Learn English – “take the day off” vs “take a day off”

It's not an article, but it's a determiner. At least, that's how Collins lists it.

Also from Collins, a determiner is:

a word, such as a number, article, possessive adjective, etc, that determines (limits) the meaning of a noun phrase, e.g. their in "their black cat"

Informally, one could say a determiner is like a article, only more powerful.

EDIT: As for the difference in meaning, there is none. The difference is pretty much only semantic. If I saw someone prowling around my neighbor's yard, I could say:

There's a robber at the neighbor's house!

or:

There's some robber at the neighbor's house!

and the meaning is the same: an unknown robber as the house next door.

When the noun that follows is singular, some and a mean the same thing. But some is flexible enough that it can be used with the plural, too:

There are some robbers at the neighbor's house!

We couldn't use a in place of some in that sentence; however, we could omit the determiner altogether:

There are robbers at the neighbor's house!

or we could use a different determiner:

There are three robbers at the neighbor's house!

These are all cases where the idea of having more (or less) than one Earth is considered. The other ones are of course hypothetical, but the indefinite article is used to indicate that there are other such hypothetical earths.

So, intuitively, you can consider this as 'an element from the set of hypothetical Earth-like planets' instead of the strict sense of a proper noun (although, of course, it still is a proper noun, but used as if it were a normal noun).

The same story could of course go for any other proper noun for which hypothetical alternatives could be envisioned. For example, one could write about 'a United States that never became independent of the British Empire'.