What "definition" are you talking about? I talk about conceptions and models. The concept of a planet is necessarily much simpler than the real relations between planets and practically everything else in the universe. We don't understand anything fully unless we think of it in the universal context that includes everything that will ever exist. Naturally, we don't think of anything that fully, so we simplify to make our knowledge useful, and where we draw the lines in those simplifications depend partly on the type of sense organs we have, on our cognitive faculties, and our our genetic, personal, and social purposes. That subjective arbitrariness is comparable to the sort we see in the aesthetic evaluation of art.

Nothing you've said in these comments touches that argument.