___ Planet (travel guidebook series) (6)

I believe the answer is:

lonely

(Other definitions for lonely that I've seen before include "Solitary and sad" , "Short of companionship" , "Forlorn, solitary" , "'I wandered ..... as a cloud' Wordsworth )" , "Unfrequented" .)

I've seen this clue in The New York Times.

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