Planet Definition:: In Search of Planet's Intrinsic Quality (23rd August 2006)
Space.com published (22nd August, 2006) an article on the issue of planet definition, where they mention my work (coauthored with Christopher Broeg, AIU Jena) on the sujbect.
I argue that a body must fulfill several criteria to be called a planet. In accordance with the majority of the scientific community, I agree that a cosmological aspect is important (a planet must orbit a star or a stellar remnant), and that a dynamical aspect must be satisfied (a planet must be gravitationaly dominant body on its orbit). In addition to those two criteria, planet also must posses some intrinsic physical property which distinguishes it as a "planet". Because we do not fully understand planetary formation process yet, choice of this intrinsic property is arbitrary, but should have a physical significance.
Most popular such intrinsic property is roundness of the body caused by gravity. However, there are several problems with this criterion; different structural composition (ice, rock, metal) will require different object size (mass) in order for object to become round. Additionaly, roundness can be a product ont only of gravity, but also of the body's cosmogony (e.g. primordial melting of the iron meteorites, or violent kinetic event turns rocky body to rubble pile). It will be very difficult, for the bodies on the planet-minor body boundary, to distinguish between gravitational roundness and cosmological roundness. Especially if objects in question are exoplanetary candidates. Roundness is important, but obviously it is not enough.
Hence, my preffered choice for the intrinsic property of the planet is its ability to hold volatiles against the vacuum of interplanetary space. Smalll bodies cannot do that, and those that can are for certain big enough to be rounded due to its own gravity. Previously, other researches also tried to connect the (presence of) atmospheres to planethood, but their concept was not quantitative, nor were they able to discriminate dilute atmospheres from vacuum. I avoid/sove these issues using the manifold concept I developed (Pečnik & Wuchterl 2005) previously. Note that only the ability to keep the atmosphere is required, not the atmosphere itself (i.e. Mercury could still be a planet).