There is a widespred anecdote that "Plan 9" name comes from the
movie "Plan 9 From Outer Space".

Since I didn't find anything more than a reference to this
anecdote (see https://en.wikipedia.org/wiki/Plan_9_from_Bell_Labs
for example), I forced myself to watch the movie until the end
(what a pain!).

Guess what? I couldn't find the link between the film and the
beloved OS.

I'm sure there are people here who know more. Thanks in advance
for sharing your knowledge with us.

Pierre