What does “little earth” mean when used as an adjective?
Syntactically, “little earth” isn’t used as an adjective. Only “little” is. In the sentence fragment
familiar, friendly, well-known, little earth
there is an enumeration of adjectives (“familiar, friendly, well-known, little”) all applying to “earth”. So this earth that the author is talking about (presumably capital-E Earth, i.e. our home planet) is “familiar”, it’s “friendly”, “well-known” and, in relation to many observable planets, it’s also “little”. But besides this literal meaning I agree with TypeIA’s answer that in this context the word is likely used as as a term of endearment. Compare also the colloquial American English phrase “little old ‹…›”, with a similar idiomatic meaning.