"earth" in the sentence refers to the world. That's the reason why the article is not required.
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lucas21c "Are you from Earth?",It's referring to a place, a home, not a physical astronomical body.
AlpheccaStars lucas21c "Are you from Earth?",It's referring to a place, a home, not a physical astronomical body.The earth is the land surface on which we live and move about.
tamguatlay(Collins Cobuild Dictionary)That's a good set of examples to study.
tamguatlayThe space shuttle Atlantis returned safely to earth today.Note that this sentence lacks the definite article. It is natural for a native speaker. It has an emotional context referring to our "home" in the universe. ie. T