adma
Superstar
Not specifically apropos the Planetarium, but these days, "completed in 1968" ain't exactly dirty words to the preservation-minded anymore. Just consider cause celebres such as IOTP, Bata, Riverdale Hospital--and Nathan Phillips Square for that matter. Heck, even consider Macy DuBois' currently-endangered brutalist office building over at 45 Charles East. And nearby, the Colonnade (1963) was designated as far back as 1983(!)But I also wouldn't clump those structures you mentioned with something completed in 1968.
Dismiss the Planetarium as relatively mundane, featureless, and unimportant in the larger scheme of things, and maybe use the alibi of NYC having replaced the original 30s Hayden Planetarium with the Rose Center. But *don't* dismiss it because it was completed in 1968.
Also, remember that one of those structures I mentioned, Eric Arthur's Wymilwood, dates from 1952. 16 years separate that from the Planetarium. 39 years separate the Planetarium from now...