A first generation star would be made of mostly lighter materials rather than planets formed later. Since the Universe was originally made up of light elements (Hydrogen and Helium), the planets that would've been formed then would probably be mostly gas giants or planets that lack heavier elements or metals. The planets that are in the process of forming today are most likely going to be made of more heavy elements, since the dust in the nebulae have had time to condense and form heavier atoms and molecules. Also, since there are currently more stars and celestial bodies in the universe, there is more of a chance that the nebula that is forming the planets will be affected by neighboring bodies, which would interfere with the formation of
