He was a kind of an icon - it wasn't his being American so much as that he was personally quite sensitive on the issue, and back in the day he was also one of astronomy's most popular figures, sort of an unlikely hero who discovered a (presumptive) planet through a combination of astounding perseverence and good luck. The other factor is of course that "planet" isn't a scientifically useful term, so any definition would have to be based on some aspect of culture, which inevitably makes it controversial.