Capitalization (the earth? the Earth?

it is capitalized when you talk about the planet, because it is a proper noun (the name of a place) and all proper nouns are capitalized. It doesn't matter whether 'the' is in front or not. If it's the planet it's Earth or the Earth. It is not capitalized if you are talking about dirt, since that is a common noun (a thing). Again, it will not change with 'the' in front of it.