Re: planetary classes
I would say Gene Roddenberry was ignorant of astronomical subjects as much as the NBC execs. In 1965, Mariner 4 had reached Mars and was transmitting data back to NASA that revealed that Mars was not Earth-like.
Whatever Gene's level of ignorance, the original pitch was written well before the Mariner-Mars probes were launched in 1964. As far as I know, the phrase "Earth-Mars conditions" never made it into the show.
“A life is like a garden. Perfect moments can be had, but not preserved, except in memory. LLAP” — Leonard Nimoy (1931-2015)