i'm sure the difference between 'a lump of earth' and 'the lump of earth' is obvious to MG. You really didn't have to explain that to him/us
but we are not talking about the difference of a / the meaning sth is definite or nondefinite, mentioned for the first time or second, etc
I would say that we use 'THE' when referring to sth that is 'one of a kind', e.g.
the sun, the moon, the earth, the solar system, the galaxy, the universe, etc
In the case of particular planets we don't use THE (although we say the planets) MAYBE because they are proper nouns. I say maybe, because some proper nouns are written with the, some aren't.
we cannot apply a definition or a rule to every single grammatical problem
sometimes it's just the way we say sth (the Brits say sth :)) and it's hard to say why...