The idea that race does not exist because it cannot be precisely defined (in a non-arbitrary way) is absurd.
Consider:
"Strong people tend to be better at wrestling than weak people."
"Tall people tend to be better at basketball than short people."
"Smart people tend to have higher incomes than dumb people."
"Poor people tend to have higher rates of crime than rich people."
The properties "strong, tall, smart, and poor" cannot be defined precisely in a non-arbitrary way because, just like race, they exist on a spectrum. You have to either leave it loosely defined or arbitrarily pick some point on that spectrum. Does it therefore follow that all of these statements are meaningless? Of course not. Race can be meaningfully defined, and it has been for the purpose of various comparative studies, and these have in fact found significant non-cosmetic differences.
If non-cosmetic differences between the races are the product of genetics (just as are cosmetic differences), what's the theory to explain this? As some posters here have repeatedly pointed out, skin color itself doesn't seem likely to have an effect on behavior, intelligence, etc. The theory would, presumably, be that there is some (yet undiscovered) gene(s) which people with skin color X are more likely to have than people with skin color Y. It's not that there's any causal connection between skin color and behavior themselves.
People on planet A and B might, over the millennia, develop a variety of genetic differences due to different selection pressures in their respective habitats (both natural and social). Some of these might be visible - height, skin color, etc. Others might be invisible - resistance to some disease, intelligence, tendency for violence, etc. These visible and invisible traits are correlated, not because of any causal relation between them, but because the selection pressures which generate both traits happened (by historical accident) to coexist on the same planet - and thus the inhabitants get both traits, and tend to pass them along to their descendents, even though the traits in themselves have nothing to do with one another.