The IAU definition of a planet is:
The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.Reading this definition, most people would agree that (a) is a necessary condition for the definition of a planet. Even this, though, was not always the case; the initial definition of a planet was a "wandering star"; it is only incidental that the planets recognized by the ancients happened to be orbiting the Sun.
Points (b) and (c) are, however, entirely subjective. There is nothing in the laws of nature which necessitates that planets be round or that they clear their orbit out of lesser bodies. Indeed, neither of these two concepts can be defined "objectively", as no planets are perfectly round, or have picked up all objects which cross their orbits.
Nonetheless, astronomers have decided to retain the ancient concept of a planet, giving it a definition that will allow them to sort heavenly bodies into those that are planets, and those that are not. Indeed, they themselves do not agree on what constitutes a planet; according to the new definition, Pluto is not a planet, which would come as a surprise to many non-specialists and is contested by a minority of astronomers.
Is there really any differences between the astronomer's problem and that of the racial taxonomist? He, too, must decide whether or not a particular human individual belongs to a category, a race, say, whether or not someone is a Caucasoid. He can easily accept a necessary condition (a) that the individual must be human.
But, any additional criteria, e.g., a prominent nose or heavy facial hair are ultimately subjective. Indeed, people disagree about the criteria used to decide whether or not one is a Caucasoid or not. Borderline peoples such as some Central Asian Turks, Indians, or some Northwest and Northeast Africans are assigned by some to the Caucasoid race, by others to different races or are considered in-between races, just as dwarf-planets like Pluto inhabit an intermediate zone between the newly-hatched categories of "planet" and "small solar-system body".
We must recognize that races are useful labels that allow us to summarize information about an individual in a single word. They are not categories of nature with a reality of their own, in the sense that e.g., electrons or stars appear to be. Nonetheless, they are useful symbols that people use to describe a person's appearance (when talking about a someone who is absent), to infer the geographical origin of their ancestors (when meeting a stranger), or to limit the number of possibilities to be considered (in a forensic investigation).
So, it is time to admit that a race is what we say it is, and that, despite its arbitrariness, most well-meaning people can agree on racial definitions that prove to be most useful in a number of different settings.