have a shape determined by gravity and not by the strength of its materials, or spheroid. These three rules define both the lower and upper limits of a planet simply by its mass and internal characteristics. This is my two bits, Wade ------------------------------------------------------------------------------ NOTICE: This email message is for the sole use of the intended recipient(s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient, please contact the sender by reply email and destroy all copies of the original message. ------------------------------------------------------------------------------