The presence of "the" does not create a proper noun. Sometimes, not always, writers capitalize "earth", "sun", and similar terms which have unique referents, as in "the Earth", "the Sun". I do not believe that they are truly the names of those entities, and therefore I do not believe they are proper nouns.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Sometimes, not always, writers capitalize "earth", "sun", and similar terms which have unique referents, as in "the Earth", "the Sun". I do not believe that they are truly the names of those entities, and therefore I do not believe they are proper nouns. If they were, we could write "I see it's going to be a fine day because Sun is out", which is complete
It depends. If you are referring to the earth as in the soil or ground, it is common. If you are referring to it as the planet, then it is proper, so you would capitalize it.