Playing around with http://books.google.com/ngrams I noted that "Pluto" (blue) has a strong and lasting peak in the literature in the 1940s, i.e. 10+ years after its discovery: what could be the reason for that? And Neptune (red) peaks 15 years after its discovery - and again in ... 1951 (but not at all after the 1989 Voyager visit). Does all that make any sense?