Let me give an example. A star is defined by its ability to undergo nuclear fusion, not its mass. However, in practice, the distinction is not crucial, because we know what is happening in there well enough to have a good idea of what mass is needed to get fusion. We might be off a little, so we live with it. But ultimately, our effort is not to define stars by their minimum mass, but rather by their internal processes. It should be no different with planets, or planetary nebulae, or supernova remnants, plerions, pulsars, black holes.. need I continue?