The article is dropped because in this instance, "earth" is meant as a place name. " I was taught to capitalize "Earth" when used as a place name (third planet from the sun) and not when used as a common noun ("he filled the flowerpot with earth from his garden"). But one always runs the danger of appearing pompous when using capitalization, and as an English user, rather than an English expert, I would guess this is why the author chose to use the lowercase "e" in writing this passage.
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