the [(stressed/emphatic), ee(unstressed)(before a vowel), ee(unstressed)(before a consonant), ə]
CORE-MEANING: an adjective, the definite article, used before somebody or something that has already been mentioned or identified, or something that is understood by both the speaker and hearer, as distinct from “a” or “an”
Encarta ® World English Dictionary © & (P) 1998-2004 Microsoft Corporation. All rights reserved.
Deriving, I think the answer is probably because this is our home; this is what we understand. The other planets are more mysterious. The same reason we say "my home," instead of stating the address. "The," a subject identifier for Earth, also gives the impression that we connect the word because we feel the Earth is more than a title for the planet we're on -- it's the label we give to our niche. I think this makes better sense if you consider some sci-fi movies, where they distinguish some planets as "New Earth".
If someone asked where you are, you would say, "I'm on Earth"/"I'm on Mars."
But when we talk about Earth, we talked about The Earth. Just like when we talk about Rock, we talk about The Rock. It's a subject identifier for the purpose of further distinguishing and compliment.
Also, like Tai Chi said, the subject identifier helps us distinguish between the ground beneath our feet. But many times, people still, even though it's obvious what we mean, refer to our planet as "The Earth".
*passes out from redundancy*