For nouns like "Mars", is the first letter supposed to be always capital letter? And also without definite article "the"?

Native speaker’s answer
Rebecca
"Mars" is a proper noun, so it's capitalised! Other planet names are also capitalised, such as "Earth," "Saturn," and "Jupiter." "The" isn't used with planet names. Sometimes people use the article "the" before "Earth," but it isn't necessary unless you're talking about it as a substance or surface - it also wouldn't be capitalised in this case. Ex: There are rings around Saturn. Ex: The earth is soft enough here to grow plants. => soil or substance Ex: Earth looks so small from space.