"Welcome. You've come to Earth." (The Writer’s Almanac)
There seems to be a definite article or not in front of earth in dictionaries. For which meaning, there isn’t one in the example? For what meaning, is there one in other cases?
It's another peculiarity of English. The planet on which we live can be referred to as both Earth and the Earth (and sometimes also the planet Earth).
The presence or absence of the definite article doesn't imply any difference in meaning; it's largely a matter of the writer's taste.
Earth is unusual in being treated this way. Mars is always Mars or the planet Mars but not the Mars.