Frequent Meteorites is a very similar planet bonus that gives that -20 population debuff. It has the same +30% extraction rate bonus for both types of minerals.

I can see how people can get confused.

But one thing that confuses me is that you can only get Frequent Meteorites on asteroids...

planets have powerful shields called atmospheres that burn and destroy all but the most massive of meteroites while the poor asteriods do not (considering you need a substantail gravity field to get an atmosphere) so when they are in an area that is thickly populated with meteors they are not going to fair well 

true, I think that the multiple of meteors in said area would limit the amount of population in such a station to a minimum (remember the numberas we see represent billions) so that mining still goes on and at a much higher rate due to the proliferation of resources but you dont get any taxes because the is not enough of a population base to give such (though you could build good enough facilities to allow a cost free enviroment)

as for a planet have a thin/nonexistent atmosphere, yes it is possible, both Mars (thin) and Mercury (nonexistent) are perfect examples and have problems with meteors (especially Mercury)

As to the OP sorry for diverting, but yes that is what is going on, and as far as I know there is no known source for planet bounus aside from the wikia and the coding (though you can go into the coding and look it up, cant quite remember where, think Game Info folder, just download the forge tools (they are not up-to-date yet but no changes have been made to planet bonus so they will be unchanged) and open up Game Info)